Item 3: Pre-War DOHC Works Cambox
Bikes and Project Builds
A lovely reproduction of the first (Works type) Norton DOHC Cambox
This 3rd article is the final scribblings in a short series about a collection of Works spec parts I have recently acquired and is unashamedly of my favorite item in that collection – that part being a DOHC Longstroke Norton cambox - an item I have been hunting for for some time.
And not just any DOHC cambox – but this one being of the very first design type that Nortons first used on their Works racing bikes way back in 1937 – yes, a Pre-War DOHC Norton cambox!
Actually, this particular cambox is a newly manufactured item (although it has some original parts) – but it is identical to the original item in almost every way and is a work of engineering art, manufactured to the very highest standards.
I do need to do some final small engineering tasks on it, before it is ready for final assembly – but as you can imagine, I feel extremely fortunate to have been able to acquire it – and it is for a very specific purpose, so read on;
What Project Is It For?
For anyone that has followed my previous ramblings in old Newsletters or blog articles, you may know that I have been on the lookout for an early DOHC Longstroke cambox for the last 3 or 4 years. This is required for one of my longer-term projects – a 1950 DOHC Gardengate Manx restoration. Although I already had the correct spec engine for this project (albeit, admittedly – it does need to be swopped from another bike first!), the cambox currently fitted to that engine is a SOHC magnesium cambox, while the chassis on this 1950 Manx is a genuine DOHC type – and I really wanted to rebuild it in that specification.
Pre-War Works Type Cambox - Wow!
I dont mind admitting - I was like a cat with two tails on the day I finalised the swap/deal with my friend and went to collect this fantastice and completely accurate reproduction of the first DOHC Works type cambox Norton made - as used on the Works bikes from 1936/37
Although visibly similar to the later (1950) production Longstroke DOHC cambox - there were some distinct design and visual differences. Read on to see what they were
Brief Encounter With A Fortunate Outcome
Well a few months ago while visiting a good friend and race Norton collector, we were discussing other parts (see the other two articles in this three piece series of articles) – and I happened to mention again, that although I had a couple of close calls, I had still not managed to find a Longstroke DOHC cambox. I had already asked if he had anything available he may want to part with on previous visits? but of course, LS DOHC cambox’s are not exactly common, and he had told me then he had nothing available in that department!Well, on this more recent visit, when mentioning it again, he must have felt sorry for me – or maybe just wanted to rub more salt in the wound!, because when we went out into one of his workshops, he went into a corner area and pulled out a box I had not seen before – and opened it to reveal the main components of the cambox you see here.
First off, he asked me if I knew what it was? and I had to look it over a couple of times before it dawned on me that what initially looked like an early Production Longstroke doubleknocker cambox – just didn’t look quite right. And then it clicked – the main visual identifier to its real identity being the flatter shape to the top curve of the main cambox shell – this being a visual indication of the first Works developed typed DOHC cambox’s first used in 1937. The production cambox’s that followed in (circa) 1950 had a more rounded profile to the top casting, as indeed did the later shortstroke cambox’s – see the comparison photo later in the article, to see what I mean.
Of course, if you were to come across one of these in an old barn, or at an autojumble – it might be difficult to believe it was what it was – but in the case of its current surroundings, it was not too difficult to make the connection – as I knew my friend had been heavily involved in many Works bike projects - including an immediate pre-War Works rebuild.
So, I tentatively started to ask questions about the cambox – was it possibly available? or was he just showing it to me, to make me feel even worse about not yet having found anything myself – and now having shown me it, was he going to put it back in the box and never let me see it again!
As it happened, I had said something to him in our previous conversations that had pricked his own interest. This cambox – while not really for sale, he knew was the second of two he had gone to a huge amount of effort to manufacture some years ago, with this one being held back as a spare or possible swop.
But he himself was now in need of a later shortstroke cambox for another project, and I had mentioned that the second of my two shortstroke cambox’s – although missing a couple of bits, was available for possible swop.
Therefore, if I wanted to show him that cambox – there might be a possible part-exchange or deal in the offing. So, with a glimmer of a hope that there may be a chance to acquire this cambox, albeit dependent on what I had to offer - I went home with that wonderful feeling we all get, when we know we have found a project part we never realistically thought we would find - but I also knew our piggy bank situation was not at its best, in fact - all available Piggy's had already been hit with a hard hammer to afford the other bits he had offered me in recent months - see my two previous articles.
But you know what it's like when opportunities arise - think how much you will regret it in 10 years time if you dident make the effort now - and of course, you never regret the money you shouldent have spent - just the deals you decided to pass up!
Fast forward a month or two, and we had another nice Sunday morning session – with a cup of coffee and a catchup chat – before having a ‘you show me yours and I will show you mine’ session! As it turns out, without going through all the details, we were able to do a deal and I became the very lucky owner of this cambox. As It happens, the shortstroke cambox I had swopped was quite interesting in itself, it being a possible original but later type Works cambox, with an extra oilway for a cooled exhaust valve, and as I had reason to believe that that cambox may originally have come from Rex McCandless (designer of the legendary Featherbed frame, whom had access to a number of Works parts in the 1950s), that could have been the case.
Anyway, as well as having a nice catchup – it was one of those few and far between occasions where you know you have been very fortunate – and I came away unashamedly feeling like a dog with two tails, and a sigh of relief that I had finally found a suitable cambox for my DOHC Manx project!
Evolution of the Works Bikes and DOHC Engines
By the mid 1930’s Joe Craig’s racing department were doing a lot of development in all areas of the Works bikes – engine, chassis and materials, in an effort to keep the Norton race team competitive against some exotic competition, particularly multi cylinder bikes from abroad – like Moto-Guzzi, Husqvarna, BMW and later, Gilera.1935 was the last year of the rigid framed Works bikes (a frame which continued on for some production side valve models all the way through to the mid 1950’s!), but for this year the Works engines gained extra finning and a twin spart BTH magneto was tried for some races – although I gather it did not give any major improvements (I have one of these very rare magneto’s which has gone on the semi-Works spec engine going into my 1939 Gardengate Manx).
However, another feature of the 1935 TT Works Norton engines – although not much of it was talked about by the press at the time – was that an enclosed SOHC cambox was tried. More of this in the next chapter but save to say that this type of cambox was never offered to the public and little was known about it, and only a couple of pictures exist I am aware of.
1936 saw the introduction of the first ‘sprung’ chassis – this having large vertical castings with internal plungers at the rear. This frame was sometimes referred to as the ‘Pepperpot’ chassis – due to the rear plunger castings being liberally drilled on the Works bikes. I say this, because a limited number of this early type sprung frame were offered on production racers in 1937 and 1938 – I am fortunate to own one myself – but these production bikes were never sold with the rear castings drilled like the Works bikes, although I have seen it done on a couple of customer bikes – undoubtedly a lot of Work!.
I prefer to refer to these early sprung type chassis production bikes as Big Plunger frames, to differentiate them from the Gardengate frame that replaced this type in 1939
1936 - Sprung Frame Works Bikes
But going back to 1936, as well as the new frame – the Works bikes also had further development in a number of other areas, including an improved and bigger (8” for the first time) front brake - with a distinct cast brake plate which was similar to those introduced on Production Manx’s in 1937/38, bigger engine finning again, and the first changes to bore for both engines. For the 500cc, they were still the traditional longstroke 100mm stroke, but bore was inscreased fratctionally to 79.62mm from 79mm to bring them out to 499cck from 490cc. However, the 350cc engine was changed to a slightly shorter stroke, from the traditional 71 x 88mm to 73.4 x 82.5mm – the first of many changes to bore and stroke of the Works racing machines in the years that followed, a couple of these changes being reflected in production racing machines – and which resulted in a split commonly referred to as Longstroke Manx or Shortstroke Manx.
1935 TT - Rigid Frame But Enclosed SOHC Cambox
1935 was the last year the Works team used a rigid frame, but the engine had larger finning and most castings were in magnesium alloy
But look carefully under the petrol tank and see that although still SOHC - the cambox was enclosed on that years TT bikes
See the contemporary line drawngs from the weekly rags of the time below (although probably from 1937) showing two types of enclosed SOHC cambox's - and the first DOHC type in the middle
SOHC Enclosed Camboxs - Two Types
These two unusual photographs show both designs of the SOHC enclosed cambox's used by the Works team in the mid 1930's
I have seen an example of the cambox shown above - as I know Stu Rogers had one, and I think this may be the type shown in action above.
However of the second type, with removable rocker covers - the photograph shown below is the only one I know of an actual bike built with this type of cambox, it being a 350cc model and a sprung frame, so assume 1936 - a lovely period picture
1936 - DOHC Cambox Prototype And Variations
As per the 1935 bikes, 1936 engines were also using lots of magnesium alloy for the castings – and I believe this included the vertical bevel castings and both inner and outer timing covers. I have seen a Works type vertical bevel gear from this period, and it looked very different to the production type, with the shaft running in needle rollers. I do however have one set of genuine original bevel castings in magnesium, so I suspect there were some made in magnesium for the standard type vertical gears, with plain bushes.I gather that the BTH twin spark magnetos continued to be used in some races in 1936 – but photos of this magneto being used are rare by this time, so one assumes they were not seen to give a major improvement (and I know myself – the small spark plug is very difficult to get to behind the top bevel casting.
And of course – then there was the cambox – one of the most distinctive features of this iconic engine design, and which had been updated on the Works bikes with an enclosed SOHC type for 1935 and was continued to be used in 1936 in this form.
These SOHC enclosed cambox’s supposedly used solid tappet pushers (which can just be seen in the accompanying photographs. But what was not widely known – was that Norton’s had also designed a prototype DOHC cambox, which was taken to the TT in 1936, but only used in practice. It is believed that because it was still a prototype only, that Joe Craig decided it would not be used in the race for that year, until further developed over the winter period.
According to Mick Woollet in his excellent book just titled ‘Norton’, this prototype cambox was slightly different to the actual DOHC that was used in anger in 1937, and employed the earlier SOHC enclosed type solid pushers – while the 1937 type (and indeed all subsequent DOHC pushers, including, shortstroke camboxs), all used hollow pushers. I am indebted to Mick Woollet for capturing so much of this history in his excellent and detailed book and highly recommend it, if you find a copy available.
How The Norton Cambox Developed
The SOHC cambox, first developed for the Arthur Carrol SOHC engine in 1930 used bevel gears to provide the drive to the single overhead camshaft, and two rockers sat over this camshaft, with the inboard part of the rockers sat inside the main cambox casting – but the outer part of the arms exposed in the air.The camshaft bevel gear and cams were also cleverly designed with a series of radial holes and dowel pins to allow for minor variation of both cam timings (the exhaust being changed by selecting a different bevel gear dowel location, and the inlet cam by changing the small roller location that kept it locked to the exhaust cam. For the 1930’s, this was a very advanced and clever design – although more complex than many other engines of the time to setup, requiring accurate shimming and careful attention to the valve timing process – something any of us that have built one of these engines is only too aware of!
As an aside – although this SOHC cambox design looked visually almost unchanged in the long period it was offiered (approximately 28 years, if you consider that some roadgoing models were still using this cambox when production ended in 1958) – it actually had some minor changes in its first 3 years, its central cambox shape changing just slightly at least twice, and it rocker pins being enlarged in diameter, as well as the bevel cover changing in shape slightly.
But throughout the lifetime of this cambox design, one of its major drawbacks was that it was difficult to keep oiltight – particularly in a long race like the TT, and it not unusual to see many race Norton motorcycles fitted with fabricated panels over the rear mudguard behind the saddles, all the way through to the late 1940’s, in an effort to keep oil off the rear tyre!
So for 1936 there were two major changes experimented with for the Works bikes – with the introduction of two variations on a fully enclosed cambox for the SOHC cambox – in an effort to keep the oil in the engine and not on the rear wheel, and also the first experiments with a DOHC cambox.
But for 1936 the DOHC cambox would not appear in an actual race, but it seems that both versions of the enclosed SOHC cambox shells were used - and although maost action photographs of the 1936 season are difficult to make out - you can clearly see an enclosed cambox shell was being used
Guthrie on Works Bike 1936
1936 Saw the introduction of the Pepperpot type) Plunger frame, as shown here with Norton's legendary rider of the time - Jimmy Guthrie
The DOHC cambox was supposedly developed in this year as well - but was felt did not have enough development and was only used in practiice. The race bike instead being SOHC - but with enclosed rockers - of which two designs were developed. Click on photo to see larger version - notice it is fitted with twin spark BTH, tiried in 35-36 - I have one on my 39 Manx build
And below, Freddie Friths Junior TT winning bike in 1936, showing well hidden enclosed cambox
More Pictures of the 36/37 Type 'Pepperpot' Sprung Works Bike
Another lovely picture oft the 36/37 Works 'Pepperpot' Sprung frame bike - this being a 500 and carrying the No1 plate (Jimmy Guthries bike?) at one of the European GPs
Below is a lovely picture of the 1937 Works Norton team, wtih Jimmy Guthrie on bike, Joe Craig - team manager, and then Freddie Frith and Crasher White - arguably one of the best Norton team line-ups ever
Works 1937 Type 500cc Bike Details
The picture above is another rarely seen picture of the 1937 DOHC 500 Works bike - but shows some other interesting details - note special front brake securing rod and magnesium rev clock drive type ribbed outer timing cover
The picture below is one of my favourite - and shows what I am guessing was the final version of the 1937 Pepperpot Works bike - as it shows the additon of what I am supposing is an oil cooler behind the gearbox, and DOHC cambox just visible. Note also the first signs of the full square head - possibly my favourite spec of Works bike, and the last year to use this style of crankcase and chassis before major change in 38
1937 and 1938 Works Bikes Comparison - Showing Major Change
Two great pictures of one of the best riders of the pre and immediate post-war period - particularly in the Junior class - Freddie Frith
But what these two photos show to great effect is the major redesign of the 1937 Works bike (above) - with its 'Pepperpot/Big Plunger' chassis (which was offered in limited numbes as an option on Production racers in 1937/38) . . .
. . . And below is the 1938 bike - with a very different fabricated tubular frame, where the bottom frame rails encompassed around the crankcase, which became the first signs of the later Featherbed type Manx Norton engine design. This chassis type was never offered to the public and was quite different to the 1939 Gardengate production Manx chassis. Both models are fitted with the DOHC cambox though
1937 And the DOHC Cambox Is Used In Anger
So moving into 1937, saw the second year of the Pepperpot sprung frame type Works Norton’s and the first year where the DOHC cambox was used in anger on the Works bikes – from this point forward, the Works bikes would continue to use DOHC cambox’s - but these prewar type were still quite different from the Production Longstroke type that were first fitted to some customer Gardengate Manx Nortons in 1949-50.Visually on the outside, the cambox shell had a distinct flat top to the centre of the casting – while if compared directly to the production versions that followed in 1949-50, those had a slightly more curved appearance.
The rear cover casting looked externally similar to the later production version on the outside – and like the early production Longstroke DOHC cambox, the external finish was plain in the centre section of the casting, with no provision for a rev clock drive – unlike the later Featherbed DOHC cambox’s – that had the rev clock drive driven off the central bevel camshaft (see the photo of the Longstroke vs Shortstroke cambox at the end of this article to see what I mean).
Instead, on those 1937 Works engines, a rev clock was adopted (I believe for the first time that year) – but that being made available from a ribbed outer timing cover with a provision for the drive off the oil pump drive gear.
Incidentally – I believe that 1937 year was the only year a ribbed magnesium outer timing cover was used (with rev clock drive) on the Works bikes – a rare item we have also replicated in magnesium on our online catalog – and I have fitted one of these to the semi-Works spec engine going in my ‘39 Gardengate Manx.
But it is the inside of this early Works type DOHC cambox that is quite different to all production type DOHC cambox designs that followed, and particularly the magnesium rear cover design – in that the central bevel gear shaft and idler gear shafts are located directly into substantial casting bosses on the rear cover itself. If you were to look at this (pre-war) DOHC rear cover having never seen the later production type DOHC cambox – it might seem a perfectly logical decision to design the cambox in this way – after all, the rear cover bosses look very robust – and on removing the rear cover, it gives immediate access to all the gears.
First DOHC Works Internals vs Production Type That Followed
The picture above of the pre-war Works design shows that the rear location for the idler pins and central bevel shaft bearing are both located directly into the magnesium rear cambox cover
While the picture below is of the inside of a later 'Shortstroke cambox design (of mid 1950s production type) - and clearly shows the addiional outrigger plate casting
This outrigger plate meant the rear cover could be removed without displacing all the gears
Internal Differences - DOHC Works vs Production Designs
But if you look at the design of the first Longstroke production (and all subsequent production) DOHC cambox’s – you will see that the later production versions all incorporated an additional ‘interim’ casting piece, i.e. an outrigger plate – designed in such a way that if the rear outer cover was removed – the central bevel gear shaft and both intermediate gears would still be positively located and retained between the main cambox shell and the outrigger casting (referred to as the ‘Outrigger Plate’ in the 1950 Spare Parts catalog). Look at the two photographs shown on the left (and click on the picture for a larger version) and you will see what I mean, the top picture showing the rear cover of the pre-War cambox rear cover, while the lower picture shows a later production Shortroke cambox - with a very different back cover, and all the imtermediate gears retained securely within the confines of the outrigger plate on the later production typeI can only suppose that this was a case of the old saying that ‘racing improves the breed’, and maybe the Works mechanics found that with this first 'Works' design – that the effort of removing the back cover (which probably required some careful tapping with a mallet - as it is closely machined to the main cambox casting and held in place by dowels) could also quite easily result in an idler gear, or the caged rollers dropping away too easily, if the cambox needed to be stripped in the heat of battle.
I can only guess at this – but certainly this is one of the key differences to the later production type Longstroke cambox that came in from 1950.
I have not had a chance yet to compare this early Works design to a later Production type Longstroke cambox - but visually, much of the other details seem similar. Notably - the cambox feet are visiually similar on the underneath of this Works cambox shell to the 1950 Longstroke production type - particularly rhe rear feet are cast into the main cambox shell. This meant that small 'cotton reel' shaped platform supports were required between the head and cambox shell to mount the cambox on. That practice continued through to the first production Longstroke DOHC cambox design postwar, but as you can see by the comparison photos below - the later Shortstroke cambox shell dispensed with the platform spacers - instead haveing a built up cylinder head casting, machined horizontally, for the cambox sit on directly - and the bottom of the later cambox shels also more subtstantial, and then machined flat - to directly mate on that cylinder head platfomr, without spacers.
1938-39 Bikes Looked Very Different
So it was clear that a lot of development had taken place between 1937 and 38, and the new Works bikes looked very different in detail to the previous years bikes, and although the DOHC cambox was carried through with little change, the bottom half of the engine had had a major re-design, and most notably, the crankcases had been heavily modified - with a bigger and deeper throat area to better support the barrel. There was also additional cooling fins cast vertically into the front and bottom of the crankcase external walls - and this design was later carried through to the Featherbed Manx Norton engines.But the major engine revelation for 1938 was that a change to the bore and stroke of the 500cc engein was announted - this being changed from the time honoured 79mm x 100mm Norton 'Longstroke' dimensons - to 82mm x 94.3mm. Still Longstroke - but now moving closer to a 'square' engine design, as many other race designs were developing towards. Likewise the 350cc Works bike was amended to 75.9mm x 77mm.
Chassis Re-Design
As can be seen from the photos on the right - as well as a tubular plunger chassis, replacing the Big Plunger type, 1938 was the first year that telescopic forks were used (no doubt because BMW and other racing marques were starting to used them) - but initially used springs only, with no hydraulic dampening (unlike the tele forks postwar), but still were found to give excellent results - greatly helping to reduce fork chatter.But even with those major changes, there were other less obvious improvements visible. The Works petrol tanks were already being made in alloy prior to 1938 - these were modified slightly in 1938 with small cutaways, to allow for the new tele forks, but now for 1938 a new design of larger aluminum oil tank was also introduced, but this no longer being a 'wrapround' design - instead it being mounted on the drive side of the bike.
This allowed for two further changes - what looks like a much larger Amal carb - rubber mounted to the cylinder head with a long inlet tract and twin floadchabmers.
And in addition, below the carburettor, the new design of oil tank allowed room for the introduction of a large cylindrical oil filter.
Works 38-39 Detail
Look closely at this picture of the 38-39 Works bike (also used immediately post-war) - reveals many fundamental changes - including Tele forks, magnesium gearbox, different crankcase castings, conical hubs and a super large carb on a rubber mounted inlet tract
The close-up of the drive side of this bike below shows quite clearly the early DOHC cambox, visible below the (alloy) fuel tank
Pre-War 1938-39 Woirks Bike - A Rare Photo
First of all - I would like to thank well known SOHC expert (and NOC marque specialist) Barry Stickland - for permission to use these previously unpublished photographs, that came from the family of Joe Craig.
These two lovely photos show lots of details of the Pre-War factory Works racers - I am guessing either 1938 or 1939. This bike would have been fitted with a DOHC cambox of the type talked about in ths article Click on thumbnail to see a bigger version, showing all the differencs between the Works bikes of this period and producton Manx spec models.
And Even More Changes On The Works Bikes
As I mentioned in the previous 'Works Trinkets' article, although 1937 marked the introduction of a conical front brake for the first time, 1938 also saw the introduction of a rear conical brake - both brakes having a larger brake surface than previously and were supposedly much improved.Final major change for the 1938 Works bikes was a redesign of the 'upright' Dolls Head type gearbox, and although looking visibly similar to the production Manx type Dollshead gearbox - in fact the main gearbox shell was very different, having a square platform base, to allow it to slide alond the new frame rails, and a gearbox cover now cast in magnesium and with subtle differences to the production type. I have seen one of the pre-war Works gearbox's in the flesh and they look quite different to production gearboxs. They also had very different shafts and gears and Works Norton gearboxs employed roller bearings and a six spring clutch with alloy cover.
As you can see from the photographs above and to the left - Nortons had been experimenting with megaphones since the mid 1930's, but for 1938 they had increased in size again - and were starting to look much larger than the smaller type starting to be used on customer race bikes.
Incidentally, if you look at the post-war version of this bike (which I am not entirely sure - but may have been the pre-war bikes held in storage through the war years) - they also had large megaphones, but had extra strengthening plates on the outside mounting points, probably to reduce the risk of them splitting through vibration in longer races.
Lots of other small changes are evident on the 38-39 bikes, but overall it was clear that Norton's were still fighting to stay competative in the top tier of Grand Prix motorcycle racing, against the might of European competition, particulalry BMW, Gilera and Moto Guzzi - and the increasing threat of multi-cylinder and supercharged competition.
It is for this reason - and the genuine rarity of pre-World War 2 Works Norton bikes and parts still extant, that I think this remains my favourite of all racing motorcycle era's and the Works Norton's of that period between 1936-1939 my favourite Norton race models (although I confess - the early to mid 50's Works bikes come a very close 2nd!).
Norton SOHC/DOHC Cambox Lineage
As part of the process of writing this article, I thought it might be worth giving a bit of additional information about the lineage of the SOHC and DOHC cambox development - which of course started with the first creation of the Arthur Carrol designed SOHC version in 1930.The photograph of the camboxs below show how the SOHC evolved through to the DOHC designs, in an interesting side by side comparison. And although that is an early SOHC cambox itself (approximately 1932-33), it’s general shape and profile remained almost unaltered on roadgoing bikes until the roadgoing Featherbed International bowed out in approximately 1957.
However, although the cambox design looked visually much the same throughout its lifetime – there were a few variations, and particularly, from the mid 1930’s – the racing version was normally cast in magnesium, used a central camshaft oil feed (with different positioned oil feed boss), and employed a smaller roller bearing in the cambox backplate, with a noticeably smaller external bulge visible than the aluminium backplate casting.
But actually – in its pre-war guise, there were a few variations and changes made to the SOHC cambox – particularly in its early years of use – and although less noticeable they show some early development did take place and that also helps identify the approximate year of the early cambox’s.
The very first cambox’s 1930/31 – as shown in the two photographs on the right, had a slightly different shape to those that followed – and a different casting for the front bevel cover, more rounded than the ones that followed (from approx. 1931-32). The earliest camboxs also had a different cylinder stud spacing I believe – but many years since I had discussed this with anyone in the know, so cannot remember exactly how different this spacing was!
As you can see, in this earliest guise cambox, the shape of the shell around the front bevel cover is also slightly different in profile, but also, there seemed to have been an additional external oil pipe on the right side of the bevel tunnel. I am supposing this was to help drainage?
Earliest SOHC Cambox Design -1930
The first (Arthur Carrol) type SOHC cambox had many subtle differences to the cambox followed, and its detail shape and features changed a few times in the first 4 years. The cambox above shows the earliest bevel cover and cambox shape
The picture below shows the seperate oilway on the earliest models. This is Phil Newsteads beautifully restored 1930 3-stay frame model
Norton SOHC and DOHC Cambox Evolution
This picture shows quite clearly almost 30 years of development!
On the left is an early (aluminiun) SOHC cambox of approximately 1932 type.
In the middle is the first (Works type) DOHC Longstroke cambox - used by the Works team from 1937 to the late 1940's.
On the right is a Production type Shortstroke cambox, which remained almost unchanged until Manx Nortons ceased production in 1962-63.
SOHC Magnesium (Racing) Cambox
The picture above shows the production version of the racing (magnesium) cambox - which had a central oil feed through the bevel cover to the camshaft and directly onto the cam lobes
The cambox in the photo is an original pre-War Manx cambox (on my 1939 Gardengate Manx) - there were a couple of subtle machining differences on the postwar type - but they continued to be fitted to Manx models until circa 1950, but it seems remaining stocks were then fitted to Featherbed road Internationals - Nortons did not like to waste stock!
SOHC Cambox Early Development
So coming to the comparison photograph of the SOHC and DOHC camboxs on the left – although the SOHC cambox in the photo is is still an early cambox (approximately 1932), and had not reached its final development stage yet, you can see that its general shape and front bevel cover looks to have changed to the more familiar profile that was standardized thereafter. Although actually this cambox shell does have little cast lumps (ears?) on the top surface of the main shell – almost like Nortons were considering putting rubber adjusters in there, which of course they did not – and they almost certainly would have fouled the frame top tube – but still one of the major shortfalls of this cambox design . . . as top rubbers could not be further tightened once worn, unlike the lower rubbers, where there was room for adjusters to be fitted.But the main difference on earlier camboxs (i.e. pre-1933 if I remember correctly) was that the rocker spindle shaft diameter was smaller than on later cambox’s – and therefore the rocker arms were a different forging to the later rocker arms (which I am guessing was in 1934-35). The SOHC Cambox in the photo on the left (which actually is a lovely timewarp cambox, which has sat unrestored for many years) is one of these early camboxs with smaller rocker arm shaft and the early rockers. But from about 1933 onwards, the rocker pin was enlarged and standardised, although pre-war most rocker arms just used flat shims either side of the roller caged bearings, while later on this was changed to cup style side washers (normally seen on post war cambox’s).
And the other visible difference on some cambox’s (i.e. CS1 rather than International spec cambox’s) – was that the CS1 variant often had the threaded hole for the exhaust lifter cable adjuster cast into the frontmost rear cambox leg. Most International cambox’s had a peg screwed into the cambox back casting – although racing cambox rear castings were sometimes cast with no provision for the exhaust lifter spindle at all!
However, the one feature that seemingly all the SOHC cambox’s shared was that they struggled to keep their oil inside their central camshaft chamber! particularly when slightly worn or well used – a Norton International with a rusty oil tank is a rarity!
But although there are lots of reasons for an oil leaking cambox – one of the most common is the rubbers and cork washers that are packed around the rocker arm central boss have become worn and ineffective. This is compounded if the rocker arm central bosses have become scored with scratch marks, where the rubbers rub against them. Careful cleanup of that rocker arm radius and careful fitting of new rubbers and corks (which we can supply), often make things much better – remembering that if on fitting new seals - if you can move the rocker arms easily – you probably haven’t fitted them tight enough!
So although I talked earlier about a SOHC enclosed cambox developed for the 1935 Works bikes, the final racing development of the SOHC cambox was the introduction of a magnesium alloy version sometime about 1934. Norton had already developed a central oil feed through the front bevel cover, the camshaft - and then onto the cam lobes directly on earler racing cambox's - but this magnesium version had this oil feed as standard, and it was this cambox that was fitted to the Racing International (M30/M40) and then Manx Norton going forward, until the DOHC camboxs were introduced in 1950.
Incidentally, most pre-war versions of this SOHC magnesium cambox I have seen have looked slightly different to the postwar versions under the main shell - the postwar versions often being milled across the base of the central cambox casting
The Longstroke DOHC Manx Engine
The picture above (a lovely picture of two original 1950-51 DOHC Gardengate Manx engines from a late friend of mine) shows two eary LS Production DOHC cambox's - note the idler shaft retaining nuts on either side of the bevel cover - not present on the Works LS cambox. Engine on the right is a super rare DOHC 600 sidecar engine
Below - Note that neither the (earlier) LS Works type cambox, or the later Shortstroke production type cambox have the idler shafts coming through te main cambox shell
LongStroke DOHC Engines In F3 Cars
And of course - when Formua 3 500cc racing became popular in the early 1950's - the 500cc DOHC Longstroke Manx became the engine of choice for the top runner. This nicely prepared engine is sat in a Kieft - which along with Cooper became one of the major manufacturers of Formula 3 race cars
As you can see here - not only was the Manx engine being used - but also a (pre-Featherbed) 'upright' type Manx Norton gearbox and clutch. These gearboxs were well known for their reliability and robustness
DOHC Cambox differences
The next cambox, in the middle of the photo showing the 3 cambox comparison photograph above, is of course the Works pre-war type DOHC cambox, this being the version first used in 1937, but not offered to the public until much later in approximately 1950. But as I have already explained earlier in the article – when it was offered to the public (on Longstroke Gardengate Manx’s initially in 1949-50), Nortons had amended the design from the Works one in the comparison photo, most notably adding an outrigger plate inside the cambox, to keep the interim gears in place.Visibly, this first production type 'Longstroke' DOHC cambox was easily identifiable from the earlier Works type because on the main cambox shell, timing side, on either side of the bevel cover was a nut visible, which was to retain the idler shafts. Neither the earlier Works type cambox, or the later Shortstroke production cambox that followed this design (from 1954), had these external shaft retaining nuts. This can be seen clearly in the photograph on the left of two original production Longstroke engines, both with the first production type (Gardengate chassis) DOHC camboxs.
And the other noticeable external feature of these first DOHC production cambox's was the slightly more rounded top profile of the cambox shell, rather than the slightly more angular Works type - and this more curved profile carried through to the shortstroker version
DOHC Manx Engine and F3 Cars
Another interesting part of the history of this evolution of the Manx Norton engine was that in the late 1940's through to the mid 1950's Formula 3 500cc car racing became very popular - indeed it was that class of racing that allowed Stirling Moss as well as many other aspiring car racers to first show their skills. And very quickly it was the DOHC 500cc Manx engine, with the 79.6mm x 100mm Longstroke dimensions that was the preferred engine for top runners.Alongside this, the popularity of these engines with the car racing brigade provided further demand for those who had the skills to build one properly and keep it well maintained - and this provided much work for some of the well known Norton tuners, and particularly those like Francis Beart and Steve Lancefield who became two of the most in demand tuners of F3 engines at that time.
As an aside - supposedly this helped the popularity of the Triton motorcycle - i.e a Featherbed Norton chassis with a Triumph engine, as Norton's would not sell just an engine alone to the car people - so a number of Manx Featherbed chassi's were sold, and had Triumph engines put in them!
The Formula 3 500cc Class continued through much of the 1950's, and one of the most well known exponents was Jim Russell (who later opened a racing school) - and he continued to win National championships with a Norton engined F3 car until 1957, but by this time interest in the 500cc formula was dwindling and in 1959 the new Formula Junior class, of 1100cc, using mainly purpose built car engines such as those from BMC, signifying the end of the Manx Norton engine's long period of domination in this popular class.
One final point before we move on from the F3 cars, although by the time the F3 formula came to an end (which I believe was 1958), the shortstroke Norton engine had already been out for approximately 4 years - but supposedly the F3 car people only used the Longstroke engine, and normally running on Dope fuel (methanol), the shortstroke engine supposedly not being well suited to these cars. I would be interestd to know if any shortstroke engines were used in period though - I could understand them having less torque, but having ridden both - I would have thought a well prepared shortstroke engine in an F3 car could still be highly competative?
!950 and The Featherbed Gives Norton a New Lease of Life
The Longstroke DOHC type cambox went through a couple of iterations after its introduction on the production Gardengate Manx in 1949-50. But of course the main change to the Manx Norton was the introduction of the Featherbed Manx in 1950 (Works bikes) and this then being offered to customers from 1951. This chassis of course revolutionised bike racing in the 1950's and gave Norton a much needed 'second wind', when being raced against increasingly sophisticated foreign opposition. And from 1951, when offered to private customers - it provided a fantastic race mount for anyone that could afford one, and remained competative all the way into the late 1960's - a fantastic achievement for any motorcycle
Longtroke DOHC vs Shortstroke
these two photographs show an interesting comparison between the Longstroke and Shortstroke camboxs
The shortstroke cambox had clearly evolved and the front and rear cambox castings were now machined together to give a very rigid base platform for fixture to the cylinder head. Note also the rev clock gearbox on the cambox backplate - although this actually was introduced for the Featherbed Longstroke DOHC cambox
Top Vertical Bevel Castings - LS/SS
Another interesting direct comparison - Longstroke top vertical bevel casting on left and Shortstroke on right
To my knowledge - the shortstroke version was always in alloy - I guess they did not think it was big enough to warrant the small weight saving magnesium alloy would give
The first production Featherbed Manx's offered to customers in 1951 closely followed the previous years Works bikes - at least in superficial appearances - the Featherbed Manx engine had a very different bottom end to the Gargendgate - the crankcases being based on the Works plunger type, originating pre-war, but looking very different to the Gardengate Manx crankcases which could trace their lineage and shape back to the International.
However the Featherbed DOHC Longstroke cambox was pretty much the same as the earlier production Gardengate Manx and shared the big 'square' head of that engine. The Featherbed Manx engine no longer had its rev clock gearbox on the timing cover - this instead now being cast into the DOHC cambox rear cover
Incidentally - although not widely appreciated, although the Featherbed chassis replaced the Gardengate on production Manx's from 1951, the last Gardengate Manx's were listed in the Norton records as being produced in 1952 - albeit for Daytona races, the home market already having standardised on the Longstroke Featherbed Manx for customer bikes by this time.
Longstroke Manx Becomes Shortstroke
And then, in 1954, came the next major development in the evolution of the production Manx engine - with the introduction of the Shortstroke engine, the bore of the production 500cc unit finally being changed from the longstandng 100mm stroke to a much shorter 86mm bore and 85.8mm stroke, to give a capacity of 498cc.To go with this revised 'shortstroke' type configuration, the production DOHC cambox also received a further redesign - and most notably, the rear mounting vertical bolt locations (i.e. the drive side) were now incorporated into the rear magnesium cover and the mounting onto the cylinder head became a much larger area, with the main cambox casting and the rear cover, both machined as a pair.
Likewise the Shortstroke cylinder head was now cast with two large oval shaped platform supports to provide a very rigid base for the cambox.
You will see in the photo comparison on the left that the hollow pushers on the shortstroke engine were of a wider diamter than the earlier Longstroke cambox. And like the earlier cambox design - they could be fitted with different thickness shim caps.
Top Vertical Bevel Casting Redesign
Another component to get a major redesign for the shortstroke engine was the top vertical bevel housing - and the photograph on the left shows this well. The Longstroke DOHC vertical bevel casting on the left, does not look disimilar to the SOHC version in general design - but its mounting flange is deeper and it has additional drainage holes at the rear of the casting (and by the way, if you look at genuine SOHC racing cambox's - they often have more drainage holes around the front bevel chamber casting than road cambox's - which indicates drainage may have been an important factor on a hard worked engine). On the earlier Longstroke DOHC cambox's this casting was always in magnesium and as far as I can gather - was part of the original Works cambox design in 1937 (although the one in the photograph is a genuine Norton item from a slightly later date).But you can see that the newly designed Shortstroke cambox has a much shorter alloy vertical bevel casting fitted, with a circular flange. This was to allow it to fit directly into the new shortstroke head, which had the vertical shaft tunnel cast into it - dispensing with a seperate bevel tube at this point for the first time. The alloy casting was fitted with an O-ring to give it its seal, no doubt making cambox removal quicker than had previously been the case - as the vertical shaft could stay in-situ
DOHC Cambox Final Changes
If you look closely at the actual shaft of the bevel gear, protruding from the bottom of this shortstroke type alloy top bevel casting (above) - you will see that there is an alloy ring around the shaft, but the vertical bevel shaft still has a slot - indicating this cambox is mid 1950's and still using Oldham couplings, the alloy ring helping to centralise the coupling in the shaft.In the later design of the Featherbed Manx engine, Oldham couplings were finally dispensed with, instead being replaced with a splined vertical shaft running in needle rollers - this being adopted on production Manx Nortons from 1959. The actual teeth on the bevel gears had also been modified in 1957, making them slightly coarser and therefore stronger than previously - the top vertical bevel gears always being a weak point, and increasingly so with the increased revs of the shortstroke engine.
There was of course one further design of the Manx Norton cambox experimented with by Norton's in the late 1950's - this being a Desmodromic cambox developed by Doug Hele. This cambox was taken to the Isle of Man TT and was used in practice in 1959 and 1960, but was eventually dropped, with Hele saying later that although the 350cc Desmo would safely rev to over 9000 rpm, it was no more powerful than a standard Manx engine and the complexity of setup would make it unsuitable for fitting to a Production model, which were the main stay of privateer racers and maintenance would have been difficult.
I have seen one of these Desmo cambox's close up - and they are an amazing piece of engineering, I may do a small article on them sometime in the future.
Are LS and SS DOHC Camboxs Interchangeable?
So, to go back to my own projects and cambox's - as I said at the beginning of this article, I had actually started looking for a DOHC cambox some years ago, for my 1950 DOHC Gargendgate Manx project - which had a correct type Longstroke 500cc engine lined up, but in its current guise - with a SOHC cambox on it. I had then managed to acquire two very nice and original DOHC shortstroke cambox's about 5 years ago - and of these, the one shown in the photographs above had been sympathetically restored and had new pusher bearings fitted (by Andy Molnar) and is ready to drop onto an engine. The second cambox was also cleaned and reassembled and ran well, but required cams fitting - and this was avaialable as a possible p-ex or swop for a Longstroke, if I could find one.But my reasoning at the time back then was - if I could not acquire a Longstroke cambox - maybe I could fit the shortstroke cambox to the Longstroke engine? I had been told by a couple of people in the know that a shortstroke cambox would not fit a Longstroke head - but on further questionning, those friends had not actually tried. However, I knew this was likely to be the case as of course the Shortstroke engine is very different in design to the Longstroke and I suspected valve angles in the heads were steeper.
But as this project has not been high on my priority list in the last 3 years - and actually I have not had any spare funds available to seriously look at DOHC cambox options, instead, it had just been another back burner project, although I had mentioned to a few people I was on the lookout for a Longstroke cambox at some point.
More recently though - I did make enquiries with Andy Molnar (who must be considered one of the foremost experts on the DOHC design, and has manufactured both LS and SS cambox's, and can supply parts for both types - see his site here: https://www.tga.co.uk/shop/) - and he was able to confirm that the two cambox types - although visually similar, were definitely not interchangeable. In fact there were lots of differences, but most importantly the valve angles of the Shortstroke head are different (steeper) than the earlier Longstroke heads.
A Backup Head - Just in Case!
But, with that in mind - going back to the time I had restored the two shortstroke cambox's, I had also managed to acquire from another Norton friend, a most unusual Longstroke Manx cylinder head he had acquired many years ago. The head was in almost mint condition, but had had some pretty serious surgery on the inlet tract and valve angles at some time, and had been built up with (suprisingly small) valves already.My friend only had scant details about its history - although he knew it had been professionally done, but we suspect it had been converted to fit a Shortstroke head, and the inlet tract looked very similar - so this was my backup plan at the time, but of course - I am hoping now I have the LS Works head, it wont be necessary. It may be useful for a future project though - after all, I still have the Shortstroke DOHC cambox!
Norton Literature for DOHC Cambox's
Above shows the only known line drawing I am aware of from Norton Motors for their DOHC cambox. This being an exploded drawing of the (first production type) LS 1950 cambox - easily identifiable because it has idler gear shafs protruding through the main cambox shell
I do have a rare article on servicing the later cambox from Norton Motors (I will be reproduing that in our Facsimlle catalog section shorttly) - but I never saw a formal exploded spare parts list diagram of the DOHC cambox appear formalliy after this one
Below - my backup plan if I had not found a Longstroke cambox! - a rather special Longstroke cylinder head, which has had heavy surgery on the inlet tract and valves - I think to make it suitable for a Shortstroke cambox
To go with this new cambox - the cam profiles change
As a final point - and actually, the reason why I was looking for an earlier Longstroke DOHC cambox for my 1950 build anyway - I believe the valve angle on shortstroke engines was different to the Longstroke engines - and therefore the pusher angle on the two different cambox types are different and dont line up the same. Therefore, supposedly they are not interchangeable. I was able to acquire and rebuild the shortstroke cambox shown in this article some years ago - and was my possible 'backstop' for my 1950 build, if I could not find a Longstroke cambox - but although I had loosely tried it on an earlier LS head, it was not with correct length valves fitted - and I could not be fully sure.Frankly - I had it in my head, that I would somehow try to make it work - and have a special LS head put by to go with it, which I think had had its valve angles modified to shortstroke configuration. But hopefully, now having the earlier cambox for the 1950 build, I now have other plans for the shortstroke cambox, for a possible future project (I am sure I not the only one that likes to daydream on future projects - even if we never get there!) the Works version shown here – but in 1954 came the introduction of the Shortstroke type DOHC cambox – and this type is shown on the right of the photograph.
Allowing for a couple of Works type cambox’s (i.e. in 1953-54, with oil cooled exhaust valve, one of which I had until recently – I think!, and an inlet cam driven fuel pump) or the Desmo cambox experiments – I believe that Norton did not deviate past this final form for the remainder of their Manx lifetime. To my knowledge, this final Shortstroke cambox design – with a large flat machined surface, and dowel pins, that located directly with the Shortstroke cylinder head, remained pretty much unchanged until the Manx ceased to be offered – the last ones being put together in early 1963.
But if you have ever looked at one of these camboxs closely in detail – you cannot help but be impressed by the engineering detail and prowess that resulted from 30 years or so of racing development – so much ahead of what was being offered by 90% of the British Bike industry at the time – they really are an engineering marvel. And of course, to have ridden a well put together Shortstroke Manx Norton . . . I am still amazed, the Norton race department knew what they were doing – they go like stink!
DOHC Cams - Longstroke vs Shortstroke
I am fortuntate to have a choice of cams when I assemble this Works type DOHC cambox.
Above are 2 types of original Norton Longstroke cams (identifiable by the profile number - but also LS cams mostly have a raised lip around the shaft hole). These are 350 profiles, but I have a set of 500 LS cams set by for this cambox.
However, I am told by someone in the know, that Shortstroke cams work well with the LS DOHC cambox, and I also have a set of NOS SS 1955 cam profiles reserved for it - as per below. I can make the choice later
Inside The Cambox - A Lot of Parts!
Open up this early Works type DOHC cambox by removing its beautifully machined rear cover - and the key difference to a later Prodution type cambox is obvious - this cambox has no triangular shaped 'Outrigger' plate
Instead - the rear cover is machined to take all the shafts for the central bevel shaft and idler gears directly - very different to the later camobox's
Well - What About This Works Cambox?
So, coming back to the DOHC cambox which I have recently acquired - what will I be doing with it?Well as I mentioned at the start of the article - this cambox was as a result of chance conversation with a good friend of mine, when mentioniing I was still looking for a Longstroke DOHC cambox - but actually, this was to go on a Production spec 1950 Manx Norton project I have.
And having now confirmed that the possible fitting of a Shortstroke cambox on my 1950 engine would be difficult - in not impossible (my original backup plan), I am very pleased I was able to this earlier type cambox, albeit of Works pre-war design - and although it still needs some final engineering work to the internals, before final assembly can be attempted, they do not look too ardous and I am looking forward to getting to grips with it. For the moment, as I am still heavily involved with my 1939 Manx project, I will put it aside - and look forward to tackling it in earnest a bit further down the line - and no doubt will give a follow up article when I do.
As you can see from the picture on the left, all the internals are of a very high quality - most having been newly manufactured, or being 'New Old Stock' but I know I am still missing a few parts - so I imagine I will also be paying Andy Molnars site a visit at some point, but that can wait now until I get to its final build. I also have a friend who intends to build one as well - so I imagine he will want to have a peek inside this one as it is!
I am sure there are purists out there who would be asking - shouldent I be trying to acquire and build a full Works bike? - and I did recently have the opportunity to acquire some other Works parts of the same era, but I know what a huge task that is - and I have a real need for a DOHC cambox for what is already a very nice and intersting production Manx project.
Yes, this Works cambox was of the type first fitted pre-war, but to my knowledge it continued to be used on the Works bikes postwar (I am not sure at what point the Works teams redesigned it to have an outrigger plate) - and I dont think it is entirely outside the realms of possibility that one could have found its way onto a production Manx of immediate post war period!
Thats It For Now - But More To Follow With This Build In Future
So thats about it for now with the DOHC cambox update. Having had a hiatus in the last two years from acquiring any substantial Norton items (quite a bit had been happening on other fronts, and keeping the normal bills paid had been my main priority) - it was great to be able to do a deal with a very good friend to acquire this, and to help him out with some parts he needed as well.
If you had read the other two articles, that preceded this one - you will know that this was one of a few very special parts I managed to acquire from this friend - and I have certainly been very lucky to have been offered them - but for the moment, my main priority (as I write this update in May 2025), is to move forward with my 1939 Manx 500 - and that is now progressing well.
I have not given any update on that recently - but the engine and gearbox are now both complete and installed, and next task is to finish assembling the (Manx type) girder forks, which will complete the main chassis build.
I already have the front wheel ready for that bike - but the Works pre-war hub covered in the previous article will then get painted and laced to an alloy rim - and I will cover that in a future article - I was already half way through a Youtube update on that project last year - so maybe I need to go back and finish that next!
As for this DOHC cambox, and the 1950 Gargdengate Manx project it is intended for - well I am sure I will be giving another update on that further down the line!
Future Cambox Build Work
So for the moment, the Longstroke cambox will be packed carefully away, along with all its parts - until I am ready to do the final remaining engineering tasks (i.e. the special DOHC cam bearings need grinding to size) - whch may be a while yet, I want to complete the 1939 Manx first - but it is great to have been offered it and been able to do a deal that helped us both out
The other big benefit of having been able to acquire a Longstroke DOHC cambox - is that it means I am hoping I wont have to put into play my 'backup plan' - i.e. to use the later DOHC Shortstroke cambox I had previously rebuilt (shown above) - and it now leaves that cambox available for another potential engine project I have in mind for the future - something for a possible very different purpose - but more of that in a future article!
1950 Gardengate DOHC Manx Project
This final picture shows the project build that the Longstroke DOHC Cambox is reserved for - this being an original 1950 DOHC (but Production - not Works!) Manx. In the photo is shown a 350cc Prewar Manx engine, installed- for trial fit only which will be swopped with another Manx engine of the correct 500 post war period - ex Les Archer, unit in the near future
Although it has many original and correct parts for a 1950 Manx - including correct alloy petrol and oil tank - I am also not too fussed about complete originality. So although I have a correct SLS Manx brake plate and shoes, I dont have the correct 1950 hub.
But I do have this lovely Beart TLS front wheel .. . so do you blame me for fitting this later type - who could resist?!
