Two 1930's SOHC Engines Get Some Attention - At Last!
Bikes and Project Builds
Recent Work On Two Engine Builds - After Years of Sitting Patiently
Back in early 2024 I had my normal 'annual' conversation with an old grasstracking and trials mate - whom has been triallng a 500T for a few years and whom I occasionally go and watch at the local VMCC Taverners trial events - those being events I used to partake in myself (rather badly, I must add) many years ago on my old RE Bullet Works Rep - and am now missing. The conversation therefore used to follow a familiar theme, of my saying something like - 'I really must put a spurt on, and finish my own Trials Norton project' , and then get back home enthused, do a little bit more work on the chassis or some other part - then move off to something else again. Well, on this particular occasion (lets say early 2024, but actually I cannot remember exactly when) - I did decide on getting home that I would try and make a better effort - and particularly - would try and do something to get the engine together. The engine in question was actually based on an early 1930's Inter engine I had had in bits for very many years, and with is rather unusual spec - would hopefully make an ideal early 1930's Trials mount.
But more of that engine later. First, let me cover some progress I have made on the other engine build I have had 'on the blocks' for the last 5 or 6 years - i.e. it has been waiting in the wings at the side of my engine build bench, waiting for my Norton single engine stand to free up from its previous occupant (my 1938/39 semi-Works spec M30 engine, which be covered in other articles when I get to them).
So, as that 30M engine was completed about 18 months ago, and finally pulled off the engine stand, and into the awaiting Gardengate Manx chassis it was intended for - so it allowed me to move the next engine onto it - this being one of the two engines talked about in this article and the one I will cover first - this being a lovely, untouched for many years - 1936 40M 350cc Racing International engine.
Engine 1 -1936 350cc 40M: Its First Stripdown in 50 Years??
Well I will be the first to admit - there is a little bit of 'Clickbait' dramatism in the heading above!, because actually - I have absolutely no idea how many years it has been since the engine I am referring to here last had its crankcases split!, but I am guessing 50 years would not be a bad guess, gauging by the amount of time I have had the engine (approximately 20 years) - and that when I acquired it, it looked like it had already been sat untouched for 30 years or so - gathering from the thick layer of dirt and grime that was covering it.
The engine in question is actually one of the earliest genuine SOHC magnesium racing engines made - the engine number signifying it is a 1936 40M 350cc engine, with the magnesium crankcase models (the design that later became called the 'Manx' models), having only been introduced in small numbers for privateer riders the year before - in 1935.
I had acquired the engine in the early 2000's from a dealer who had managed to buy most of a deceased Norton collectors parts, and along with this engine was a lot of other SOHC and DOHC Manx parts. By the time I had found out about this, most of the 500cc engine parts (which I had been looking for at the time) had already gone - but this engine had a bottom half that looked so original and straight - I could not bring myself to walk away from it at the time, and a deal was done to purchase it.
1938 Big Plunger Manx - Back in 2009
So above is a picture taken back in 2009 with the great Les Archer and my 1938 sprung chassis Manx Norton - commonly referred to as a 'Big Plunger' chassis shortly after I had finished its build. This chassis type was only offered in very small numbers in 1937 and 1938 to privateers (alongside the rigid framed racing version).
At this time I had built the bike with a slightly later 500cc 'Square Head' Manx engine - which was not really fitted to privateer Manx's until 1939, so am now keen to fit the correct type engine and maybe use the bike at some events. By the way, the engine fitted was actually a post War 500 originally owned by Les Archer, so it was a privilidge on the day to reunite Les with the engine and listen to some of his own stories of when he was road racing in the late 1940's - and a lovely person to talk to he was
Early 350cc 40M - Sits Waiting
And here is the engine that will eventually go in the 1938 Big Plunger Manx, here sat loosely built under my engine workbench about 2 years ago
The engine number shows it is a very early magnesium (SOHC Manx) M40 engine of 1936 vintage - probably less than a year after this iconic racing engine was first offered. However, it would have been of pretty much identical spec in 1938. Note alloy round barrel and bronze/alloy (bi-metal) cylinder head
1936 350cc 40M (Magnesium) Engine
But by a fortuitous coincidence, this turned out to be a real stroke of luck - because the bike I was just starting to restore at the time was my 1938 Big Plunger Manx, for which I was building and fitting a slightly later 500cc Manx engine (actually the crankcases on this engine showed it had been a post-war engine, having originally been sold to that well known Norton racing and motocross exponent Les Archer), and although I knew that strictly speaking - the big square Manx heads and barrels were not fitted until 1939 - I could not resist fitting that to the '38 bike - as like many Norton enthusiasts, those big square engines were a bit of a turn on! But on stripping the chassis down of this rare 1938 Big Plunger Manx (of which only a few were made as customer bikes - the frame being based on the Works chassis of 1936-37 - those bikes sometimes being referred to as 'Pepperpot' frames - because of the liberal drilling the Works team applied to the rear plunger castings - I found that although the bike was being fitted with a 500cc Manx engine, it was actually a 350cc 40M (i.e. Manx Grand Prix Spec) model, and would have been fitted wtih a 'Round fin' bi-metal head and barrel magnesium engine of exactly the specification of the 1936 engine shown here. and as it turned out from the chassis number - actually the first of a small batch put together for the 1938 ISDT event - which at that time was very prestigeous event - and by the late 30's had a fast speed event as one of its sections - therefore it seems the GB team had decided something quite sporty would be required. ike sh from 1936 - it being a oint WWII put a hold on all competition development for the next 5 years). drum and magnesium brake plate and shoes.Objectives of the Engine Build
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It is already fitted with bearings - and although I have not checked sizes yet, will expect they will be similar to the post war (production) conical type. Sprockets I already have a number of available (but need to check they fit!)
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Pre War 350 Manx - This a Rigid Model
This original photograph provides a great view of an original Pre-War 350cc Manx specification racing Model - in this case, taken at the TT with the Wembley race team - one of my favourite period photos
Note that this particular bike is a rigid framed models - only a few sprung models were made for customers in 37/38, and alongside them the rigid framed race model still continued to be offered
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350CC M40 Engine - What Needs To Be Done
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Lets Swop Engines! . . .
This photo taken about 5 years ago shows my 1950 DOHC Gardengate Manx project - which eventually will have the Square Head 500 engine from the 38 Big Plunger bike put back in. However for this photo I had temporarily fitted the 1936 350 Manx engine as a loose assembly - it did not stay in for long though!
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Engine 2 - 1932-33 350cc/500cc Hybrid Trials Engine Build
An unusual engine - but an interesting project
I suppose that as with many like minded motorcycle enthusiasts - I am fortunate to have built up enough parts for a number of future bike builds - a couple of these being longer term builds I tell myself I am saving for my retirement! One of those projects is the bike I talk about here with this engine - which is a intended as an early 1930's Trials Norton - but unlike the later postwar production 500T OHV model - I am building this bike to a most unlikely specification - an early 1930's SOHC Trials model. It is worth pointing out to those purists that might feel I might be 'making things up as I go along' - and there neebike projects lined up for – and quite right too!, they must have come from one of the deals I did a few years ago – but although I vaguely remember the bag – I thought at the time they were ‘fronts’, so am over the moon to find they are actually ‘rears’ and exactly what I am now looking for!As you can see from the photograph on the right - this engine
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How To Tell An Early Engine - A Couple of Giveaways
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Aqua Blasted - Looks Better Already!
This picture was taken in July 2025, having just had the crankcases, barrel and (unwelded) head back from Aqua Blasting - and it looked terrific already!
A shout out to my new found aqua blaster - who did a great job and I will be using again, Leicester based - XXXXXX
Did They Make A Pre-War SOHC Trials Bike?? - Yes They Did!
When you mention a Trials specification Norton to most people - they naturally think of the postwar 500T Norton (which actually - was the specification I was originally building my bike to), but actually Norton's were already well versed in the trials scene (as it was then) going back to the early 1030's
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The rear shoes fitted to Manx conical (racing) rear hubs had a much bigger brake shoe area than the normal plunger rear hub (fitted to production Manx's up until 1949)
What Specification Is The Engine - And What Needs To Be Done?
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1932-33 Early Crankcases
The crankcases for my trials engine are a very early set and are of the type that did not have the filter in the bevel chamber (i.e. pre-34)
They came to me many years ago with the barrel stud holes drilled out and plugs fitted. This may indicate they were the earliest type (1930'is?) which I gather had slightly different cylinder stud spacing. Many years ago I took out the alloy plugs and asked Arthur Sosbe (a very well known Leicester welder back in the day) and weld up the holes to take the normal stud spacing
Engine Bare - But Cambox Fits!
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Which And What Head To Use?
This particular project has gone through many guises - in my imagination anyway! - but the general principle I have had for it in all that time was - this was to be a usable 'Trials Iron' when done - and I was not to get to hung up about originality, or using up parts that may be considered 'less than perfect'!
With that in mind - I had a number of heads which were of the correct 'racing' pedigree (i.e. bi metal - alloy/bronze or just alloy) - but not good enough condition to put in one of the really nice builds I have. This head shown here was prime candidate - but as can be seen - will need much work to look like an original head!