Original Norton DOHC Cambox(s) Stripdown - Part 2: July 2019


** Click on any picture in article to see a larger version **
Cambox with gears removed
A reminder where we had got to at the end of the previous article - the camshafts had been removed, along with the outrigger bearing and idler wheels - all of which were in good condition, and just needed a thorough cleaning.
Next task - tolook at the old Pusher bearing replacement



DOHC Cambox Stripdown - Next Task?

In the previoius article I had covered the first part of the stripdown of this very original and unmolested mid 1950's DOHC cambox - which had got it to the point where the camshafts had been removed, along with the outrigger casting and the idler gears.
The most obvious next step would be to remove the central bevel shaft assembly - which could be seen still in place in the accompanyning photo on the left. However, having had a close inspection of the rear roller bearing, the front bevel gears and the gear wheel (that drives the idler gears) I was having doubts to as if this was really necessary? True, I could nott get a good look at the main front bevel bearing . .. which would be single row ball bearing, as it sat behind the big front bevel gear - but I had had it soaking in degreasing oil for a couple of weeks and it was spinning nice and freely now with no obvious side play or rumbling. I know from past experience of SOHC cambox's that these shafts are tightened up very securely (one of the few areas of a Norton engine build where I will use a torque wrench, rather than relying on just 'feel', as it requires +50lb per sq inch), and can sometimes be difficult to undo, particularly as the rear nut is a LH thread and needs a very firm grip on the square of the shaft. I had no problem doing this, but was wondering if it was really worthwhile - as I could clean pretty much all of the assembly in situ - other than the back of drive gear, where poking my finger around the back, I could feel a bit of the treacly old Castrol R still adhering to it. So having got to this point by early June 2019, while pondering the benefits of taking the central shaft apart - I switched tack's and decided to look at the replacement of the Pusher bearings - the main overhaul task I knew needed to be tackled on this cambox when I purchased it.





Old Pusher Bearings
This picture shows the underside of DOHC cambox shell, with the exhaust camshaft pusher bearing visible and still in place in the housing. Clearly the previous owner had tried to remove it - but this is a difficult bearing to remove without special tools, as it sits flush with the shell and the only means of purchase are small blind holes in the bearing around its circumference, about 4mm in depth. As can be seen in this photo - it looks like the previous owner did not have a 'specialist' tool and had used a punch - as many would, and it had not budged - instead chewing some of the holes.

Luckily, this bearing was a '6 hole' type and some of the remaining holes were still in reasonable unmolested condition. In the small box of bits that came with this cambox when I purchased it, was another old bearing - which I assume was the inlet side bearing that had been successfully removed. Interestingly - this bearing has just 3 holes, but although they were a bit chewed - it seemed to have come out ok

Pusher Bearing Thread

As a useful comparison, this is the pusher bearing housing on Inlet side, where the bearing had already been removed by the previous owner.
As can be seen, there is a machined recess first, which the lip of the bearing sits in and will be fully flush when screwed home. Then beneath the recess is a deep threaded area - which the phosphor bronze bearing is screwed into.

The Issue of Removing and Replacing Old Pusher Bearings
Only just visible in the photograph above on the right - where the Inlet side pusher bearing thread is showing - at the base of the thread is a scolloped area - where the bronze bearing has a curved scollop machined from it, once in place and fully screwed home. This is done so that the cam lobe does not foul the bearing at its highest point of lift as it rotates. As this scollop can only be machined on the bearing when the bearing has been fully screwed home - it follows that the bearing may be left with some sharp protruding edges from teh cutting tool - with the possible issue that on removing the old bearing - it can damage the housing thread with the sharp edges. As you can see - this thread looks fine, but I was worried that for the knackered exhaust bearing - as well as now being 'well seized' into the cambox shell, it might also damage the thread in the process of being removed.

As a footnote - although the use of cam 'pushers' is still considered 'current' technology for high performance engines over 50 years on from when this cambox first appeared, I cannot help but think this is the one weak part of design on the Norton DOHC cambox. The main difference beween the Norton DOHC cambox and modern design is - this cambox was almost a seperate external unit from the main engine below - with its own oil feed been fed from the oil pump and expected to return to the crankcase sump via the vertical bevel housing. Unfortunately, where the pushers acted on the valves was external to the engine - in the open air, while on modern engine designs (i.e. most automotive DOHC in line fours), these pushers are just part of the broader head casting - so oil is kept in the engine (and I believe also helped with modern 'O' rings on some pusher designs. So as well as being quite difficult to fit, as described above, one of the main issues was that they soon started leaking oil badly in use - resulting in a messy engine and often the rider getting his legs sprayed in a race! I know with my own 1955 DOHC Manx, I have to place swabs around the hairpin valve springs to catch oil - and it is normally quite wet around this area after a run round the racetrack.
Therefore, to help overcome this leaking issue - I gather it is considered normal practice when fitting new bearings, that they can hardly move when the engine is cold - because when the engine is ran and gets hot, everything expands and they loosen off. If they were free when cold, they will leak heavily. I know this all a bit of a necessary compromise - but it just seems a bit inelegant compared to the rest of the design and not good engineering practice. I am sure those with far more experience than of this engine than myself will have a better view on it . . . .

Removing the Old Pusher Bearing

Pusher Bearing Removal tool
Having looked carefully at the DOHC Cambox casting - to ensure I had clearance around the cambox base area - I came up with the design of bearing removal jig you see here, with a large hex head, which would allow a big socket to be attached. As can be seen - its lower section was a slide fit into the old bearing bore, and then there are 6 holes bored around a lip, which correspond with the 6 holes in the old bearing. Silver steel pins are then inserted into each of the holes to locate with the bearing holes. The silver steel rod is in the background. I quickly wrote a program to machine this from Hex bar on my Emco CNC lathe, so boring the circumference holes would be accurate and easier to do it on a manual lathe or mill

Making a Pusher Bearing Removal Jig
xxxxxx -
<Feb 2020 - still writing this article, will publish as soon as I have finished>

Cams Are Missing!

And here is another picture of the finished jig - now with pins inserted and also an alloy collar and lock bolt added. The most important thing about this design is that the jig can be fed into the old pusher beairng, but then with the use of the alloy collar can be locked in place, so that the pins can not slide off the bearing when heavy spanner force is applied

Text still to be added here

Close Up of Old Bearing Holes
Here is a close up of the old pusher bearing to be removed. If you click on the picture above to see a larger version - you will see the old bearing looked quite knocked about in the cambox housing - which was why I was a bit nervous about how easily it would come out - the main objective being not to damage the main cambox casting in doing so,
If you look carefully you will see that I had also chased the old holes with a pilot drill to try and clean them up a bit - as they had been chewed by the previous owners attempts to remove the bearing. The original holes were quite shallow, so I knew I might just 'touch' the magnesium behind them - which would be ok, but must be very careful, so as not to risk boring through into the main cambox inner recess - which would have been a disaster! All went ok

Bearing Removal Jig In Place

And here now is the jig fitted.
In this photo you can see why the jig had to be made so long - it had to be deep enough that the large Hex head did not foul the feet of the cambox shell. You can also see that the lip of the alloy collar is up against the base of the old bronze bearing, but due to the scollop milled in it, it is only touching for part of its circumference. However enough was making contact as to give good purchase. The lip of the collar is machined to the exact diameter of the inner bore of the thread - so it will pass through the cambox casting thread bore as the bearing is removed . . . we hope!