The cambox and bevel drives are certainly the most distinctive part of a SOHC engine, and although not as complex as the DOHC version, they still need great care when assembling to get the best from them.
They have a reputation particularly for puking oil out from the exposed rockers, all over the rear of the bike and the luckless rider’s legs. I have not had enough experience of owning different engines to confirm how true this is, but from talking to seasoned engine builders such as Stu Rogers, I know that it is possible with careful assembly to keep this to a minimum (as a postscript to this, having now ridden the bike in 4 or 5 events I have found this particular cambox to be remarkably oiltight, but freely admit that the mileage up to now has been small).
|The Cambox Castings
The cambox that came with the engine was a standard alloy version (not the racing magnesium version), and overall it was in very good condition. As such, it was fitted with the road going side oil feed as used by non-racing International’s, of which more later.
The main cambox casting particularly, did not show the normal horrors that can befall this component. It is not uncommon to find that the rear feet of the cambox have been broken off and re-welded. Even more common is to find that the aluminium ring that surrounds the cams (and is designed to capture oil being span out from the camshaft), has been broken away. Although this second fault is not too difficult to rectify (normally by machining a new ring and securing it in), it is still not a task to be taken lightly, so I was pleased it was not required. The rear cambox casting was also in good condition and as bought, the engine came with a new rear camshaft roller bearing for this backplate, which only required fitting. Interestingly, this bearing is actually bigger on the International than it is on the magnesium SOHC Manx version. Not quite sure why this is, but I do know that it is not cheap and becoming harder now to get hold off (postscript July 06 – I now stock these and other SOHC bearings if you require them, please go to my For Sale page to view my catalog).
The castings had even been cleaned up ready for assembly making my life even easier. So, other than ensuring the threads were ok and that the mating faces were made true (easy on the front face as this can be emery’ed on a surface plate, but more difficult on the rear face, as the rear legs get in the way), the only real job to do was to provide the means for a central oil feed.
|Camshaft and Central Oil Feed
As a dead giveaway on a SOHC Norton to differentiate a racing engine from it’s road going counterpart, is to look for a central oil feed to the camshaft, identifiable by a blind Hex nut in the centre of the front bevel inspection cover. The standard road going model does not have this central feed through the camshaft, instead relying on a side feed, that passes oil onto a grooved spacer ring in the centre of the cambox, where hopefully(!) it is passed down to the cams.
Both road and race going cambox’s look to use the same casting (although I have seen a few minor variations on the same theme), but the casting has provision for two separate oil bosses. The road going cambox takes its oil feed from the rear boss (closer to the central spacer, while the racing version uses the front boss (closer to the front bevel cover, which is fitted with an oilway from the side of the cambox’s mating face, to the centre of the instpection cover, from whence it feeds the camshaft.
As I had always intended to use this engine for sprinting and serious speed work, I knew this would be a ‘must have’ mod, and should result in less cam wear, as it gives direct oil feed to the camlobes. By the way, just in case you might think that high lift cams don’t have a difficult life, you should see some of the old ones I have in my possession, where the lifting face looks like it has been pressed up to a grindstone.
Luckily it is relatively easy to drill and tap the blank front boss to take an oil union, then very carefully drill the cambox with two oilway feeds, one from the boss side, the other from the front mating face. Hopefully, if you get it right!, they will both meet up in the middle, rather than drilling into the central cambox. This I was able to carry out successfully, even with both fingers crossed!
The camshaft is also different on a racing SOHC camshaft, as it requires
drilling down its centre to act as an oilway. The oilway from the bevel
inspection cover is then mated to the camshaft oilway via a phosphor
bronze spring loaded plunger, identical to the one fitted to the big
end feed. The plunger fits onto a similarly tapered plug fitted into
the near end of the camshaft.
Both ends of the camshaft are fitted with a large shouldered nut, one left hand thread and one right hand thread, as purchased, there were new ones of both supplied. The central grooved spacer (used to pass oil on the road going version, but purely a spacer on the racing version) was also in good condition, so it was only necessary to buy a few spacer shims and a cam woodruff key.
An interesting point with the cambox casting supplied with this engine, was that someone had gone to the trouble of welding two additional ‘lumps’ to the top of the cambox, which had then been drilled and tapped to accept adjustable rubber seal tensioners. My initial reaction on seeing these when I purchased the engine was to think to myself ‘they need to come off!’ However on closer inspection, I noted they had been very well done, and actually seemed a very good idea.
For those of you not familiar with Norton cambox’s, they are very basic in their design particularly with regard to keeping the oil on the inside! The outer part of the valve rockers are exposed to the elements, while oil from the inner cambox is supposedly stopped from exiting the cambox by the use of rubber seals (proper ones also having a cloth like material running through their centre). This seal then presses against the central (bearing) part of the valve rockers, both top and bottom. Both the top and bottom seals fit tightly into a slot in the cambox casting, and if adjusted correctly should be pushed firmly against the rocker arms central area, which is basically round at this point, and whose profile remains constant as the rockers open and close the valves. Yes, I know that that probably all sounds very confusing, but hopefully the photographs make my explanation a bit clearer!
Anyway, what seems a slight flaw in Norton’s original design is that – to ensure that the rubber pad is always acting firmly against the valve rocker, they built in adjusters to the bottom pads, so they could be tightened as the pads started to wear. However, they didn’t bother to do this for the top adjusters, so as they wore there was no means to adequately take up the slop, at least not without stripping down the cambox and fitting packing pieces. With my particular cambox, someone had gone to a great deal of effort to alleviate this problem by building in the same style of threaded adjusters on top as were fitted at the bottom.
Admittedly my initial rationale for removing the lumps was for purely cosmetic, reasons, I thought they ruined the lines of the cambox and did not look original, however, after looking at their construction and realising they would not be visible once the petrol tank was in place I decided to keep them on for the moment. I did however, get my angle grinder out to them and removed some of the excess metal, so as to blend them in with the original casting more. The finished results don’t look to bad at all, perhaps Norton’s should have tried it themselves! My only concern was that I was not sure the rear cambox lump (inlet) would clear the top of my frame, once the adjuster was fitted.
Postscript July 06 : This concern turned out to be well founded, as the adjuster did foul the frame. After much trial fitting of engine in and out of frame, and some judicial metal removing from the cambox boss and top adjuster , I eventually managed a compromise. The front (exhaust) adjuster is fine, but the rear boss is now much shallower and the adjuster is almost flush with the cambox contour, without even room for a locknut. O.K. Norton’s – that’s probably why you didn’t bother trying this!
Side Washers, Cork
Seals and Rocker Bearings
The rocker bearings themselves are also quite unusual, consisting of
a harderned steel outer cage pressed directly into the rocker arms.
Then a phosper bronze cage holding 12 small rollers, in two parallel
rows of 6 rollers each side by side. The rollers act directly on a hardened
steel rocker pin that feeds through the inner and outer cambox cover,
and is tightened in place using a special castellated nut, that can
be pinned if needed (haven’t bothered doing this myself).
The side thrust of the bearing rollers is taken by hardened steel shim washers that press into the rockers from each side once the cage and rollers are fitted. Finally, over these on either side fits unhardened lipped washers, that as supplied with my engine looked to be brand new. Once the bearings are oiled and assembled these lipped washers are placed over each side of the rocker before assembling in the cambox.
The final component, which I assume is another aide to keeping the oil in, is the fitting of circular cork washers on either side of the lipped washers. These are fitted directly into each cambox casting and in my case were retained there by a light application of sealer.
|Overall, the arrangement is probably considered
quite unusual today, in an age of cheap and available sealed roller and
ceramic bearings, but I would imagine that in the 1930’s it was
cutting edge racing technology.
In its favour, I have seen quite a few of these cambox’s stripped down, and although the hardened side washers are often worn, very rarely do the rocker pins, rollers and cages look to have much wear, so I assume the design has stood the test of time.
I vaguely remember that JAP Speedway engines were of a similar design with their rockers, using loose rollers as well, but in J.A.Prestwichs’s case they did not bother with the bronze cage, those engines being slightly less exotic!
Incidentally, if you are buying one of these engines, the other problem area to look out for is on the rear cambox casting, around the tapered area of the exhaust rocker pin. I have seen these cracked, particularly on Manx versions and supposedly this is not uncommon. With magnesium this can be extremely difficult to repair.
All the components for the rockers supplied with my engine looked to be in very good condition. As I have mentioned in previously, one of the big selling points of this engine was that so many of the smaller parts had already been replaced, which can add up to a lot of money very quickly when you are faced with replacing them.
When I came to assemble them I first fitted the new cork washers into both cambox castings. I then assembled the caged rollers and pressed in the hardened side washers. However, when I fitted the lipped washers and then gave them a trial assembly in the cambox, I found that the rockers seized solid before finally tightening up. Eventually I was able to trace this fault back to the lipped washers, which turned out to be too thick. I have subsequently built another cambox with a similar problem and gather from Stu Rogers there are actually a couple of variations here. Anyway, after much trial and error and very careful skimming of the washers in my lathe I was able to assemble everything up ok.
The most important point to remember when the rockers and rubber seals/cork washers are fully assembled (without camshaft fitted I find easier), is that the rocker arms must be almost solid and very difficult to move by hand. If they are a sloppy or loose fit in the cambox they will almost certainly leak oil!
As I mentioned earlier in a previous article, the engine as originally purchased came with a very nice set of ‘International’ profile cams. However, as I was intending to use the engine for competition, running on Methanol, I wanted to replace these with full blown longstroke Manx cams. Luckily, Stu Rogers is able to supply these off the shelf, so getting hold of these was not the problem I was expecting it to be (and actually, considering the work that goes in to a set of these cams, complete with vernier holes, the price was not excessive either). One thing I have found after years of playing with the older British bikes is that half the battle is actually being able to find the parts, irrespective of cost. You only have to own an Italian bike like a Ducati for a short while to make you put price into perspective!
When I was racing Norton ES2 pushrod engines, in desperation I ended up making my own cam forming tool to grind cams to my own patterns, as they were not commercially available. Much as this was perversely rewarding (when they worked), quite honestly I would much rather just have bought them if they had actually been available!
I will deal with assembly of the vertical bevel castings in another section, but save to say that both top bevels that came with the engine were original and in nice unblemished condition. Original bevels have a very distinctive oval trademark stamped on them and I have been told this is actually the same manufacturer as makes Coventry Dieheads – of which I also possess a few of in my workshop!
The large camshaft bevel has twice the number of teeth as the top vertical bevel and this (obviously!) gives the reduction in gearing of 2:1.
This top bevel is also very unusual as it has a series of holes around its inner circumference. As the camshaft has a similar set of corresponding holes at a slight number of degrees offsetb, it is possible to vary the valve timing just fractions by removing the pin that secures them together and changing holes. Very clever and effective. Mind you, it must be remembered this is only useful if the valve timing of both cams is out by the same amount, as both cams will move together using this adjuster. If only one cam is timed incorrectly then a similar vernier arrangement is also used to pin the cams to each other.
|Order of Assembly
As I write this article it is difficult to know just how much detail I should go into to describe all the fun I had trial assembling the upper bevel gears and cambox, until I eventually got everything right! Save to say, you will breathe a sigh of relief to hear, that I will give you a much abated version!
If memory serves me right, I must have assembled and disassembled the cambox close to 15 times, until I had got everything right. But from doing this, what I did find out is that there is a logical order of assembly and fitting, that if followed in sequence should result in a cambox with gears that correctly mesh and is at least partially oiltight!
Cambox Bevel Cover
Incidentally, I could not resist holding both the magnesium and aluminium
covers together side by side. The difference in weight is actually quite
noticeable and you can understand why Norton’s employed this material
for their serious racing machines.
As a footnote, Stu Rogers cannot understand my fascination for all
things magnesium and likes to remind me of the term coined to describe
it by a well known senior manager at Norton’s – ‘Electrified
This I have found to be the best
order of assembly:
Even Stu admits that he has never felt comfortable about exerting so
much pressure through the bevel drive train (obviously it is the bevel
gears that stops the camshaft rotating), but there is little alternative
if you want to ensure the nuts are really tight. Ahhh, you are probably
saying, but isn’t there a square on the end of the camshaft for
holding it in a vice when tightening? Well yes there is, but this is
not ideal for the torque setting’s we are talking of, and in many
cases when stripping cambox’s you will find this square badly
Postscript July 06: I am
jumping ahead slightly here, as I found this out through hard experience
the first time I ran this engine for any length of time. The two nuts
loosened within minutes and wrecked the camshaft. I thought I had it
locked them up solid, but clearly it was not enough.
Next section : confirming the valve timing and setting the compression ratio