Racing Inter Restoration - Part 6: Cambox Assembly (September 2005)

Confirming the Valve Timings and Compression Ratio

Fitting Piston and Barrel
As I mentioned previously, early on in the rebuild I had managed to get hold of a very nice Arias forged dope piston from Ken McIntosh in New Zealand (see my Links section for his contact details). I had given Alpha’s the weight of the piston when I had the crankshaft made, so they could get the balance factor correct, which had been duly done (see Part 2 of the restoration).

Barrel Spacer

Fitting barrel spacers, to give the correct Compression Ratio before fitting barrel and piston - See my utilites section if you would like a handy tool for calculating the correct thickness for any given comp ratio

Anyway, the next task in the rebuild was to fit the piston and ensure that the piston to valve clearances were adequate, given that I was going to use full Manx longstroke timing, that being much ‘lumpier’ than the road going ‘International’ valve timing.
This bought back memories of my earlier racing days, when I spent endless hours farting about with overstressed pushrod Norton engines, trying to ensure my home made cams did not result in the valves clouting the top of the piston, or even worse – playing pattacake with each other! I have a clear memory of missing a gear down the back straight at Mallory in the mid eighties and getting no second chance – a crescendo of exceeded revs, followed by instant loss of power and on sheepishly returning to the pits, finding I had no compression whatsoever. Subsequent strip down revealed the expected ‘cleaning’ of the piston’s inlet valve cutaway and a valve (brand new one as I remember – bugger!) that looked like a plate being wobbled on a stick, i.e. it was well bent!

Measuring valve cleaance

Having turned the engine over for two full revolutions, it is possible to measure piston/valve clearance quite accurately with the depth gauge element of a digital vernier caliper, poking it into the plasticine

From this I learnt that a minimum safe distance between each valve and piston should be 0.060”. In practice I found there was only one easy way of setting and confirming this distance, which was to place plasticine on the piston head, assemble everything up, set the correct valve timing, then very gently turn the engine over from the crankshaft – stopping as soon as any resistance is felt (just in case there was negative clearance and the engine would not even do a full four stroke revolution without valve coming into contact with piston and locking the engine).
Providing this stage works successfully the next stage is to strip the top half down again which should reveal a piston with ‘marked’ plasticine where the valves have come into close proximity. It is then simply a case of carefully prodding the plasticine with a depth gauge and hoping you have at least 60 thou clearance at any point. If not, it will be necessary to mill larger valve cutaways for any shortfall – see my Vincent rebuild section for an example of doing this.

Anyway that’s the theory, but I’m jumping ahead a bit here, first I needed to fit the piston, assemble the head, then set the valve timing in the cambox correctly, so read on.

Piston Ready for Fitting

Arias piston - in this case after the extra clearance has been machined (see also last paragraph)


Fitting the Piston

Fitting the piston is a straightforward operation, I bang it in the oven for about 10 minutes (I’m not going to tell you what gas mark – this isn’t a cooking program . . .), which ensures it has warmed evenly, then the gudgeon pin should slide easily. Before fitting though I scribe a small mark somewhere on the crown that indicates which way is front. This is very important on a racing piston, as often the cutaway required on one side is different to the other (my inlet valve is noticeably larger than the exhaust).
Having fitted the piston and let it cool down, I then fitted the barrel. I haven’t bothered to fit the piston circlips at this stage, just in case the piston has to come off again. I would recommend though that you place these circlips somewhere where it is impossible to complete assembly of the engine, without stumbling across them again. It is all too easy to forget about them if the piston does not need to come off again, and the consequences of this do not bear thinking about.


cambox from front

With the head fitted the main bevel ring can be set so the valve timing can begin. Note that at this stage the vertical shaft is fitted without the tube at this stage


Cambox Inside

This photo shows the cambox as first placed on the head, with both cams 'off' their lobe, so as not to impart strain on the cambox legs when tightedned down

Actually, that reminds me of a mildly amusing story from my motorcycling past, this time from about 20 years ago. At the time I was a single bloke with very little money living in a dilapidated rented flat (paradoxically though, I remember that period being a very happy time, isn’t that always the case?). Anyway, at the time I was using a very well worn Honda CB125 which was already the best part of 15 years old when it came my way (cost me £55 from a mate at work as I remember, complete with one and a half spare engines – both with the cambox’s knackered!).
This bike was much abused and used to run on the oil that had come out of the race bikes – yes I know, shame on me! Everywhere was travelled to at full throttle and generally it never complained. On this particular morning though I was travelling out to see Titch Allen and had noticed it was labouring a bit more than normal. Anyway, don’t worry, push on etc etc. Eventually though while trying to ascend a steep hill it became apparent it was not going to make it, so rather than stopping and it not restarting, I turned round and gently coaxed it back the next few miles to home. By the time we arrived back the poor bloody thing was vibrating like a Jackhammer on steroids and strange fumes were emanating from parts of the engine that should normally never see the light of day!
On stripdown the cause of the problem became clear, one of the piston circlips had broken, which had allowed the gudgeon pin to work itself out, where it had gauged a furrow the whole length of the barrel. Amazingly though despite such abuse, it had still carried me the 10 miles home, despite the depth of the furrow approaching 6mm by this time! Say what you like, but old Mr Honda certainly knows how to build bikes!
That old hack was a bit like one of those poor abused donkeys that worked down the mines and never complained, but I still sold it for more money than it cost me and had a queue of people ready to buy it at the end, oh happy days.

Finally, fit the previously built up head, complete with valves etc, and tighten down, but again don’t give the sleeve bolts quite the grunt you would with final assembly, remember this is just a trial fitting.


Setting the Correct Valve Timing.

Ok, here I am going to go out of order a bit, as I know I haven’t included a section yet on how to assemble/set up the Oldham coupling (incidentally, my wife like’s to spot this kind of error when watching movies, I think she calls them continuity errors. Mind you, if she was here now she would probably correct me on that as well - that’s one of her other favourite hobbies, correcting my grammar . . .). Save to say the Oldham coupling will be covered in a (near) future article, so lets assume this is already assembled and you have placed the cambox on top of the sleeve bolts and bolted it down using the four cambox bolts (long and short). Incidentally, you should ensure first the two cam lobes are positioned such, that the rocker arms are both in the valve closed position (akin to top dead centre).
Dependent on if this is a made up engine (as mine was) or an engine you are just refurbishing, it may also be that you need to make up new spacers between sleeve bolts and cambox base, to ensure valve to rocker arms are good, again I will cover this in the Oldham coupling section and assume at this stage they are already in the correct postion.


Cambox after initial fitting, now ready for final setting of the valve timing


Cambox Spacers

These are the cambox spacers I machined, that fit on top of the sleeve nuts. The cambox sits on these

Cambox Spacers
Before moving on to the valve timing it is worth mentioning the fitting of the cambox on top of the sleevenuts. It is very important to ensure that the cambox sits evenly on all four sleevenuts, otherwise there is a very real risk that you can break a leg off the cambox.
It is also important to ensure that the rockers are acting correctly on the valves. By this I mean that (broadly speaking) they should be at 90 degrees to the valve stem (and in line along the centre line of the valve) when in the mid lift position. I am not sure if the method for doing this changed over the years, but in the 1948-49 catalog it shows nothing more than a washer under the sleevenut, which l assume could be varied in thickness to achieve the correct position.
I decided that I would make up spacers between the sleevenuts and the base of the cambox to achieve the ideal setting. Something I forgot to do at the time though is mark each of these spacers, so that when I strip the cambox next, I ensure the spacers go back on the correct sleevenut. I will need to do this the next time I strip it down.
Incidentally, as I write this article (which is admittedly some time on from the original assembly), I am currently building a SOHC ‘square head’ Manx engine. On this engine I have made up titanium spacers that fit underneath the sleevenuts, rather than on top of them. Not sure it makes much difference really!


Setting valve timing

At this stage the engine is assembled and a timing disc is fitted to the crankshaft. I have fitted a dial gauge on the Inlet valve so I can see exactly when the valve opens and closes


Final Assembly of Cambox – Setting the Valve Timing

Back in Section 3 I described the order of assembly for the cambox. Having then set the cambox aside, I worked on the final build of the rest of the engine, to get to the point we are at here – where the bottom half is complete, the barrel and piston is fitted and the built up cylinder head has been fitted and tightened down. This last operation has only been done to a point of being semi-tight, knowing I need to set valve timing and valve/piston clearance before the final assembly.
First task is to set the piston at TDC, as this will be the point all other timings are taken from.
To confirm this, it is necessary to first to fit a timing dial to the crankshaft and revolve the engine gently, until you have found top dead centre with both valves closed then set the pointer against the dial to reflect this. I have seen some lovely tools for screwing into the spark plug hole to help with finding TDC, but have never got round to making one. So I always end up putting a screwdriver down the hole and put a mark against its shaft with a marker pen, to show easier when the piston is rocking on TDC.

Although there are various valve timings for SOHC Norton’s, below I have detailed the most commonly seen timings. All timings are shown in degrees:    
Inlet Opens
Inlet Closes
Exhaust Opens
Exhaust Closes
x x x


International 30/40
Long Stroke 30/40M (megga)


The timings shown should be obtained with a 4 thou clearance on the tappets set (normal running clearances are 10 thou inlet and 20 thou exhaust). I have found the best way to measure when the valves open is to use a dial gauge, held in place using one of the cambox bolts. However, if you don’t have a suitable dial gauge to hand, a more basic method is to use a cigarette paper between valve tappet and the valve itself. When the cigarette paper goes tight, the point of opening is reached.

Main Bevel Wheel

This picture shows the main crownwheel bevel, with its 12 holes, and the camshaft whose corresponding shoulder has 11 holes. Above it can just be seen the spacer, that on the roadgoing cambox feeds oil to the cams via the slot along it. On the central fed racing camshaft this acts purely as a spacer

            Setting the Exhaust Valve Timing
So Having fitted the cambox in place, and ensured the top bevel gears meet up with the punch marks on both vertical bevel and camshaft crown wheel, next step is to set the exhaust cam closing point correctly. As it is the exhaust cam that is keyed to the camshaft, it is this cam that must be timed correctly first. The inlet cam is pegged to the exhaust cam, so there is no point trying to time on the inlet until the exhaust is correct.
By the way, I know this is pretty obvious, but don’t be tempted to rush and only fit the cambox bolts finger tight. They do need to be tight enough to ensure tolerances are correct, and if not fully tight there is a slight risk of breaking off one of the back cambox legs (it is not uncommon to find magnesium cambox’s in this predicament).

The main crownwheel bevel gear has 12 holes around its inner circumference. The camshaft shoulder behind it has 11 holes; so there is a few degrees difference between each. A peg fits through both of these holes to secure the two together. In turn this peg is held in position by the camshaft retaining nut, which is lipped for this purpose.
Therefore, placing the peg through a different pair of holes will result in a slight variation in the exhaust cam lobe position in relation to the bevel gears. In this manner it is possible to adjust the exhaust cam timing slightly, to account for any slight variations in the opening/closing position of the exhaust cam lobe in relation to its key slot on the camshaft.

It is very important to ensure the peg is a good slide fit, with no wear, as this can result in erroneous valve settings (and even lead to shearing). I stock them myself if you need a new one.

Cams and camshaft

And here is a view of the cams, with their similar method of keying to each other with offset holes. The exhaust cam is keyed to the camshaft and a small roller is placed in the holes to join the cams. Note also the central oil feed on the camshaft

So, to initially set the correct position of the exhaust cam, having ensured the engine is at TDC and the top bevels are both lined up with their punch marks, revolve the crankshaft forward until it is 42.5 degrees beyond top dead centre. Please note that while doing this, the camshaft should not be pegged to the crownwheel (and therefore the lipped nut does not need to be fitted), so it is not rotating while the crankshaft is being set.
Now, while ensuring the bevel gears do not move, gently rotate the camshaft clockwise ( the camshaft runs backward, so this is in effect finding the exhaust lobe closing point) until the exhaust rocker comes into contact with the valve. When this point is reached, look for the point around the crownwheel where two holes are in line and push in the peg. You may need to gently tap this in, using a soft punch. You can now fit the lipped nut, semi-tight, and rotate the engine to confirm the opening and closing points of the exhaust cam are as per the valve timings above. Repeat process multiple times until you get it right!
If you find you cannot get the valve timings exactly right because there is a couple of degrees difference between the actual cam and the book settings (I have found they are rarely spot on), then I tend to even it out between the opening and closing settings. By the way, if you find your initial timings are close, but not spot on, remove peg and gently turn crankshaft a couple of degrees until the next hole on the crownwheel lines up. If this has moved the timing further out, go back to the previous position and repeat the process in the opposite direction. Using this method, you should find it possible to adjust the timing a few degrees at a time.
This process takes some time to carry out, but in the case of this engine there were no real problems and I soon had the exhaust timing pretty much spot on.

Cam Timing 1

Close up of dial gauge fitted to the cambox that allows valve timing to be accurately measured


Setting the Inlet Valve Timing
While all this has been going on, what has been happening with the inlet cam? You have been watching haven’t you?! I did mention right at the beginning of the article that as we have not set the valve/piston clearances yet, we need to be extra careful and watch out for any resistance when rotating the crankshaft, so this is another consideration that requires that at this stage extra care is taken.
Some people recommend that while performing the initial setting of the exhaust cam timing, you don’t bother to fit the inlet cam, however, I am not fond of this approach and prefer to fit it, as I know then that all the tolerances along the camshaft will be as per the final assembly. However, as a safety measure it is a good idea to omit the inlet rocker when setting the exhaust timing, and then you don’t have to worry about the piston clearance.

I did not mention earlier, but the inlet cam uses a similar method to the exhaust cam for varying its timing, a vernier system, that pegs it to the exhaust cam. The inlet cam is numbered from 1 onwards in a clockwise rotation, with number 1 being in the 12 o’clock position. The exhaust cam starts in the same position, but the numbers rotate in an anti-clockwise rotation, therefore number 5 is approximately in the 8 o’clock position.
The inlet cam is held in position to the exhaust cam by a small roller, with a nut at the rear end of the camshaft holding them both together. Once the valve timing is complete, this nut should be torqued up to 55 pounds a square inch, which can only really be done with the engine assembled in the bike.

Anyway, the normal starting point for setting the inlet cam is to locate the roller in position 1 on the inlet cam and position 5 on the exhaust cam. If adjustment is necessary you should then try 2 and 6 and so forth. Like the exhaust cam pegged to the crownwheel, this will have the affect of varying the inlet cam by a few degrees in relation to the exhaust cam, allowing you to subtly adjust the timing.
In my case I was fortunate in that the initial 1 and 5 position was pretty good and the timing was almost spot on to start with. I seem to remember it was a couple of degrees out, but quite honestly I could have sneezed and made that difference!

All in all, setting the timing on one of these cambox’s takes a considerable amount of time to get right, but it is not an altogether unpleasant process and you cannot help but admire the subtle logic of the design. I am sure it must have been considered cutting edge when it was first designed back in the early 1930’s and far in advance of other designs at the time.
With the timing correctly adjusted, the final job is to tighten the large bevel nut and also the rear nut (although as previously mentioned, this will need further tightening once the engine is in the frame).

Valve to Piston Clearance
Once I had the valve timing set correctly l was able to set the tappets to their normal running clearances and then make a full (gentle) revolution of the engine.
Luckily there was no solid resistance felt, which at least told me at this stage that the valves were not coming into contact with the piston. I made a couple more revolutions just to be sure the plasticine had been well moulded, and then stripped it all down for inspection.
The results were quite pleasing, with nicely placed indents in the piston crown, sitting nicely in the intended valve cutaways, which could then be measured by the simple means of placing the end of a digital vernier on top of the plasticine and then using the depth gauge element of the vernier to press through the plasticine at strategic points to confirm the depth, which would indicated the clearance between valve and piston.
On this particular engine, with the compression rato at approximately 12:1 (running on methanol dont forget), the exhaust cutaway was fine, but the inlet cutaway was marginal, and more importantly, was not quite large enough in diameter to facilitate the larger inlet valve I was using in this engine. However, the plasticine makes it very easy to gauge where the metal needed to be removed and the final jobe was then to remove the barrel and piston one final time, mount the piston on my Cemtec milling machine and fly cut a slightly larger and deeper cutaway. Job done!

With this final task completed it concluded the process of setting the valve timing and ensuring valve clearances, so I could now commence final asembly. But before moving to this, the next article will now go back one step in the sequence, and cover the process of building the Oldham coupling assembly

Cams showing Holes

This is the original Norton workshop manual drawing showing the numbering of the cams. The numbers are referenced in the text


Plasticine on Piston

Here you can clearly see the impression left in the plasticine by the inlet valve, and the marks left where I have gone around the circumference, testing the clearance afterwards


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