Tuesday 29 March 2022

AJS 33/7 Trophy Model Restoration: The Cambox

Before continuing with the camshaft drive, ie chains, sprockets, Vernier adjusters, Weller tensioner etc.  I really needed to check that all was OK in the cambox …and fix anything that wasn’t.

The picture below shows the cambox before stripping

I immediately noticed that it was nearly impossible to rotate the rockers because they were so tight and there were 2 small screws that appeared to be inserted through the rocker spindles.

I removed the rocker spindle screws and found that they were actually threaded taper pins. This is not an AJS fitting and had presumably been inserted by someone to stop the rocker spindles rotating in the cambox casing.

The picture below shows the myriad of bits and pieces from the stripped cambox.

Apologies for using a page of the Sunday Times as a background; it doesn't help clarity.

A few other points were noted:

Someone has gone to a lot of trouble to replace the rockers skids (these are the hardened “ends” that bear on the cam) and new ones had been beautifully made and engraved “IN” and “EX”.

The rockers had been polished and re-bushed – but not reamed to size, which is why they were so tight on the (original) spindles.

The use of taper pins to lock the spindles is really the wrong solution to the problem. This was remedied by making new spindles (silver steel oil-quenched at 815C and then tempered at 230C) that were exactly 0.500” rather than the originals which were 0.499” to provide a good tight fit in the cambox body and reaming the bushes to give the proper fit of the rockers on the new spindles. New deep groove ball bearings (which would have been fitted originally) were also acquired to replace the 2-row self-aligning bearing and the roller bearing that had been used on the camshaft itself.

There should also have been end caps on the ends of both rocker spindles (my R10 and K7 both have these) as a spindle could, theoretically, fall out of the end of its hole in the cambox. The original spindles were threaded to accept these but the end caps themselves were missing. 4x new end caps were therefore made and also 4x reduced hex head 3/8” BSF bolts to allow a spanner to be used in the restricted space where the bolts are inserted. All were chemically blacked prior to fitting.

The cam had been reground and was in excellent condition.

For some reason, somebody had drilled a hole in the top cover of the cambox and fitted a brass union.

This would serve no purpose whatsoever; there is already a targeted oil feed that provides oil to an annular chamber which, in turn, feeds into a drilling in the camshaft through to the cam lobes; there is an oil scavenge drain at the bottom of the cam chamber which is connected to a scavenge pump driven by the camshaft and there is already a breather in the inner timing case. This superfluous non-structural hole was therefore blanked off using Devcon aluminium putty.

Finally, new felt strips were inserted into slots above and below the rockers and a gasket made for the top cover. The felt strips help to prevent oil that is thrown around by the cam from exiting the cam box and pouring out of the ends onto the cylinder head; in my experience these are only partially successful. 

Before bolting the cambox down it is vitally important to check that it "sits" properly on the top of all 4 threaded cylinder head studs. If there is any sign of rocking then this must be remedied to avoid putting a bending load on the cambox structure that could result in it cracking. There are 2 possible ways of addressing this; the first (and easiest) is to insert a shim between the top of the appropriate cylinder head stud and the cambox; the alternative is to adjust the lengths of one or more cylinder head studs (bear in mind this is typically ~0.005” maximum) to eliminate the rocking. I carefully measured the lengths of the existing studs and found that there was already a few “thou” difference between each and by selectively assembling these I could completely eliminate any rocking. The studs are now numbered 1 through 4 so that I know which way to replace them in the future. 

Refurbishment of the cambox is now complete and it can temporarily be put back onto the engine for setting up the chains, sprockets, Weller tensioner et.

And, finally, a bit of "nerdy" information. AJS stamped their company moniker on a number of major components, such as the oil pump, but they even did this on items such as a simple bolt head. This is a picture of one that I removed from the engine and was used to bolt down the cam box onto its support pillar.


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