Although I have worked on 3 Mk 1 OHC Velo camboxes in the past none of these have been for a Velocette engine; these were for the AJcette and the AJS V-Twin, both of which were converted to chain-drive OHCs rather than bevel gears. Whilst many of the requirements are the same because the internals – the cam, rockers, rocker skids, tappets etc are the same, the Velo engines require the bevel gear meshing and large bronze bush, K-12, to be set up carefully.
Please bear in mind when reading this blog that this particular cambox has been assembled from a collection of parts that have not previously had the pleasure of each other’s company. I would expect (or at least hope) that a cambox that has been stripped for inspection and then reassembled would not require as much attention.
One fundamental requirement is that the cambox does not “rock” when placed onto the threaded cylinder head bolts and the 4 points of contact should contact simultaneously.
Why is this important? Because, if there is a problem with the cambox seating properly, then it will be stressed when it is bolted down and liable to break or crack at the weakest section. Damage invariably occurs at the lugs on the side opposite to the camshaft drive (the left side on the bike) and I have inherited a number of old camboxes with broken or cracked lugs that are a testament to a failure to address this.
There are 3 possible reasons why there might not be simultaneous contact – uneven spot facing of the cylinder head holes or machining of the mating surfaces on the cambox (resulting in non-coplanar contacts) or different lengths of the internally threaded section of the cylinder head bolts – or a combination of all 3! I am reticent to correct this by machining any material off either the cylinder head or the cambox and generally this can be addressed by adding shims – typically 0.005” to 0.010” maximum between one of the head bolts - to - cambox contact points.
There are various versions of the K-15/X camshaft. The earlier shaft has a larger diameter to support the K-17/2 cam whilst the later shaft has a smaller diameter for the K-17/5 cam. The picture below shows both variants.
Additionally, KTTs have a slot in the threaded section at the end into which slots an Oldham coupling that drives the cambox scavenge oil pump (and the shafts have a different part number – K-15/2 S or K-15/5 depending on which cam is used).
Although KTT 55 would originally have had a K-17/2 cam I decided that I would use a K-17/5 cam, especially as I had gone to the trouble of making one. However, I didn’t have any shafts (except on KTT 305) that had a slotted end and so the first step was to set the shaft up in the milling machine to add a slot using a 4mm end mill to engage a yet-to-be-made Oldham coupling.
The camshaft was put in the lathe after the studs had been fitted to machine off the small amounts of protruding studs to ensure the ends were all flush with the surface and then centre punched, as the originals, so that they will never shake loose.
The large bronze bush, K-12, needs to be “sandwiched” between the cambox and the bevel gear housing, ie it must be positively clamped. Why? Well, if it wasn’t then it would be free to rotate…. I am aware of 2 recommendations for the gap between the cambox and the bevel gear housing when assembled: 3 to 5 thou (0.003” – 0.005”) (Peter Miles - Velo Club K series specialist – FT 141) and 8 thou (0.008”) used by Jeff Clew on a Mk 1 KSS (FT 93).
So, what do I have? I took 3 measurements: first, the thickness of the flange on the K-12 bush:
and secondly the depth of the recess on the cambox
This indicated that I would end up with a gap of 0.252” – (0.122” + 0.112”) = 0.018” when assembled and which I also checked with a feeler gauge by assembling the bevel gear housing onto the cambox. This is too much and was cured by very carefully removing material from both faces of the flange
to give an acceptable ~0.003” gap between the cambox and bevel gear housing.
I do not know the history of these particular parts and so have no idea why the flange thickness was so much greater than the available gap in the cambox/bevel housing. I’ve measured the thickness of the flange on a couple of other K-12 bushes and found a ~0.004” variation between them. Maybe they were selectively assembled at the factory?
Finally, the K-17/5 cam was pressed onto the camshaft.
The 2 main requirements for meshing the bevel gears are that they are in contact over the whole width of the gear teeth and that the bevel drive/camshaft combination is free to rotate. Quite a bit has been written about this in the past so I won’t repeat it here – see, for example, the excellent article by Chris Benallick in FT 319 or the section on Camshaft Drives in Tuning for Speed by Phil Irving.
The simplest way of determining tooth engagement is by looking through the inspection cap
and if in any doubt as to their relative positions then the point of a scriber is useful to gently “feel” the engagement height of the teeth. If it needs adjusting then either the camshaft needs to be moved axially in or out and/or the bevel drive can be moved down by adding gaskets (it can’t easily be moved up! – at least, not without machining some material off one or other of the mating surfaces). It is unlikely that the camshaft needs moving outwards and can be moved inwards by adding shims behind the bearing supporting the camshaft at the other end. The OD of the bearing is 1 9/16” diameter and shims for the bearing are available from the Velo Spares Scheme – part number K128.
I ended up with good engagement with 4x 0.008” shims (which I would think is about the limit) behind the bearing and one 0.020” gasket between the K-14/3 bevel gear housing and K-46/2 bevel gear bush housing. As shimming the bearing changes the position of the cam relative to the rockers it is also important to check that the rocker skids contact the whole width of the cams. In this case, they were in the correct place so no further work was necessary
New rocker skids, part number K-41/4, are available from the VOCs Spares scheme. However, it’s important to recognise that variants of the (original Velocette) skid exist with some differences to the section thickness at the “business end” – see picture below which shows an original on the left and a new yet-to-be-fitted skid on the right.
I believe the reason for this was to adapt the skid for different cams and for variants of the rocker as the latter was updated to give it greater strength during the evolution of the engine. A consequence of this is that the fitted length of a new K-41/4 skid needs to be checked and adjusted if necessary. There is also a question about how the load is transferred between the skid and rocker (at the bottom or the top of the skid) when using the K-41/4 skids on earlier rockers but I’ll leave that discussion to another day.
I have previously used the K-17/2 cam in both the AJcette and V-Twin. For comparison, I showed a picture of both the K-17/2 and K-17/5 cam on a recentblog. The K-17/2 cam has the same lift as the K-17/5 but a larger diameter base circle and the length of the skid needs to be reduced to position the rocker in the right place – it otherwise “sits” too high at the “cam end”.
Although I didn’t need to do this here, I have reduced the overall length of skids in the past by making a small aluminium sleeve into which the skid is a good tight fit and in which the sleeve has a 1mm slot from a slitting saw
so that it can be clamped tight in a chuck on the rotary indexer. This can then be set up on a milling machine, aligned with either the X or Y axis and then carefully machined using an end mill to retain the stepped feature (which positions the contact surface of the skid relative to the cam and prevents it from rotating in the rocker) but reduced in length. The skid is hard but can be cut easily with a carbide cutter.
(This is actually a picture from the V-Twin project which used K-17/2 cams).
The original adjusters are small fiddly little things to make (I made a batch of 6 of these in the past – see here) but there is an alternative that I discovered subsequently. The picture below shows, from left to right, an original Velocette split-collet adjuster (K-29 and K-40), one that I have just removed from KTT 305 in the centre and, on the far right, a new adjuster for a cammy Norton from Racing_Vincent
If I need any more of these then I will use the Norton ones.
Finally, I found it pretty well impossible to simply insert the felt washers that fit into the recess in the cambox at the end of rocker spindles and then assemble the rocker and spindle in place -the felt simply gets pushed out and damaged. I have therefore made a simple jig – just a short length of round aluminium bar that slides over the spindle
and can then be used to compress the felt washer in place.
I’ve also found that it helps if the felt is dipped in engine oil first and the whole cambox assembly put into the freezer (in a polythene bag!) overnight. The oil is then so thick that the felt does not decompress when the pressure is removed and the rocker can be slotted into position without a problem.
Finally, I have added a small aluminium disc that can be put inside the cam puller to remove the cam from the shaft
to avoid the pointed end of the screw on the extractor splaying the now-slotted end of the camshaft.
And…. I found the easiest way to remove and tighten the bearing locking ring is to use a small length of flat steel bar
and an adjustable spanner, being careful not to scour the aluminium thread with the end of the bar.
This cambox and upper bevel drive assembly (and the cylinder head) for KTT 55 are now complete.
There is one remaining job to do, namely making the Oldham coupling to drive the scavenge oil pump but I’ll postpone that until I’ve rebuilt KTT 305s cambox and then make them both at the same time.
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