The cambox that came with KTT 55 had been substantially modified because the bike came with a bronze head (which should be fitted to a later Mk IV) and, because a bronze head requires a different cambox casting, someone in the past had modified an earlier cambox to fit. Anyway, I had parted company with the bronze head and modified cambox to a buddy that has a Mk IV fitted with an iron head and can make better use of it. I had already rebuilt an iron head (see here) for KTT 55 and now it was time to rebuild the cambox.
There are a lot of details to pay attention to in a Velo cambox. The picture below shows the bits that need to be assembled to make a functioning cambox – but it’s not just a simple matter of bolting them together and expecting it to work, at least, not when assembling a cambox from a collection of parts.
Many of these early cammy Velo camboxes have structurally damaged castings, usually to either one or other of the lugs at the bottom where they bolt onto the cylinder head studs or the section between the holes where the rocker skids enter to contact the cam – the latter can be seen in the cambox in the above picture where the thin “bridge” has disappeared. The other damage that is often found is to the 2BA threads that fix the bevel drive casting or at the other bearing-end of the camshaft. The above cambox also has some damage to the corner when the exhaust rocker exits.
Anyway, I decided that I would repair another cambox that was structurally intact (regarding lugs and threads) except for a gaping and jagged hole where the rockers enter the cam chamber.
Apart from the obvious unsightliness of the damage, the question arises: why repair it – after all, it’s hidden inside the cambox? The reason, quite simply, is that oil “escapes” from the cam chamber – thrown out by the rotating cam - into (and over the top of) the gutters on both sides of the chamber ….and then to the outside world and invariably ends up covering the bike. For this reason, the gap that connects the cam chamber and the cambox through which oil can escape needs to be minimised – 2 small holes in the original design. For anyone that has read my previous blogs, I have had plenty of experience in trying to solve this problem on the AJcette and the AJS V-Twin, both of which use the same cambox.
The first step in making a repair was to machine a piece of solid Aluminium into an annulus,
clean up and remove the jagged edges from the casting and mark out, as best as possible, the shape of the hole on the new piece of aluminium (by inserting it inside),
chop off the top section of the annulus and cut out the shape to fit the hole
and with support from a piece of thin steel bent to fit inside the cam chamber
use oxy-acetylene and Lumiweld to fix the new piece in place.
This was then cleaned up on the milling machine
and finished by hand with a file and Dremel.
After vapour blasting
…..not perfect, but a reasonable repair to the damage.
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