Wednesday 3 June 2020

The Cambox

The cambox is a relatively simple affair. The camshaft is supported with a bearing at both ends and has inlet and exhaust cams to operate rockers that actuate the valves. Oil is fed into the bottom of the cam chamber where it is then thrown around by the moving parts – cams, shaft and rockers, and then eventually finds its way either through the bearing on the drive side and then back into the timing case or escapes via the rocker spindles and ends up on the outside of the cylinder head. There is a “breather” on the top of the cambox but it is questionable how effective this is.

The picture below shows the K7 cambox before disassembly

It is interesting that getting oil out of the cambox seems to have been a much greater problem in-period for both AJS and Velocette. Although there are substantial differences in the designs of each, both Companies introduced a pump at the end of the camshaft in an attempt to scavenge the excess oil and return it to either somewhere in the engine (AJS) or the oil tank (Velocette). Later Wolverhampton AJS engines, such as on my 1930/31 S10 engine shown below, have a gear pump on the end of the camshaft opposite the drive. This has been set up to deliver oil back into the timing case.

whilst the slightly later 33/7 model (1933 350cc OHC) from the Collier brothers has an externally mounted pump on the top of the timing case.

The K7 cambox and all the internals were in remarkably good condition. All castings and threads were sound and there was no discernible wear on either the cams or rockers. The bearings and rocker shaft seals were replaced as a matter of course, all parts thoroughly cleaned and then the cambox was reassembled.

It is reported that the K7 was not initially as fast as the Big Port that it was destined to replace and that the reason for this was the loss of control of the valve timing on the Big Port engine, leading to an extended inlet valve closing that was serendipitously beneficial for breathing at high engine speeds. AJS carried over the same cam design that had been used previously on the very successful OHV engine but, with the lower inertia of the OHC valve train, the valve motion was "as prescribed" by the cam which proved not to be optimum.

It is certainly true that the first incarnation of the K7 cam is remarkably similar to the Big Port cam of the same period. The picture below compares the K7 cam (RHS) to the Big Port cam (LHS); it is hard to see much difference.

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