Saturday 4 July 2020

The Camshaft Drive

The AJS SB6 OHV engine was never designed to take a shaft with a gear and sprockets to drive an overhead camshaft and there is insufficient thickness of material in the crankcase wall to accept a bearing. Space is quite tight in the timing case; nevertheless, there is just about enough room to squeeze in a complete housing containing a bearing the same size as the original K7 (1/2” x 1  5/16” x 3/8") and the 3 support pillars on which the outer bearing end-cap is mounted. The picture below shows the nearly completed bearing housing.

Although at a much later stage of the project, the following picture shows the complete bearing housing installed into the crankcase. The housing is positively located into the recess machined into the crankcase casting and secured by 4 high-tensile screws inserted from the inside of the crankcase.

The 20 tooth crankshaft pinion and 40 tooth drive gear were made of the same material (O1 tool steel) and machined in the same way as the K7 project (see K7 gear details, K7 gear machining and K7 gear heat treatment). Internal keyways were spark eroded and a series of lightening holes were machined in the 40 tooth gear to reduce unnecessary inertia.

The main ingredients – the spaced crankcases and the camshaft drive, are now available to make a first mock-up of the complete drive to the camshaft. With the addition of a temporary brass plate on the Velocette cambox to replace the usual bevel drive housing and to support a dummy camshaft plus a few washers under the cylinder base to get reasonable chain tension, the first incarnation of a chain-driven OHC engine with Velocette cylinder barrel/head/cambox and an AJS crankcase is realized. 

Whilst there is still a lot of work ahead, the basic idea would appear to be sound and a big incentive to continue with building a “proper” engine.

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