Saturday 18 July 2020

Machining the Timing Cases

The first machining operation is to face the inner surface of the inner timing case, ie the mating surface to the crankcase. This provides a datum for subsequent boring of the holes for the securing screws and machining of the outer face and internal surfaces.
Internally, the surface on the tension side of the chain must be machined to accept a flat strip of low friction material to avoid the chain hitting the aluminium surface and the slot for supporting the chain tension blade support (approximately midway between the chain centres) must be added.

The picture below shows the inner timing case on the milling machine.

After machining the internal surfaces of the inner timing case and the inner and outer timing case screw holes and threads, the timing case can be assembled on the engine. 

The original material that was riveted to the tension side of the timing case appears to be some kind of hard fibre. I have rebuilt a number of original 350 and 500 engines which have been fitted with exactly the same material and I find it amazing that this fibrous material has survived, in good condition, for 90+ years. Unfortunately, I don’t know what this material is or even if it is obtainable today. I therefore chose to use a modern low-friction and tough nylon-based material called Ertalon 4.6  which has desirable properties for this application. However I could only buy this in round bars and it was therefore necessary to make a ~1” wide flat strip from a round bar.

This was then riveted to the timing case. At the same time, the chain tensioner support and retaining spring were fitted.

Note that a small hole has been bored into the flat surface on the outer timing case, shown below, to locate the oil pump drive. This has been made by drilling from the timing-side crankcase and through a dummy camshaft drive spindle to ensure concentricity with the oil pump drive spindle.

Finally, the housing for the Velocette oil pump is machined and 2 aluminium covers for the magneto and camshaft sprockets are made. 

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