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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