Friday 18 December 2020

Timing Case Machining – Part 2

There are a total of 33 2BA screws for which the inner timing case needs threading and the outer timing case needs clearance holes.

This was divided into 2 stages: in the first stage, the 21 holes/threads, excluding the 4x holes at the extremities of each “arm” in the inner and outer cases, were machined and in the second stage the 3 sets of 4x holes at the top of the case at the camshafts and the magneto were put in.

Why split it into 2 parts? The reason for this is that the first set of 21 holes/threads “merely” has to be in the correct position to align with the bosses in the castings. But, the second set has to additionally support 2x scavenge pumps that will be driven from the ends of the camshafts and a magneto drive cover plate. It is desirable that these holes are on the same PCD because this makes it easier to machine the scavenge pump housings and also that the magneto end cover can be fitted in any one of 4 positions, ie every 90 degrees, rather than in just one position

Each hole of the first 21 was set up individually on the milling machine and drilled and tapped. The screws were inserted as each hole was made to firmly fix the timing cases together and ensure that the inner and outer cases were correctly mated for subsequent holes.


It was a fairly laborious job but eventually they were finished.

For the second set of 4 holes at the top of each camshaft drive, in addition to wishing to have the same PCD there was also a requirement that the centre of the camshaft could be accurately located so that the scavenge pumps could be positioned in their housings.

To do this, the first step was to machine the holes/threads in the cases.

The next step was to make a couple of aluminium disks, one for each cylinder, that could be screwed onto the outer timing case and to drill these to locate the axis of the camshaft accurately. This was done by making a couple of long drills – a centre drill and a 5/16" drill inserted into a hole bored into a long shaft that could be passed through the smaller camshaft bearing and the dummy brass disk and which could then be turned with a hand-held electric drill.

The 3 pictures below illustrate this.

These aluminium disks then effectively became surrogates for the camshaft/timing case combination. In due course and after the pumps had been machined and attached, short shafts with slotted ends will be made to connect the camshafts with the scavenge pumps and also the main oil pump which is driven from the end of the camshaft drive spindle at the lower end.

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