Part three - by Ramon Wilson

An expanding mandrel made for the previous ETAs was used to act as a mounting spigot using a bolt and washer as opposed to expanding it. Reason for this is that due to the bore for the cylinder the mandrel is not gripping on its entire periphery. It will be okay for later ops when there is much less to come off but for roughing was considered to be the safer bet.

Another angle plate was brought into play to lend support and the lower case was roughed back to the mounting lugs. The lower face of the mounting lugs was then finished and the crankcase brought to width with a larger dia cutter.
The case was then swung through 90deg and the tops of the lugs finished and the case brought to width using a 1/2” cutter that had had the corners rounded off to put a radius in the top corner of the lug.
It was then time to start reducing the weight. A thin piece was left upstanding to act as a 'foot' on the next op to shape out the exhaust boss
So after a good day’s handle wringing - no power feed I'm afraid - they look like this, a semblance of a case beginning to materialize,

(In case you're wondering about the felt pen colour - the top rings, though interchangeable, were colour coded to keep them to their respective machined part).

The rotary table certainly earned its keep with a long series 4mm end mill. Though unintended those feet left on proved ideal for clamping down to the R/T for bringing those bosses to width.
The cylinder bolt holes were aligned with the spindle by using a 60deg pointed dowel (the bolt holes were recessed slightly so the point was aligning with a true circle).
After setting the stops on the R/T a witness cut to make sure everything was in the right place.
This was quite a time consuming op as each hole had to be set up and two cutters were used - a slot drill for roughing and that LS 4mm EM for finishing - 16 set ups and 32 cutter changes - time to actually machine each station about 30% of the set up time - but panned out as planned.

See part one here  part two  part three  part four  part five  part six