Part three - Bad Day! - by Ramon Wilson

Next stage was to turn up an expanding mandrel and mount the 'closed' case on, to bore out for the backplate. At this stage it still has .5mm to come out before screwcutting. The rear face was skimmed, too, to bring it exactly square to the bore ready for the next op on the case.

It was also used to face both sides of the 'open' case.
A necessary diversion into the cylinder heads came next so two blocks were roughed out ready. A 32 TPI thread gauge was made for the cylinder heads and for the front housing at the same time (I've decided to cut the rear thread at 24 TPI - I'll turn the other end of the gauge when  the change wheels get swapped). A plug gauge for the heads and the crankcase liner bores was also turned up.
The length of thread inside the cylinder heads is 7.5mm and with such a small groove to run out in it was decided that this would be much better to do in reverse. With such low cutting forces involved it's most unlikely the chuck will unscrew and it certainly makes screw cutting such threads a lot less stressful. By keeping to 32 and 24 TPI the leadscrew (8 TPI) can be engaged anywhere which helps things even more. An old tool was pressed into service to turn the groove....
..and an internal thread tool ground up from a discarded long series 6mm slot drill shank also found in the tin of previously used lathe bits.
The screw cutting was done from the inside groove working outwards - hence running in reverse for a R/H thread. The saddle was brought up to a stop and the leadscrew engaged - it sure takes the worry away of having to stop 'in the groove'.
The threads were made a nice fit to the gauge and these in turn will be used as a gauge on the crankcase thread. The bore was taken out until the plug gauge was a nice slide fit and the hole for the compression screw drilled and tapped at the same time. These will be mounted to a mandrel at a much later stage to turn the outer profile and cut the fins.
The two threaded rings incidentally are for use as protectors once the threads are cut on the cases themselves.

This first one went well so a change of the change wheels back to fine feed and it was time for the second one. The bore diameters incidentally were done to the same plug gauge as the cylinder heads.

Another change of wheels and the last screw cutting. This went just as well as the first until trying the head on. The thread was a little tight so it was gently chased trying the head, the fit improving each time. Constantly aware of the possibility of the ali 'picking up' this was done quite gingerly until, with a slight tightening with less than half a mil to go I felt it grab!  It was at this point with a rapidly deepening sick feeling that I knew this was not going to be my day and kicked myself for not using some tallow that I had thought about doing earlier but overlooked. Some releasing fluid was squirted over in the hope it would penetrate and the head gripped with emery to gain a good purchase. Very tight it moved about half a turn. To improve the grip anti-slip matting was then tried - then heating it - then, in desperation, a pair of chain grips used - the only thing achieved was to undo the face plate. Reality dawned  - it was well and truly stuck - big time - so for the first time, though I've read and heard about it, I could quickly see that the only way out was to turn it off. (Expletive deleted, as they say).

The OD was cut back using a parting tool then parted off to leave the 'ring' ...

.... which was very gingerly turned until it began to break up.
As you can see most of it fell or was pulled away save this one small piece stuck between the thread - absolutely solid.
But after checking the bore was running true and re-aligning the tool it soon 'turned out' OK  - Phew!
Though it's down on diameter by .1 mm  that's not so much an issue as the new cylinder head can be made to match. So, though one of them is there as a back up there are still two viable cases.

part nine  part ten   part eleven   part twelve