Do you have a 'rear chain oiler' fitted to that, Mark?

On some models it is a small channel protruding from the back of the case. If so, it should have a tapered threaded pin with a spring fitted in the relevant case screw hole. Adjust as required.

Not really satisfied with my measurements, but the sprocket which was in place has a 5mm dish with a 3mm boss. The new sprocket has a 5mm dish and 3.5mm boss. Still looking at the rest of it. The bearing in the cover does not turn by hand, the cover has been off for some years. There is no sign of t...

Hi Graeme, yes, I've been doing a bit more thinking, too. You will see that the sprocket I bought from Bruce is 27 teeth. The thinking I've been doing is that the backs of both sprockets I have seem to be of similar geometry. I really need to do a dry fit to determine exactly what was wrong. I had n...

The castellated nut is available on ACME Stainless.

I don't know about hardening, if any. It is a very different item from the pressing shown on Simes' Amelia blog. The relationship between sprocket, shoulder on whichever sleeve you use, and whatever surface the assembly tightens onto will need careful thinking about to get sprocket alignment. The ge...

Hi Graeme, this ought to be a photo with arrows and scribbles, but that's beyond me. Sleeve length looks like 1 27/32" or a shade under 47mm. Washer OD 2", .. OD to fit inside spring 1 21/32", .. ID of nut recess 1 1/2", .. hole for narrow section of nut 31/32". Overall dept...

The top photo above showing the specially shaped washer and castellated nut does not show the order/orientation when assembled, before anybody points that out. ..

I should also say mine is MkII, but I believe these items would fit MkI.

Before you resort to the lathe or Black-Country screwdriver, these photos should show that the sleeve will pass completely through the sprocket. It can be inserted into the sliding member from the engine side until stopped by the splines in the sliding member. There are different sets which can form...

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Graeme, I believe the sleeve is wrong, as Simes suggests.
I cannot tell from your photo, but the sleeve seems to have a wider rim at the end.
It should be uniform OD at both ends. That sleeve looks too short.