Just a foot note to report some success!
With the nuts and tab washer reassembled correctly and also with the spring cup being tapped gently into place I now only have only one spare thread showing. The inside of the chain case has had its weld high points ground down and all was reassembled.
It started first kick (they all do that sir) and I did notice that the primary case dome became warm instantly but with no noise. I think that there might have been the slightest contact at first but it all seems to be ok now.
I did notice that the clutch basket is quite slotted so I will look for a 'new old stock' item to replace it.
Today I've had several rides through the Devon countryside up and down hills and nothing has blown up. The new piston and valves are all swishing away like a Swiss watch and apart from the need for a little clutch adjustment the whole plot seems very happy with itself. Brakes are very powerful, the handling and steering are very natural and the overhauled charging system and magneto are all spot on.
Hurrah! Happy Arieling to you all.
1938 Ariel Red Hunter VH1
2011 Moto Guzzi V7 Cafe Classic
AOMCC Member 3400
Looking for a well sorted and original Black Ariel, 1928 OHV or 1930 OHV (or a nice Velocette)