domi 99 crankshaft thread size

Dynamo or Alternator  crankshaft???

One is 7/8"and the other 5/8"......both 20tpi

As an addition to what Philip said - don't forget that the timing-side is a left-hand thread.  Good luck, Howard   

See attachments for info.

I think that Dommies are 55 degree Whitworth form but Commandos are 60 degree UNF (form).