To those using the Tekno mount...

[JThiessen]

Send it to Tekno. If they measure it and its "good", then they can send you a 42mm mount for your motor. You could then wrap your motor with some aluminum tape to get it snug if you want. If its bad, they'll send you a good one. Much better than trying to mill out the one you have.

Only other real option is to take it to a machine shop and they could possibly mount it to a lathe and open it up some.