measure the width of the mounting lugs on your gearbox
good chance some one has fitted the early narrow box where a latter wide box should be fitted.
read the article in the technical section.

The mounting lugs are exactly 2.5” which I think is right. The frame section that attaches to the plates is slightly less at 2.475”. Should I shim it or was that intentional?

email (option): Micah.leinonen@gmail.com

I’m pleased to confirm that straight gearbox plates and correct spacers fixed my primary sprocket alignment problem.

email (option): Micah.leinonen@gmail.com

In the parts list for the RAF outfits, a 17T sprocket was used. But they did tend to under gear stuff back then. I think I'd try the 18T. Ron

email (option): ronpier@talk21.com