Sorry to resurrect an old topic but I've been on for days trying to get my gearbox to work . It was selecting the gears no problem with the outer cover off but as soon as that was re-fitted the gears wouldn't select. I bought a new selector claw but it made no difference.
With Colsbeeza's theory in mind I noticed that the claw was made from material thicker than 3/16" and this seemed to be stopping the claw from presenting square to the quadrant. With the cover off and the selector shaft in place on the inner cover it's easier to see than explain.
using a grinder I reduced the thickness of the claw where it touched the stop plate to 3/16" and it works a treat now.
. The gearbox has never run but would appear to have been re-built by the previous owner and it would seem that he fitted a new thicker claw so I had no original to compare. When the claw arrived it had a note in the package saying that it might need "some fettling" to get it to fit on the shaft.
An exercise in character building if ever there was one. If there had been a skip handy the box might've ended up in it.
All well in the frozen north tonight but I won't test it again in case it was all a dream and it stops working.