I think when you built the gearbox, the mainshaft needs a litte play, sideways. Not much.
And the thickness of the gasket between the housing and the middle cover is very important.
Thanks for your message Bruce, I'm surely gonna try to keep this washer out. I have been told before that it is not always good to keep the rules too strict, so that's what I will do..
I am not ready to give up my single spring clutch becays it is working fine. It can take a while but I will come back with the results.