We've all been at a dead end sometimes! Have someone else build the diffs for you, and then see exactly how they've done it. You're probably just missing something.
Sounds like you have too much play.
Good luck!
Just for the record: When the B44 came out, some had problems with gearbox havocs all the time. Others never ever. Clearly that was just incorrect shimming. There may be production tolerences as the moulds get worn, so you should always properly check the clearences, not follow the manual blindly. May be wise to start with it too tight, then take off 1 shim. If it's then free, you know you can't make it any tighter... And correct slipper adjustment of course, check that on the track. Make it too loose, then tighten until it slips just slightly. Don't use the "manual/measure setting".
Keep in mind that these are sophisticated racing machines, not toys. So you very seldom get a refund when something breaks. But some guys are extra nice, and give a refund/free parts even when there's been a user error. Doesen't hurt to send the parts and politely ask for new parts. But I'd be a bit humble about it!
