My bike is slipping out of 3rd gear. Thinking of buying another gearbox complete so is it only 1975 and 1976 gearboxs that fit or is there any other years that interchange. Feels like worn dogs on the gear …. any ideas
Thanks

I agree it's most likely worn dogs [on both meshing gears male & female] & the selector will have suffered too. Only other thing that might contribute is if the sprocket nut was 'very' undone so the shaft can move to the right a little which will reduce engagement. So far a what fits--can only guess any of the same shape motor cases & even the 250 which is probably the same box anyway. I'm sure you know but 3rd is 19 into 20 plus the corresponding selector in another page. Plus check the drum for chipping at the high point of in gear. Cheers. & detent spring & plunger but if it's still there then won't be that. People [not you] take that out to drain oil & the bits end up in the oil tray & cap gets refitted lol. But then all gears are up the creek.
www.partzilla.com/catalog/yamaha/motorcy...ission-250b-c-400b-c

Will whip engine out and split gearbox as I am still running with worn dogs or damage to selector drum as all other gears are running perfect. Think if there was movement in either shaft other gears would be affected.