Been there , there is nothing wrong with the worm drive but you need to up grade the cover as well , most are so worn that they have a Cpl of mm play . Ive used the inserts off the 500 and 750 4banger Yamahas and re machined them to fit , Im just saying that the whole unit is well worth it , 1 it makes the YZ a and b legal to race ,2 all parts are there to get if you bugger it . Iain