have a super nice yfm 100 that broke the rear axle shaft and searching for a new or used replacement part. where would I look?

 

According to Babbitts there is no such model as a 96 Moto 4 YFM100, in fact the only YFM100s I can find are Champs from about 1990. So find out what you have got, then check the on line parts books, axles for early Champs are available from Babbitts. The shaft drive rear axle set up looks the same as the later YFM80 Raptor.