I could use a little help. I'm looking for a cross reference parts for a 88 yamaha yfm 100 top end the head is good but the piston and cylinder is bad I can't find any parts online and I been looking for three days now if anyone can help it would greatly be appreciated

 

Unless you can find old stock at a Yamaha dealer, or one of the aftermarket makers like Namura and Wiseco still make pistons for them, used parts or the Classic car restorer's tricks are your only options. Check the bore of your cylinder, check the bores of other engines of similar capacity, check other Yams, including motorcycles, first, but there will be a Hondas and Suzukis with a bore same or slightly bigger. Next bit is more difficult, you may be able to check with Namura/Wiseco etc, if the piston for this similar bore engine has the same gudgeon (wrist) pin to top of piston height, if it has, get your cylinder bored to suit this piston. If not, check other pistons for the same bore, until you find one with the correct dimensions. If skirt length is too long, the engine machine shop can alter it to match old piston. If small end in piston is too small, it can be machined to suit, or con rod bushed. Some Classic car/motorcycle specialist machine shops will do all this for you, or stock "piston blanks" that they can machine to suit your cylinder, but will charge accordingly.