When looking for a mountain bike to motorize, look for bikes with normal round tubing. Any sort of oversized tubing or non-round tubing will give you trouble. Here's the frame I'm using, to give you an idea : http://www.nashbar.com/bikes/Product...09_-1___202337
Also, when buying an engine kit, make sure wherever you're buying it from carries replacement parts. Different sites/vendors carry different kinds of motors that have small differences from one another. If you stick with the same place you bought your engine from, chances are there won't be a problem.