I ran an o.s. 18 tm in mine for a bit then I decided to put the trx 2.5 back in it. IMHO the trx 2.5 is a perfect match for the rusty, it generally has more power than I need in that light little ST, but not so much as to make it difficult to control. With the .18 tm in it all I ever did was spun out, even when running pretty fast, opening the carb up would break the back end loose and make it tough to keep under control. So I put the TM back in my max and stuck with the TRX 2.5 in the rusty.