Put one on a 2001 XR70, its the perfect blend of power and fun for a kid. The bike has had stock 70cc, 88cc big bore(no felt difference from 70cc), 88cc with head/cam v2 (big step up but was blowing through stock semi auto clutch), yx160 (way too much for this bike, engine was is too twitchy and dangerous on trails), and now this piranha 125cc. This is the one that will live on it! Its my 12 yr olds bike and he is faster on it than our 160cc pit bike or our 65cc 2 strokes simply because he is so light. It also has an amazing exhaust tone with stock xr70 muffler.