It all depends on what track extension you are looking to do. I just extended my '01 sxr 600 from the stock 121" to a 136" rail and tunnel extension. Ive heard of some guys doing one size bigger on the tunnel extension versus the the track extension, (ex. 136" track with a 141" tunnel), just to get extra clearance for the rear heat exchanger. Im happy with the way mine came out, and i have not had any issues. Im running the 1-1/4" ripsaw with 1-1/2" studs. Also, I put 8t anti ratchet drivers for extra bulkhead clearance.