I had always heard of people cleaning up the stock headpipes. Well, since it was raining all day I decided to pull the head pipe and clean it up. I measured it before, about 1.01" diameter, both sides. I started griding and cleaning. Ended up with about 1.12" diameter both sides. Just over .1" extra diameter. That's a worthwhile effort. Just some quantitative proof of what can be gained in case your interested.