Of course, the rulebook! We argued this out a few months ago over BigKs Husky pipe. The rule is there for sure but in my 22 years in vintage it's never been seriously enforced. If it had been enforced, 50% of the bikes racing would have had reason to be disqualified. In my opinion rule 18.3.0.4 needs an overhaul. The exhaust rules need to be there to prevent modern fatty pipes and such but to prevent changing a pipe with another period piece is silly.