Mitani Special titanium front exhaust for Montesa 300RR - 301RR 28 internal diameter for standard rear exhaust 28,5mm. S3 and standard headers are of a thicker gauge tube and so their outside daimeter is 30mm, but internal is just over 28mm, so same performance.
Big bore header that fits straight into the standard rear silencer on the 300 and 301.
These are pure class, and to correctly flow the exhaust gases the smoother the header the better. Straight angles and joins lose performance and interupt the air flow.
Also the lightest header available. The best, bar none!
Mitani 220g
S3 360g
Standard  443g
Does not come with a gasket.
Uses the standard Honda exhaust gasket - 18391-MB0-891
(in the above picture for reference is the S3 on the left and Mitani on the right)