I have read on this forum that the exhaust porting is different between MC21 & MC28 barrels, whereby the MC28 barrel is in a lower state of tune. Guys - Need some input from others on this point.....

Obviously you need to compare like to like, 21R to 28R, 21SP to 28SP.
Another question out to the forum....any difference between SE & SP barrels??

Then there are the 3 cylinder / piston pair matches, with cylinder sizes A, B + C corresponding to a specific piston size.

Honda UK online parts are saying the MC21SP B cylinder P/N has been superseded to / replaced by the MC28SP A cylinder, with no mention of availability of the MC28SP B cylinder.

I am currently trying to track down a brand new MC21SP B Cylinder (for spares). If the MC28SP B Cylinder has the same porting, OK, but I'm not happy about simply switiching to an A size cylinder, just when I've bought 2 X Top End Piston kits for a B Cylinder.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30