There are differences in the porting, but it's negligible. The MC18 porting does appear to have been finished better and more consistently than the MC21/28 barrels, but it's the overall package (top-end, bottom-end, and possibly exhausts) that (often) makes the MC18 produce better power in a standard form.

The later barrels are unquestionably stronger than the early hexagonal type though, with better thermal properties.
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Andy.
NSR-WORLD.COM

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