Bottom end wise almost everything is interchangeable. About the only thing that got altered was the oil pump from early to late '21's. The late '21's being the same the the '28.

Barrel and head wise the two are pretty much the same though the porting on the '28 cylinders is just a little less peaky compaired to the stock 21. However the difference is almost nothing. The '28's do seem to make alittle more in the middle over a 21 of the same spec though.

I can never remember which way around it works, but R cylinders I think are shorter than SP cylinder so while they can be swaped you must use the right head with the right cylinder. Best way though is to measure them up and replace with the same hight. Just because some says the cylinder is off an SP or R doesnt mean it is lol
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If I have to take the carbs off once more...