They're the same (that's a lie, but they're a direct swap) for mc21/mc28 ones.
Measure the barrel and buy them to suit or send the piston off with the barrel for a replate and ask for the clearance you want.
Post by Matt@Tyga here with the sizes.
mc21 barrels and heads (togeather) also fit. As do mc28 ones but you would contaminate an mc18 that badly would you?

I won't mention what you said about us on the PB forum (oh, did i say that out loud?) and please remember I'm allowed to post off topic, hijack threads and hold personal conversations in General...you aren't
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Please do not PM me technical questions, if you can't find it on the Forum start a thread