Standard cylinder opposing a P10 cylinder

I'm being told by an aviation parts supply shop that I cannot oppose a P10 (oversized) cylinder from a standard cylinder even if the Pistons weigh the same however a reasoning could not be supplied as to why that is so... does anyone have any information on this.

Application is rv4 IO320 fixed pitch...i was considering putting the oversized P10 in the number 2 position and a new standard cylinder in the number 1 position.

I'm assuming because displacement is larger on one it's actually producing that more power and therefore there's an imbalance.... but perhaps there's more to it

Any thoughts appreciated

Tom

If you were building fresh overhaul and had no price advantage to mixing, I agree. But practically you're looking at less than a 0.5% difference in cyl power and piston mass. I wouldn't worry about it.
These are not very precision engines. But if you're worried, I bet you could easily find a reconditioned oversize cylinder from a reputable shop.

I have seen it done on both Continentals and Lycomings and they ran fine. I have knowledge of a V8 Indy car engine that had 7 cylinders .125" over and one that was standard ( it isn't cheat'n if you don't get caught!) , it won the Indianapolis 500. Your plane is an Experimental.