So as clearly I was bored yesterday I tried an A15 crank in an A12 block and there are issues.

First the block in question was a 73.75mm bore. My intent was to use 75mm Honda Pistons in my A12A block

I used the standard A12 rod and piston (knowing th piston was going to stick up 2.5mm), the connecting rod fouls the bore. I didn't measure but the top of the rod bolt contacts the bore so I'd say it needs at least 5mm more clearance.

Next I pulled the rod and piston off the crank; the crank throw contacts the block about a 1/3 of the way along the surface of the throw.

So a couple of things; an A14 crank may fit without these issues given the shorter stroke, the A15 crank might clear the A12A 75mm bore block but I doubt it so I won't be trying it.