I think the heads were all the same as well. I'll confirm later.
Tonight I cleaned up the pistons a bit more - #1 was pretty sludged - I doubt the oil ring could function properly - and that would be if the oil holes hadn't been totally clogged. Here it is towards the end of my soaking & scrubbing:
In the process of cleaning the underside of the piston, I discovered they are in fact Toyota pistons (note the tiny hole going to the oil ring. There are 4 such holes per piston, 3 of which were totally clogged on piston #1:
My frugality is costing me quite a bit of time. It is incredibly difficult to get precise bore readings with cheap Amazon telescoping bore gages and a cheap micrometer.
Did I expand the bore gage the same amount? Was it in the same location? Did I line it up in the micrometer the same way? Did I tighten the micrometer the exact same amount (it has a ratchet, but it is inconsistent)? All of those factors introduce variance so I end up taking ~5 readings per location to try and get a better feel for it:
Without fail, the bore on the piston-pin axis ("A") appears to be like new, but the piston skirt axis ("B") has worn/ovalized about 2 thousandths. There does not appear to be much taper between depth 1 (10mm from top) and depth 2 (40mm from top)
The piston skirts all appear to have had about 1-1.5 thousandth of an inch oil clearance when new (and now have about 3 thou max clearance on the ovalized portion of the bore). 5 thousandths is max so in theory I could probably get away with honing the bore and using new rings - assuming everything else checks out.
For funsies, I checked the ring gaps for the #1 piston. They were all towards the tight end of the standard range, which surprised me. I would have expected more wear. Top ring appears to be cromoly or otherwise plated. Middle ring seems like a cheap/standard ring. the oil control ring is quite a bit different than the ones I'm used to from Supras and MR2s - by contrast it appears to have much finer holes that are easier to clog...
The crankshaft would definitely need a polish. The rod journals for the middle cylinders look pitted in places (but you can't feel it with a fingernail):
