Yes, you are right. The cam and other factors will affect the PSI. The main thing is if all the reading are similar, with no cylinder way lower than the others. Your test looks good.

I'm looking at my 1972 B110 parts catalog. It lists two types of pistons: All ("reg gas") and GX-H ("high gas"). It also lists two long blocks:

10001-H2350 GX high gas
10001-H2356 GX reg gas

It only lists one type of GX head and camshaft, so presumably the difference is in the pistons. The Datsun 1200 group 1 FIA recognition amendment doc lists 10.0:1 and 29.1cc combustion chamber but the earlier engine (non-GX?) had 36.6cc combustion chamber.

Interesting. I assumed all GX's were 10:1.

Mine has flat top pistons and the small heart shaped combustion chambers.

I assume the numbers on the side of my block would tell me a bit more. The block says 253 19 A12 on the exhaust side of the block. That doesn't seem to match your numbers.

My factory manual states compression should be 178 to 206 psi with a compression ratio of 8.5-1 this is a US spec car by looking at your photos looks like you have a later a12a engine with distributer in middle of block. if you just put the motor together your rings may not be seated yet. usualy takes two to three hours of engine running to seat the rings. my a12 with 13.5-1 has 250 psi compression. also I will crank the engine untill gauge stops climbing ussualy about ten compression strokes on cyl being tested,this procedure will even out any air flow differances between cyl. should use screw in type gauge with throttle wide open and possilbly prop open air valves on SU carbs
Bob

How do I tell if I've got an A12A or an A12GX?

Can someone tell me what numbers to look for?

My head is the double valve spring, high oval port, heart shape combustion chamber type - and I have flat top pistons.

My engine number is A12 058259 B

I can't find any numbers anywhere in the head casting.

You probably don't have an A12A, which was a US-only engine and didn't come in "GX" style, it was the fuel economy motor. It is 1237 cc, with a slightly bigger bore than the A12.

The 71-73 A12 has the distributor under #1 spark plug. The 74-up A12 has it under #3. There is an ID mark on the flat boss near the distributor that should say "A12" or "A14".

a12 is engine type, 058259 should match the vin # of the car it came from, unsure what the B stands for. there is no marking on the block that says in is a gx engine, your engine apears to be a gx engine with the gx style small chamber head and flat top pistions. if you degree the cam you can tell if matches the specs of the gx cam 256 duration, stock is 248 degrees duration both cams have the same valve lift
Bob

On the dissy side of the block it says a12 in a funky font on a seperate flat area beneath plug 4. The block also says nissan japan on it.