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A LOT.

Apparently I have laboured for some time under the mistaken belief that the A12A was, like the long stroke A13, a North American market-only item.

I'll give myself an uppercut [ouch] & add that item to my meagre store of knowledge.

A12A is not unique to US market. It was sold in Japan and other Asia markets. But apparently not where Dodgeman lives.

do you mean A12A is US only market? my b310 has A12A in it, and none of my friend use A12A. the car is a little different, as my car have the heater. we don't use heater in Malaysia, the weather is scorching here all year :)

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So what exactly is a squillion?

For all practical intents & purposes the later [rear distributor] A12 block is interchangeable with the A12A with the only significant difference being the bore sizes.

If one is going to go 'big bore' in Australia it is probably better to go to the 76mm bore size & use the readily available A14 pistons as A12A pistons [75mm] for A series Datsuns would need to be sourced from the US.

73mm vs 75mm isn't it?

A12A big bore.

A12A block and A12 block, is there any different?

cheers guys.. especially dodgeman thanks for all the extra info and hints.. i will certainly keep it in mind

As already written, standard pistons were 73mm with three factory oversizes in half mm increments [73.5, 74 & 74.5 mm]

If you use A12A or late model A13 rods you can bore the block to 75mm which is A12A size along with three oversizes in half mm oversizes.

Additionally you could bore it to 76mm & use A13 or A14 pistons. Both of these come in the same three oversizes but pushing it beyond 77mm is probably pushing your luck a bit.

Why the different rods? The 73mm pistons [& oversizes] used a 17.5mm wrist pin while all other models used a 19mm pin.

The A12A & late A13 were simply bigger bore A12 blocks but their rods had the larger eye to accommodate the larger pin.

Later A12 rods were forged with the larger 'little end' but were bored for the smaller pin, These can be bored out with confidence.

I wouldn't actually recommend boring the earlier rods with the smaller 'little end, yet this is what I did in my GX engine in the early 80's & drove it for squillions of miles without any problems.