I am probably a bit of an imposter in the datsun 1200 club. I have a 1964 Datsun 320 ute. It has an E1 series 1200 engine so technically it is a Datsun 1200.
I have been considering fitting an automatic transmission (a jatco from a 120y or sunny). I have a spare A12 engine and was thinking of increasing the capacity along with some breathing mods.
I would love an A14 or 15 but I hope that I can make the A12 perform well enough to handle the auto.
How large can I go and what about piston type, availability and price?
What do you think?

firstly i think bugger the auto!
but if u want one, here's what i think.
if u want more torque go with an A14 or 15 as they bolt right up to anythin the A12 does.
but if you want an A12 then perhaps an over bore with higher comp piston and thinner head gasket would be ok.
if you want to bore it, dont go any more than 2mm. if you do there is a possibility of warping the bore because of insufficient metal for support under normal operating temps.
ucan do the maths. if u bore the standard A12 bore dia 2mm more, u'll end up with 75mm. volume of a cylinder = 2piR^2 x stroke x 4cyl. (two pi radius squared multiply by stroke multiply by four cylinders)
i hope thats right. if u bore it 2mm u should have about the capacity of an A14 or 15. but then you need custom pistons to suit and all the other mucking around.
i would buy an A15 and be done with it!

try finding an A15 first - then buy it.

suddenly building an A12 will seem like an easier proposition.

wat kind of ccs can u achieve with an a15? couild u achieve nearly 1700cc?-sorry to drop in

80mm bore 82mm stroke
work it out

The factory offered 76mm pistons for the 1200 engine from their competition department back in the 70's & I bored my 1200 block to this size back in about '81, but I used standard size A14 pistons. I drove it for so long that I eventually wore the motor out, so I bored it to the first oversize [76.5mm] & used A14 pistons again. Once again it served a full life & was removed from service only due to a burned exhaust valve.

Plans called for one last rebore to the second oversize [77mm] but an A13 block turned up & I will use that instead.

The downside to this is that the A14 pistons use a 19mm wrist pin, while your A12 rods will have a 17.5mm wrist pin hole. You can have the rods bored, as I did, or you can find some A12A rods in the USA to overcome the problem.
A12's are generally quite safe at 76mm, & 'probably' safe at 76.5mm. After that, you're on your own.

My recomendation is the same as the others. Spend the time & money & get yourself an A15 as the additional torque will help things along very nicely. A12's & auto's are best used in the lightest cars.

Reportedly, many 1971-1973 A12s cannot go 76mm, but others can. It is recommended to sonic-test them. This bring the A12 out to about 1300 cc.

The A12A and newer (non-US) A12 engines should all to go 76mm.

The A15 can go to 1600+ ccs, you can even buy a complete engine from Pitroad.

I heard off a speedway engine that had been sleved to take it out further but thats getting expensive

the old man has a A15 block taken out to 79mm but the cylinder walls start to deflect at that point