There's not as much meat in the later A12 blocks so I suggest you have it checked for wall thickness before you try fitting 76mm (or bigger) pistons. Everyone into A-series engines here in Japan says it can't be done.

For our US friends, the best A12 block to use would be the A12a, as these already have a 75mm bore, & the factory 3rd OS would be 76.5mm Pushing these to 77mm or 77.5mm bore size [A14 2nd & 3rd OS] should be a doddle.

If using the A14 crank, use the A12a rods with A14 pistons as they have the larger wrist pin hole already.

Use the A14 or A15 head & it will just bolt right on.

was speaking to an engine builder (a mate of my mechanics) who reckons its eaiser to keep the standard crank (especially as their is only a minor difference of 3mm in bore and 7mm in stroke from an a12 crank to an a14 crank.
he was trying to convince me that by using a standard a12 block and crank and modded rods, bored out to accept 75 or 76mm high comp pistons with an a15 shaved head and a metal head gasket, it would rev harder and produce slightly better power (although less tourqe) depending on carb and cam selection... he belives that by machining and balancing the a14 crank to suit would create a few headaches (something this old man doesnt want)..
he says to buy new pistons to suit the a12 rods wrist pins instead of moddifying them to accept a14 of a15 pistons.
is he right or just being lazy like i suspect.
he does uild nice 383 308 strokers though... and the sound is fantastic.

Unfamilia
The "minor" diference of 3mm represents an increase of about 4.1% in bore size. By way of comparison, many of the late type V8 engines are limited to a MAXIMUM overbore of .040" or about 1mm. In a 4" bore this represents about 1%, yet the little A12 will go even more.
Your friend should get an understanding of what a "minor" oversize is.

As for the "minor" 7mm increase in stroke, this looks like a 10% increase to me & i don't know what he calls "major"

By using a 77mm bore & the A14 crank, it increases the capacity by about 23% with completely stock parts.

The 383 cube Holden is an increase of 24%, but requires costly special parts, so he has no problem pushing Holden engines out by 24% with expensive special parts, but doing something similar to a Datsun with inexpensive stock parts seems to be a no no.

If he can nomimate a supplier of Datsun pistons of the required 76mm bore with the smaller wrist pin, i think we would all love to know of them. Methinks that they would need to be made by a specialist piston manufacturer at great cost.

My Dodge hemi has a stroke of almost 4" & these engines, & when suitably "tuned" will rev quite happily to about 6,500rpm without problems, & the A14 will do the same, so how high does he think that your A12 will go?
For street use, anything much over 6,000rpm is really not necessary & the 77mm crank will handle that with ease.

As for the rods, just how hard does he think it is for a competent machine shop to bore the small end in 4 con rods. It seems that he thinks it is preferable to have custom pistons made. If push comes to shove, i wonder if one of our US members could provide some A12a rods as these have the correct length & pin size.

Is he being lazy? Thats not for me to say, but the fact that you asked..............

76mm A12 pistons are still made by a few companies. I have a used set. However, they are floating wrist pin configuration so there's still modifying to be done to fit them. Matsuoka and Pitroad are two companies they're available through. Both forged and cast.

2332owner
I knew that the Datsunsport 76.8mm forged pistons had the small pin, but these things cost cubic money even 15 years ago.

I wonder if either of these companies market within Australia. The price may be a factor. Even the stock pistons, when purchased new, either genuine or replacement, are not cheap. If one has access to a junk A15, then there is a good chance that the used pistons from it could be used in a strong A12 motor. The only cost would then be for rings & the rod alterations. The stock cast pistons are surprisingly robust & will handle the revs & power output of a well built street engine.

As i have said before, A12a rods should do the trick without alterarion.

after talking more to him, ive decided that he is being lazy, as all the parts for the 383 can be purchased for bolt in. he doesnt want to modify anything himself anymore, just buy it already done, and noone sells a12 pistons that size anymore, but can get 74mm. will these go okay with an a15 head bolted on? might end up being cheaper enginering this blody a15 into the ute.

74mm is simply the second oversize for this engine. I believe that there is a third oversize of 74.5mm

If you want to use this size, then check the Repco catalogue. Datsun 1000 pistons used a flat top with a longer skirt than 1200's
The 1200 had a shorter skirt to clear the longer stroke crank when the piston was at BDC, & used a dished crown.

Repco, bless em, made both 1000 & 1200 replacement pistons on the same shorter 1200 blank. They machined a dish for 1200's & made em flat for 1000's.
It doesn't take Einstein to figure that what we have is 1200 flat tops here & since the price was the same, a little extra compression could be had for free.

This is what i used in the 1200 engine that i built for my wife's '68 Datsun 1000 two door back in about '83. No tyre smoker, but a nice little grocery getter.

mmm 74.5mm flat top 1200 (1000) pistons may be a goer, especially with the a15 head.

ive also found an early a12 block now by swapping it for the later a12 block i had, so what is the mod that has to be done to early a12 blocks to put the a15 heads on them. the oil somethings.

Unfamilia
I modified the head to work woth the block.

You will need to drill two holes, but the best way to know what to do is to get an A10 or early A12 head. Borrow or steal one, but study it closely.

You will need to drill a hole from the gasket face & a horizontal one from the spark plug side. They must intersect then the one from the outside must then be plugged. I used a small gas [tapered thread] plug, but the factory just drove a soft plug in there.

The one that i did was done with hand held tools, but a drill press would be even better.

If you use 1000 pistons, make sure that they are made on the shorter 1200 piston casting. The Repco ones that i used were just the ticket