okay i have the orders " get rid of all those car parts"... but i havent been told how so heres the idea,
* a12 block bored out to accept a15 pistons (76mm bore to 79mm bore as per specs on datsun1200.com) and fitted onto a12 rods (what nees modifying again?)
* this all to be topped off with an a15 shaved head to inc compression and line up with a12 bores with a webber 32/36 (that ive almost finished.

the reasons for this are that i cant be bothered with the engineers or other methods of a15 block insertion linto the 1200 so next best thing.

will the a12 block have enough meat to be bored out that much and will the a15 pistons be okay on the a12 rods? and why has my a15 fuelpump got two lines running from the pump to the carb? never seen that one before.

Ideas and comments would be appreciated.

i dont think the A12 block would handle being bored out that much. Dont quote me on it. My A14 walls look very thin between piston 1&2 and 3&4. And wouldnt you be better off using the A15 rods for the longer stroke?.

A15 pistons are 76mm standard. 79mm aftermarket pistons are also available.

An early A12 (alternator mounted on opposite side of distributor) will accept boring to 76mm safely. I've got one. Later A12 blocks should be checked for wall thickness before trying this. I think you'll find most of them won't work. 79mm pistons aren't an option in an A12 block as far as I know.

so an early a12 block can be bored safely to 76mm (i read 79 somewhere), it could be better just getting high comp a12 pistons overbored to 74 or 75mm, thought i would use the a15 ones seeing i have them.
Will the a15 pistons fit onto the a12 rods easily?

Not using a15 crank or rods cause the stroke on the a15 is a lot longer than an a12, and wont physically fit in an a12 block i believe without smacking the head. The a15 block is taller by about 10mm or so than an a12 block. i measured it and was a bit supprised.

OK blokes, here's the deal.

The "genuine" A12 block [from a 1200] will happily bore to 76mm. I have been running the first oversize [76.5mm] for so long that the engine is worn out. There is a 1200 coupe running around here with a 77mm bore [2nd O.S.] & no problems yet. I would not risk any more without a sonic test on the cylinder walls.

Rods
The 76mm [+] pistons have a larger wrist pin & the 1200 rods can be bored. Better yet are the later 120Y type rods. I have a set fron a later type 1200GX engine & they seem to have more meat around the wrist pin area. I have been using the early 1200 rods, bored out, for somewhere near 200,000 miles & no problems yet, but i don't flog my engine like some do.

Pistons.
If you are going to use the 1200 crank, then use the A14 pistons. These will put the piston crown just about flush with the deck of the block.

Better yet, use an A14 srank in the 1200 block. To do this you will need to place the crank in a lathe & machine the dags from it. It is best to have the block bored with at least one piston & rod mock assembled to allow trial fitting.

To pull this off you will need A15 pistons as these have a lower crown & a shorter skirt than the A14. You need to machine the crank to ensure that it does not touch the bottom of the pistons. It doesnt need much.
Next, be carefull about the rod to "bottom of bore" clearance. Some rods have a larger protrusion for the bore oiler than others & these may touch the bottom of the bore, but only just. A small round file on the bottom of the bore fixes that.

You need the A12 length rod with this combo & the A15 pistons come flush with the top of the block.

So what do you get with all this effort? Well with the 77mm bore & the A14 crank combo, you get 1434cc in a rego friendly A12 block.
Thats what the 1200 coupe around here has & it runs like stink. It has an A14 or A15 head [i forget which] that has been modified for the early oiling system & a brace of carbs from a Yamaha FJ1200.

I hope that this has been helpfull.
Chris
P.S.
Please note, this whole engine is built up from stock Datsun "A" engine parts. You pick the cam, the induction & the exhaust that suits your fancy, but the core engine is all stock stuff.

Thanks for the ideas and details. I don't think this has ever been discussed here, at least not with much detail. It sounds like a good idea: A14 crank and A15 pistons in A12 block. Save a little weight too, I reckon.

i think im excited.... better wait and cancel that perscription for those little blue pills, i found the info im after... thanks fellas and ladies, there are some here... i think

The "tweaked twelve hundred" described below was built from a pile of parts that were laying around the workshop, including the new 77mm pistons that had been bought in for someone who never came back.
Even the 1200 GX exhaust manifold was found in the parts pile.

The cam was also left over from one of the race engines & is of an unknown grind, but it has a damn lumpy idle & kicks in a fair bit into the rev range. The car was dyno'ed at 80hp at the rear wheels, so i guess that's not too shabby for an engine built from workshop clutter.

two more questions,
* can you bore the later a12 blocks to accept the 76mm pistons

* please describe what i'm going to have to do to stop the a15 pistons catching on the crank/bore bottoms. im a bit worried about the grinding that will have to be done.

all this info is great though... something we have lacked a bit since the old forum died

Although i have never done it, i have no reason to doubt that the later 1200 blocks will bore to 76mm. Nissan once supplied 76.8mm forged pistons for the A12 motors.

To prevent the A14 crank from touching the bottom of the A15 pistons it is necessary to chuck it in a suitable lathe & machine the dags from it & reduce its overall rotating diameter a little. This basicly brought all the counterweights to almost the same diameter on the crank that was used in the local engine. [All counterweights are not necessarily the same on any given crank] This is a bit of a trial & error job, but the amount needed to be removed is small. There is no "grinding to be done".

Some rods have a lump of material on one side with an oil squirt hole to lube the bore. Others have only a small bump with this hole. During trial assembly, make sure that there is no "lump to bore" interference. If there is, then get a nice sharp round file & remove some material from the offending area. Only a small amount will do the trick. I suspect that most rods will not have a problem at all, but check the clearance & make sure that there is about 1mm or more. It is only the very bottom of the bore that may have a problem, & only on one side.

After you are happy with the crank, you can send it out for grinding of the journals if needed & for chrisake, make sure you get the whole rotating / reciprocating mass ballanced.

Use an A14 or A15 cylinder head as the port, combustion chamber, & valve sizes were designed for this capacity. If doing it with an early A12 block, MAKE SURE that you modify the head to the early oiling system. If using a late block, it just bolts on.

It's gotta be a good thing to take the original 1172cc out to 1434cc.[with 77mm bore] Thats 262cc or about 23% greater than stock.

Used A15 pistons will work just as well provided that they are within service limits in relation to piston skirt taper & ovality, the ring lands are unworn & undamaged, & the pin bore is within limits.
Usually the pistons are fine, it's the bores that wear. If using "pre loved"pistons, then the reconditioner will bore & hone to suit, but you will need to ensure that the ring gap is right as the new bore may be a poofteenth smaller than with new pistons & the ring gap may be a tad tight. Just check everything as you go & you will love it, especially when your mates become amazed just how strongly that dinky little 1200 motor of yours runs.