so that means an A15 crank in an A12 with A12 rods will work? with the right pistons of course. which brings up the next technical question, is there a difference in cudgeon bore centre to crown between the A12 and A15? Is it actually possible to bore the A12 out to 76 mm for the A15 piston?

All of the A series engines have the same piston crown height [pin center to top of piston] EXCEPT the A15 which is shorter.

Most A12 blocks will quite happily bore to 76mm & more [if you're game]

Why on earth would you want to fit A15 pistons when the A14 piston is the same bore size [76mm] & is the same deck height as the A12.

This is essentially what Nissan did when they made the late [1980 onward] A13 engine. The only diference was that the rod was made for the larger wrist pin of the A14 piston but the rod was the same length as the A12.
The A13 rod & the US model A12A rod are interchangeable.

I put A14 pistons in my 1200GX engine back in 1981 so it's a conversion that has been known about for a long time.

Oh yeah, forget the A15 crank in the A12 block exercise, it won't fit, but the A14 crank will & you need A15 pistons for that conversion.

"This is essentially what Nissan did when they made the late [1980 onward] A13 engine. The only diference was that the rod was made for the larger wrist pin of the A14 piston but the rod was the same length as the A12."

My experience with the A13 engine is different. In Canada and the U.S. I assume, the A13 has the same bore as the A12 (73mm) but a longer stroke (77mm vs 70mm) and a taller block. The A13 rod is longer than the A12 but keeps the smaller wrist pin. A13 rods can be used in A14's or A15's but the wrist pin has to be enlarged to 0.750". The later A12A has the taller block and the longer rods but retains the short stroke crankshaft. This was probably because of manufacturing simplicity. To further complicate things, it is possible to use the A14 or A15 block (taller & bigger bore), the A14 or A15 rods (longer for more torque) with A14 pistons and the A12 or A12A crank for a really short stroke, high revving screamer.

I am building an A13 race motor with 77mm stroke and 75.2mm bore Arias pistons for approx. 1290cc's to fit within SCCA GTLite regs.

Hi Iceracer, he is talking about "the late [1980 onward] A13 engine". You are referring to the 1974 A13. Two different animals, and north america didn't get the 1980 A13. Yes, the 1974 A13 is a tall-block with stroker crank. Yes, the 1980 A13 is a bored-out A12.

Wow. You learn something new everyday...........

Yes 2 types of A13 and now we only need TopGear to make billet A18 series engines with removable cast sleeves.

Quote:
As Ddgonzal wrote, there are two different A13's

There were three different engines built on the late [1974 & onward] short cylinder block.
1. A12 with 73mm bore & 70mm stroke for 1171cc
2. A12A with 75mm bore & 70mm stroke for 1237cc [Used in B310 North America-only cars]
3. A13 with 76mm bore & 70mm stroke for 1270cc [Used in B310 Asia-only cars]

One last edit.
The US-only A12A is simply a late type A12 with a 75mm bore size.
ALL of the 73mm pistons used the small diameter wrist pin & ALL other A series pistons used the 19mm [not 0.750"] wrist pin.
19mm= 0.748" & the difference is a lot if you are boring a rod to 3/4" then fitting a 19mm pin

so much great info, thanks Dodgeman, is the A15 crank longer or larger diameter? if its larger diameter i think ill try make it fit anyway. as for why use A15 pistons, well i wasnt actually going to use A15 pistons but i didnt want to jump ahead of myself as i wasnt sure how large i could bore the A12. I'm going to try and get an 80mm bore.

Has anyone had any success in offset boring the cylinders?

Yes, offset boring of the cylinders is good with the 79mm bore but at 80mm I think you are pushing your luck just a little bit too far as I do not know of anyone who actually did it & made the engine actually run.
Generally speaking, a 3mm overbore is the practical limit, however I have a 1200 engine at 76.5mm & it ran fine for years. I have also seen a 1200 engine at 77mm & it too was running fine but those cylinder walls had to be getting awfully thin.

The problem with the A15 crank is that it will pull the pistons into the rotating mass of the crank at BDC.
Even with the A14 crank you will need to machine the counterweights to clear the pistons & you need to machine about a half mm from the top of the A15 pistons to bring them level with the top of the block.

'O' Ring job at 79mm













A14 crank mods.

nice, i take it the meat taken from next to the crankpin is for balancing? i think i'm still going to do it, but maybe not such a big bore. am thinking of using unknown pistons if i can't trick up the A15 ones. I'll run big clearances on the pistons with light rings, this engine is purely for the 1/4 mile by the way, hopefully the pistons won't grab, next issue will be headgasket i guess, thinking of going steel.

O-ring job at 79mm bore? don't really follow that one.

may as well go the whole hog and stuff the block with grout! i wonder if running coolant through the head but not the block would cause issues.