bore flex comes to mind along with a tad of blowby

Bless me soul, at 80mm with an offset, one side of the bore must be so thin that it would almost be transparent. LOL

FYI Dodgeman, BigE on here has an offset bored A14 with 80mm bores..
and yes I have followed your lead and on the Path of Big cube A12A goodness.. :)

The crank was machined in a lathe, so the cutting tool removed any material that passed by. If you can do the same job on the counterweights without machining material from the crankpin area, then go for it as the piston is a long way from this area when the crankpin is at the top part of the crankcase & material removal is not required.

Unfortunately, when the counterweight is uppermost, the piston is at the lowest part of the bore & there is an interference fit between the bottom of the piston & the crank.

This machining job overcame that problem.

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.

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.

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?

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

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

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