No I wouldn't.

The dish in the A15 pistons is eccentric in in placement to the centre of the head, smaller in diameter but considerably deeper.

The A14 dish is concentric and much shallower but also larger in diameter.

My suggestion would be (with a budget in mind) to deck the block and the A14 pistons (to remove the dish) to bring the comp ratio up, this procedure would still be cheaper than new pistons I think. But you would influence the rocker ratio slightly and also affect the finish level of the head bolts.

The bolts can simply be spaced with an extra washer.

The affected rocker ratio would work in your favour increasing the valve lift slightly but this modification will have to be carefully partnered with the camshaft with the correct specs. You may also need to introduce fly cuts or eyebrows/valve recesses in the piston crown to prevent the two colliding. Fly-cuts also influence volume hence compression ratio.

Otherwise you could make up some little spacers that sit under the rocker shaft pedestal, the thickness of these would be determined by the amount you have machined off the block face, their introduced height would bring the rocker ratio back to stock. This would be a cheaper mod as well saving you having to address cam and piston crowns.

Other option again is to do custom pushrods to retain the original rocker ratio.

Obviously you'd need to do your own research to see if it is feasible, I think the last time I looked into it it was looking pretty good but of course what would I need another engine for?!

EDIT: Sorry I shouldn't have used the rocker ratio as a term. I mean the distance the rocker arm travels is still the same but it is starting and ending in a different position compared to stock because the distance from the rocker assembly to cam have changed. The rocker ratio isn't affected. Maybe I've confused myself.

Happen to know what the differnce is dish size is? I'd also thought about using A14 pistons with a well chosen decompression plate to select piston/ head clearance (keeping in mind the 2.5mm extra compression height). Sound whacky?

Yeah don't forget there are other (comparatively cheaper) ways to achieve higher comp, like flat top valves (diesel), skinny head gaskets and head machining. The off the shelf head gaskets range from 1mm to 1.15. So if you get a hold of a 0.6 or the like you're doing well in increasing compression ratio.

When I was doing those copper head gaskets the 0.7mm thickness were on the cards, but I couldn't get enough numbers together.

Other wise go a A12 with A10 flat-tops which are available in O/S. Or stroked A12 with A10 flat-tops, now that is a budget build with good power.

The A15 had the big dish pistons didn't it? I have a few used sets of the the shallow dished A14 ones that will fit. If you are interested.

Chris

What sort of Mazda pistons? (if you don't mind sharing)

I'm thinking about piston swaps that would bump up an A15 compression ratio up, the thread got me thinking about it. Wouldn't it be sweet to find a standard flat top piston to swap in, preferably without resizing things.

Probably just getting carried away, too many other things to do before building hi comp motors.

I have used Mazda pistons in my race engines for the last nearly 10 yrs.

They have to have about 2.5mm machined off the top.

Haven't had any problems - to date.

Sorry I haven't read the entire thread yet but what are trying to achieve Dylan?

Seems to me there'd be quite a few motorbike pistons around the 76mm size but I can't find a good catalogue like the ACL one I found for car pistons that shows compression height, profile, oversizes etc.

Any hints? This has got me curious.

What about GA16de pistons? Anyone know the dish volume. They're 76mm diameter with a comp height of 29.1mm and a pin size of 19mm. That'd put it 0.9mm down the hole.

For me I'm just thinking aloud and I only ask because you guys are such an oricle of Datsun knowledge.

The inspiration was reading about e85 and the idea that with this fuel or water injection you can dramatically increase the compression ratio. Made me wonder how feasible it would be with a piston swap.

Cheers