Need knowledge of 1.6 Miata; Milling 2mm of top of piston

Looking for someone who has intimate knowledge of Miata pistons. It appears there is enough meat to mill a couple of millimeters off the top of the piston for my application.

Background:

In order to create a big bore Datsun A-series engine people commonly use Mazda UC and or B6 pistons. These come in 77mm and 78mm sizes versus Datsun 76mm. The Datsun A15 engine has a 30mm pin height versus Mazda 32mm. (A14 is 32.5mm so people mill the block .5 for Mazda Pistons)  Several racers have milled UC pistons with no adverse results but only street guys have milled the Miata pistons.

Application:

The A15 in my  race car uses custom  76.5mm JE Pistons. I have a spare A15 street engine that I'm looking to build up, Miata pistons are available at 78.75mm which would take the motor from 1508cc to 1598cc. They are also much cheaper than the $560 for another set of custom pistons. A friend can mill the pistons so no cost.

Any feed back would be appreciated.

2mm seems like a lot to me, but I don't have any real knowledge of such things.

So you're looking at making the dish on one of these (what comes up for a 1990 MX-5) more shallow? 

I'm no engine builder but I'm guessing it wouldn't weaken it any appreciable amount unless you need to take enough off that you end up with a flat top and then keep going. Seeing the backside straight in with the pin removed would be helpful, but my hunch is the metal there is plenty thick.

The thickness of the piston dome is one consideration.

Also the amount of piston above the top ring affects the rings operating temperature and the pistons ability to maintain ring land integrity.

bentwrench said:

Also the height of the top ring affects it's operating temperature and the pistons ability to maintain ring land integrity.

Good point, running the ring gaps on the wide end of what's suggested (or maybe even go forced induction wide just to be sure) would be wise.

Basically I was indeed looking to make flat top pistons  from these but as pointed out my concern would be the distance to the ring land. The unmodified piston would need to be 12-13 thick from the crown to the top of the ring land, otherwise I think it would start to become dicey.

Don’t the pistons from automatic 1.6 miatas have a lower compression ratio and perhaps need less machining?

I'd drill a hole in a candidate piston and see how thick the middle of the crown is.  If may not be very thick.

I've ordered a piston from rock-auto so we can play with it. The compression ratio with flat tops is 11.5 with the larger bore it would creep up to 11.8 

This is for my second / back up motor hence my not wanting to pony up $$$ for another set of forged pistons. We are trying to play with cranks as well,  Nissan E16 cranks drop in the A-Series blocks, combined with the 78.5 mm piston it takes the motor out to 1700cc 

Any chance of doing something mildly stupid like only milling .050 or so off and double stacking the head gaskets?

That is a possibility; current gasket has a crush thickness of 1.2mm but there are .6 & .8. The compression ratio even with a .6mm would only be 12-1 and the current piston to valve clearance is 2mm so milling the piston less could work.

Matthew Kennedy said:

I'd drill a hole in a candidate piston and see how thick the middle of the crown is.  If may not be very thick.

Or even run one through a bandsaw once or twice.

If it's the pistons in the pic you won't have any problems. I do this all the time for another application.

I've done hundreds of them and at 12 or 13-1 and 9000 +/- RPM we have never had a failure.

In reply to white_fly :

Yeah, cutting one in half is probably a better idea.