And this is exactly what I don't want. I'm trying to understand this whole thing, but I don't want people telling me "just go do this." That doesn't help me learn anything.

And I'm not afraid to bore it. After reading up on this on the various Miata boards, the consensus is that it is unnecessary to do so on a Miata when there is no extreme power goal. Also, no point in weakening the block if I don't have to. I don't think a bore up to .020 over would do any harm really, but I just don't want to do it.

Yes, pistons come with a spec sheet. If the bores on your engine are "perfect" (I highly doubt that's the case), the machinist can hone your bore to meet the specs of stock sized forged piston. IMO the best thing to do for a "necessary" overbore with boosted engine is to go with the smallest overbore, focusing on cylinder wall thickness for boost strength.

Guys .020 or .060 from the cylinder walls is NOT Gonna hurt the cylinder walls. It's a thick ass cast iron clylinder, you will never hurt it. It's the strongest thing in your motor. It's more displacement, which our undersquare engines can use. By increasing the bore .060, you could unshroud the combustion chambers that .060 as well, which helps flow. More flow and more displacement is...... desirable. Do the math but that's surely a few % more displacement. Boring it the minimum is ok so it can be rebuilt again. They do this for commercial equipment, where they would like to get several rebuilds out of the engine for maximum service life.

Sorry Rotornut but reread my post. You were asking stuff I outright already answered.

Ok, using the factory specs of 3.2678" bore and 3.346" stroke, I get 112.25 cubic inches of displacement.

Let's punch this motor .020 over.
3.2878" bore and 3.346" stroke= 113.63 cubic inches (.000 to .020 displacement changed 1.378 inches)
1.23% more displacement then stock.
1.43 cubic inches larger than stock.


Now let's punch it .040 over
3.3078" bore and 3.346" stroke=115.01 cubic inches (.020 to .040 displacement changed 1.385 inches)
2.46% more displacement then stock.
2.76 cubic inches larger than stock


.060 over
3.3278" bore and 3.346" stroke= 116.35 cubic inches (.040 to .060 displacement changed 1.400)
3.65% more displacement then stock.
4.1 cubic inches larger than stock

Though insignificant, as bore size increases, area increases exponentially. This can be seen as the change in displacement is not the same for each .020 iteration. For example, look at the change in displacement for .020 and .040. The difference between them is 1.385-1.378=.007. However, this is not the case for .040 to .060. Here, the change is .015. Hence it's exponential by a factor of 2. This is because the formula for area is pie times radius squared times height.

An extra 4.1 cubic inches is 3.65% more displacement, or 3.65% more power. However, it will also allow you to unshroud the combustion chambers .060, which will increase flow. I would say the benefits of unshrouding will be greater than the benefit of the larger bore, but at the minimum, equal. If only equal, that's 7.3% increase in power.

That's my inexperience showing. Night night.

Alright, so since .020 over is not that much of a change, does it affect what head gasket I need to use? Or can I still use the OEM gasket?

And if the sizes of the pistons are listed as 83.5mm, 84mm, and 84.5mm, what overbore are those?

You can fit up to an 84.5mm bore on the stock gasket. As far as the bore sizes, here's the list.

83.5 = .020 over
84 = .040 over
84.5 = .060 over

Anyone know a piston mfg that has an off the shelf 9.5:1 compression ration piston in the 84.5mm size?

none of them

Thank you for that info. I assumed that's what it was, but wasn't 100% sure.

I guess it was wishful thinking! I guess I will have to do some more homework then.

have them custom made mr. high roller

out of curiosity, why do you want 9.5 CR?

I actually am de msming an msm. The stock CR on the car is a 9.5:1, so if I am gonna touch the current pistons, I want an apples to apples switch, otherwise I will just pull the current ones and coat or cryo them. FYI, I have managed to locate a 9.5:1 in an 84mm bore that is a couple weeks out if anyone else is interested. At a non Mr. High Roller price nonetheless!

On Raceeng.com, they list a 83.5mm Wiseco piston and list that it's for turbo applications, however, they don't list the C/R. Anyone that has dealt with this company know what it is?

Belfab only lists either a 8.6:1 or 9.5:1 at 83.5mm for their forged pistons. Anyone know if they can get a 9.0:1 for the same price? ($439) I called, but I was told they are at the Winston Cup race and to call back Monday, which I'll do, but if anyone knows now...

Also, any other places that currently sell forged pistons for a reasonable price? (Or will be in the near future?)

Have you found a place that sells reasonable bearing/gasket sets?

FM has a kit, but it's over $500.

You can get a quote from MiataRoadster.com or Rosenthal (www.finishlineperformance.com) has some sets on their site. You can also call them directly and get set prices. They're pretty flexible.

they can't get any other compression ratios. If you're worried about off-boost performance, then go with a 1mm overbore, and just roll the timing forward off boost more. I have the 8.6 pistons, and after you surface the head you'll gain 1-2 tenths in compression.

Here is my version of a MILD build

ETD/M-Tuned Connecting Rods
Stock Pistons (94-97)
New set of OEM Rings
OEM Bearing
ARP Main Bolts
ARP Head Bolts

I've personally tuned this combo in the 330+hp range and it works great!

I'm going to build the same for myself, but I plan on using the 99+ Pistons for a little more compression :)