On today's USA market there are few choices in cast stock-type pistons for the A15. I see Sealed Power 12182P (used to be federal, used to be TRW) and something from ITM. I am familiar with Sealed power and it's a pretty good product. I am not familiar with ITM, are they a re-brander?

I hope to make at least 100HP at the flywheel with this build. I will be using the car on the road with mostly cruising but some occasional bursts for passing or for the hell of it. 7000 RPMs max, I guess. MG Midget, about 1500 lbs (690kg). Would either of these brands work or should I really be looking at cu$tom slugs? I'm really not at all familiar with the longevity of cast pistons in a "fast road" A series build. If this were a Ford engine like a 302 I would have no reservations at all at using Sealed Power. Don't know about ITM.

Opinions? Experiences?

Oh, H89 head. Either 1980 or 1982 dish style would be OK. I'll run it close to 10:1 CR for use with our 87 or 89 octane gas (RON x MON/2 is how they rate it in the States).

100 hp is nothing for the stock Nissan pistons. No need for customs. The stockers can do 8500 RPM.

Getting 10:1 will be difficult with the large chambers of the H89. The best thing to do is get flat-top pistons -- they will take advantage of the quench too. Milling the head a large amount will raise compression, but the dish pistons won't have a good quench.

Most guy use large overbore 79mm pistons. No only will flat-tops give more power but the extra 100cc gives a bigger power boost than the compression ratio change.

Just staying with A15 parts for the moment, here is what I have observed.

The factory CR rating for the 1982 A15 (the one with the H95 shroud-port head) is 8.9:1.

(Please check my numbers. I am here to learn)

Lets assume for the moment that 8.9:1 is correct. The chamber of the H95 is ~35ccs from my measurement. I measured the H89 chamber at ~31ccs. So already that's about a .5 increase in CR. Surface the head and the block (cleanup during rebuild) to remove a little more from the deck clearance and the chamber and add a .5mm or .75mm overbore (whatever piston I find) and I would be closing in on 10:1 but not over. That would be where I would like to be for use with non-premium US gas. But...

I must consider the piston dish. I measured the H95 piston offset dish at ~6ccs. I do not know what the larger diameter, shallower and centered 1980 A15 dish measures at. Does anyone know?

Discussion reference slugs:
A15 Deep Dish pistons on Ebay
A15 Shallow Dish pistons on Ebay

Edited to add: The ITM catalog PDF, page 53, states that the CR of both A15s is 8.5:1. I dunno. Just gotta measure.

The A14/A15 heads are 31cc
The GX heads are 29cc

The 1980 80HP piston reportedly has larger CC dish than the H95 piston. Unfortunately I don't know the exact number.

Yes, if you are going to mill the block deck, then you can get any compression you want. You can also mill the piston tops too. Even with the same compression ratio, dished pistons will make less power than flat-tops. The H95 dishes are good though since they are a quench design.

From what I understand, a good quench is worth more HP than 0.5 compression increase.

By the way, in the US market, there are many choices for cast pistons. Nissan alone sells 3 different A15 pistons, plus overbore sizes.
12010-H9704 B310.FED.1979
12010-H9714 B310.(CAL.1979-1980 + FED.1980 + Canada.All)
12010-H9802 B310.USA.1981-1982 + N10.All