Sniffing around the web I found a couple of candidates for A15 flat top pistons, the most promising looking like the HD engine pistons. Same diameter, comp height, and pin diameter. (http://www.totopiston.co.jp/cat/piston/pist-da.html).

If anyone's smarter with web stuff than me I'm happy to provide the excel doc I made to filter the catalogs for the piston size. Handy tool. Would be even cooler if anyone wants to add to it....

This is the shape of the dome on this piston according to the website you linked to:




There may be a couple of issues with this piston.

It's not a 'true' flat top - it has a dome.

You could machine it dead flat - but then you'd have to worry about the crown thickness (it might still be ok, ideally you'd want to purchase 1 piston and cut through it with a band saw to see if it will still have enough thickness after machining to be safe).

If you wanted 'really' high compression, you could machine the dome to match the chamber shape, and deck the block etc, so that the piston 'dome' section sat above the engine block deck height.

If it does have to be machined dead flat, well the crown height on the listing is potentially measured to the top of the dome, not the outer edge (I don't know for sure, it might not be). If that is the case, then once it is machined, it'd end up sitting a mm or more down in the bore.

Of course you could machine the block to get zero deck height, or offset grind the crank and get a bit more stroke, and hence raise the piston a little at TDC (you'd probably only get it about 1mm higher absolute max, and even that might require the crank journals be welded up and re-ground to have enough meat for that much increase in stroke.

Alternatively you could look at slightly longer rods. (not cheap of course).

I used ga16de pistons in my a15 ,they have 9cc of dish and 29.5mm pin height

I had my block decked 30 thou and with a 24cc head and .6mm gasket I have 11.6 to 1 comp

Jmac`s sketch of the modded for wedge Hicomp
would be an awesome setup very worth it as
Daihatsu pistons are super tough & quality.

Id love to find out which bike pistons go
close to 77mm as their comp heights usually
range from 25mm to 27mm allowing much longer
conrods in stroked engines.

std Triumph Speed Triple 1050 pistons are 79mm, I have my old manual somewhere that I will try find the rest of the specs from..

Deano you thinking of boost or NA?
I would prefer no bigger than 77mm for boost.

Am a NA man..

Have had my time in boosted cars and heaps of fun but just love a crisp revving NA motor..

No idea if the triple pistons are suited, just remembered they were 79mm, no idea on pin dia and height..

How do you read the +0.5? To me that says the dome is 0.5mm because the pistons with dishes have a -ve there, I'm guessing though. Some photos of the actual piston would be good.

Edit: I found a pic here http://daiforum.net/viewtopic.php?f=6&t=176, seems to agree with my guess.

how about the gemini pistons they are almost flat tops and
some available in 28.2mm com height

An old thread I know, but I was thinking about it again. I reckon you could use the Daihatsu HD-E piston without modification, if the crown is 0.5mm above the deck it's still within the gasket height. Should get some good compression, I calculate 12 and a bit with a stock head and gasket.