looks so thin is it really worth it? even 1605 is pushing it.

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I went to the nissanforum site, found your thread & read the whole 15 pages. Quite a story & I loved it.

Nowhere in the whole story did I find any of the Justy pistons that were bigger than 79mm, & it looks like it was necessary to deck the block to use them. This gives a displacement of 1608cc
To get a displacement of 1675cc requires a bore size of 80.65mm which is another 1.65mm or .066 thou. bigger again.

Mazda pistons seem to be the better choice out to 79mm as no block decking is required.
Once you start boring to 79mm you end up with a perilously thin sealing surface between the end pairs of bores. This can be helped with offset boring, but there is only so far that you can go before one side of the cylinder wall starts geting a bit thin.

These A series engines were originally designed back in 1965 with a bore of 73mm, so there was plenty of meat between the bores back then, then the factory reduced this surface by 3mm when they went to 76mm bores, & there was no problem, but reducing this sealing surface by 7.65mm when an 80.65mm bore is used is pushing ones luck a bit. Even at a 79mm bore size it becomes necessary to use a figure 8 'O' ring when this mod is performed to improve gasket sealing.

Take it out to 80.65mm & the problems get worse. I am not yet aware of a piston of that size that could be used either, but look foward to learning of it.

The photo is of an A15 with 79mm bores that are centered [not offset bored] & as you can see, it gets a little bit tight in there.
If only the factory had spaced the bores just a leeeetle bit further apart.

it uses 4 subaru justy pistons and bores the block out some. Search the internet some. my page at nissanforums.com has several posts from someone who has done it and has links. sorry that the thread is 16 pages long.

scroll down a ways and there's an explanation of how to do it.

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What kind of stuff has caught you out and been more expensive than you thought (or just never considered when you started)?

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I'm not sure where you get your info from, but to get 1675cc from an A15 [76mm bore x 82mm stroke - 1488cc], according to my calculator, you would need a bore size of 80.65mm for a displacement of 1675.6cc
If you can let us know what pistons can be used, & how to bore an A15 block this far without problems, then you will have a good many members singing your praises, because no one else seems to have managed this trick so far.

the NA SR20 is not a cheap conversion (mine has cost heaps so far).
I would go the A14/15 with a nice carby and exhaust. then spend your money on suspension and braking. I was happy with my A14 but wanted to give my ute a good birthday present and give it another 28 yrs of life.

If 2k is about right for an a15 conversion, that is what i will do. I could always bore that out to 1675cc later on aswell and add a few goodies..

A good A15 goes plenty fast for someone who is learning. It's plenty fast for those of us who know how to drive well too.

If i wanted something that made me grin when i stepped on the gas it would be an A15 turbo.

when you learn to be fast with a little engine you'll be really fast.

a15 is the cheapest and most cost effective... get it to handle and brake and for around 2k you will be happy.

Have you considered a NA SR20?
No turbo, but more cubes & heaps of potential to massage for more HP later.