I'm in the middle of a rebuild of my A14 (still) and I've come across a problem. My original pistons have less than 1mm dish (60mm diameter dish approx). The most shallow dish I've been able to findin replacement pistons is 2.2mm, and they go up to 4.0mm. They are supposed to compensate for the dish depth by changing the pin height but the one set I've bought and returned had the same pin height (3 mm or so dish). So that would basically have resulted in a lower compression which I don't want.

So right now I've ordered the best I can find which is 2.2mm dish and the only overbore I can get is 1mm. Going 1mm over is pretty much the limit on the A14 right (meaning I'll never be able to overbore again)? Anyone know of any alternatives besides horribly expensive forged pistons? And at the same time can anyone explain the many dish depths available? They all say 8.5CR in the catalogs but there's no way the CR stays constant when the pin height doesn't change. Did the combustion chambers change a lot during the '79 model year and beyond? I'm confused.

And if I do go to custom pistons as a last resort could anyone tell me how high the CR would be if I go from ~1mm dish to flat pistons? Could I get by with mid grade gas (89 here in Dallas)? I have a feeling this is probably too complicated to answer right off the bat but hey it's worth a shot. I guess really if I do have to go with custom pistons a lot of dirty work will be involved in the design.

Is there a nice CR vs. Octane chart somewhere to use as a guide? I've looked all over the internet. (stock is 8.5CR, uses 87 octane fine here in TX, fairly low elevation).

Thanks for any and all advice!!!
michael

If the diferent dish pistons are all supposed to be the same compression ratio, then i assume that the original combustion chambers for these particular pistons must have been of diferent volumes. It's possible that your shallow dish pistons went under a combustion chamber with a larger volume.

The factory listed pistons in three standard oversizes, .5mm, 1mm, & 1.5mm. The cylinder block should cope with up to 3mm of bore size increase without difficulty.

Have you tried ordering "genuine" pistons from your Datsun /Nissan dealer?

Hey Dodgeman,
I went to Nissan yesterday. They do have a .5 and 1mm oversize part number but until they order it I can't be sure the dish will be right. Also they're $350 for a set compared to the ~$150 I'll pay for aftermarket ones. But if they work and the aftermarket ones don't (which I'm not sure yet, I've got another set on the way to check out) then I may have to shell out that extra $$. I also looked at the Nissan Motorsports catalog and they only have pistons for A12/A13 applications. I looked at a Nismo catalog too but I couldn't understand it since it's in Japanese. It had a section that looked like it could be for pistons and it had A14/A15 listed but there was no way I could figure out what it was for or the specs.

If none of these work I guess I'll have to shave the head because there's no way I can afford forged pistons right now.

If the pistons that you ordered from Nissan are for your exact model, then [in theory] they should have the same dish, but sometimes they "supercede" some part numbers to reduce the variety of some parts, so you will need to take your sample with you when you go to pick the genuine ones up.
If they are not the same, then you can obvoiusly buy the wrong ones for a lot less & should decline to purchase deep dish ones at the "genuine" price.
If they ARE the right ones, then just grin & bear it. The nasty gash in your wallet will eventually heal, the painfull memory will fade, & you will be glad that you have the right part.

Is it possible to have a look at a variety of diferent A14/15 heads? If you can find a head with a smaller chamber, then cheaper pistons & a replacement "smaller chamber" head may even be the way to go.

I paid the Aussie equivalant [roughly] of $150 US for my non-genuine A15 pistons. Fortunately for me, the dish volume was the same as genuine ones.

The Nissan parts are not available unfortunately. Also the only aftermarket manufacturer I could find doesn't actually produce them anymore. Looks like pretty soon the only thing available is going to be $600 custom sets (unless I've missed something). Can't wait till I get to my next rebuild!
Michael.

Overbore + same dish in piston = more compression

The aftermarket pistons you are looking at may have a deeper dish to keep the compression to stock spec. I suppose you could calculate the difference.

If you're doing a rebuild with performance in mind, it would be a good move to cc the chambers and the deck.

When you say cc the chambers and deck do you mean measure the volumes? I woudl like to have all of that speced out so I'll know what my comp ratio is really going to be. I'll if the shop will do it for me. I need to find a compression ratio calculator. Do cam specs come into play on the compression ratio? Thanks for the help -
Michael

The compression ratio is just a numeric value that represents the ratio of the whole cylinder, including the combustion chamber, when the piston is at the bottom of its stroke, when compared to the volume of the combustion chamber when the piston is at the top. Any volume below the head gasket, like piston dish, is included as part of combustion chamber volume & so is the volume that is encompased by the head gasket.

The cam timing has no bearing on this arbitary compression ratio number, but it doesn't take Einstein to realise that the cam timing has a very serious effect on actual compression 'pressure'. A wild cammed engine may have 12 to 1 compression ratio, but if the inlet valve is still open with the piston part way up the bore, then at low speeds it is pushing the inlet charge back out through the carbs. If the valve actually closes at, well,... say,.. half way up the bore, then the amount of gas actually being compressed is only half as much as the mathematical volume of the cylinder. Under this imaginary scenario, the actual compression ratio, as seen by the combustion chamber, is quite a bit less than the theoretical value.
The same goes for unmodified engines at idle. The valve may well close at a more sensible time, but the volume of gas actually in the cylinder is quite small. It would have been at a lot less than atmospheric pressure when the valve closed, so the piston may well travel a fair way up the bore before cylinder pressure rises to be the same as the atmosphere outside the cylinder. It is only at this point that compressing of the fuel charge actually takes place.

The engine actually sees the full compression ratio as 'maximum cylinder pressure' when it is developing maximum torque, so in a way, all I.C. engines are of the variable compression ratio type.

Thanks Dodgeman. Good info. I'm attempting to calculate my CR myself (I'll post it here when I done to see if it makes sense to anyone). But to finish I need to know how high in the cylinder the piston goes, meaning the space between the top of the piston and the top of the block when the piston is all the way at the top. Does anyone know that for an A14, or even approximate value would be ok. Is it around 1mm or closer to 1cm? Thanks
michael

I've used Mitsubishi 4G63 (i'm pretty sure) pistons in an A14 before. They had the same pin diametre, slightly higher compression height, and a slight dish, which when milled back to a flat top is spot on for an A14 flat top piston. If that engine is available inthe US, this may be an option, but it is important to calculate CR before buying a piston. I'll try to check tomorrow if this was the piston I used.