Can anyone tell me if 2mm (0.080") is too much to deck the block? I'm thinking of taking 80 off the deck and 75 off the pistons. This will make the pistons almost flat with 7 thou deck/piston clearance. By my calculation the compression will be right at 9.0.

So if you guys think 2mm is no prob I'll give the shop the go-ahead. Thanks for the help -
Michael

I've looked for several weeks nothing has come up that I can afford. No one makes standard cast pistons, nissan can't get them and so my only alternative is forged pistons which I really can't afford (cost about 4x these cast ones). It's come down to time and money and I'm running out of both.

So hopefully I'll be able to get the block decked. If there's not enough clearance then they'll probably have to deck the block AND cut the pistons back. This is just getting worse and worse. I should know by the end of the week. Thanks for the advice though. I see what you're saying about how you gain more cutting the block than the head.

Michael

Hey A14force,
By my calculation the compression will drop somewhere between 8.22 to 8.1 depending on the piston/block top clearance (haven't measured it yet). To get it back to stock which is 8.5 it would have to be cut around 1mm - to make up for the 1.2mm deeper dish in the pistons. If there is enough clearance I may want to cut a little more to raise the compression up to around 9.0. To get to 9 I'd probably have to remove as much as 1.5mm. I can't be sure until the machine shop measures the clearance between the piston top and the deck.

The motor is not assembled right now. It is completely torn apart undergoing a rebuild. That's why I have different pistons.

Michael

Here's a related thread from a few days ago: click

OK well I'm stuck with some lower compression pistons on my engine rebuilt so I have 2 options for raising compression: either shave the head or deck the block.

The rebuilder told me that on a pushrod engine it's better to deck the block than to shave the head because cutting the head messes up the geometry of the pushrod action or something. I can't see how there would be any difference between the two but maybe I'm missing something.

So can anyone tell me which way is preferred? (decking the block is $60 more than shaving the head by the way, so they the both come out with the same result I'd rather do the head...)

Thanks,
Michael

Thanks L18 but I've already got the pistons and am running out of time and money to keep returning new parts. I wish I'd known this a couple of weeks ago!!!

Here's my compression ratio calculation. I just took a guess on the space between the top of the piston and top of the block at 0.5cm.


Original Pistons:
7.6 x 7.7cm => 349.3cc displacement per cylinder

Compression Ratio = 8.5
displacement volume + combustion chamber volume
= 8.5 x combustion chamber volume

dv + ccv = 8.5ccv
dv = 8.5ccv - ccv = 7.5ccv
ccv = dv/7.5 = 46.6cc per cylinder

original combustion chamber volume = 46.6cc

New Pistons:
7.7 x 7.7cm = 358.5cc displacement per cylinder
dish is approx 5.7cm across, .12cm deeper than stock
added combustion chamber volume due to dish depth is
3.06cc

Added bored area due to oversize = 1.21cm^2 (need to know
space between top of piston and top of block to calculate
added volume due to this area difference)
WAG of 0.5cm space so bore volumem above piston = 0.61cc

so new ccv (combustion chamber volume) is
original ccv + deeper dish volume + added bore volume above piston

46.6cc + 3.06cc + 0.61cc = 50.27cc

so new Compression Ratio = (50.27 + 358.5)/50.27
= 8.13

Ignoring that little volume (0.61cc) due to the space between the top of the piston and the block it would be around 8.22

That is way too low - I'd rather be closer to 9:1. Crap.

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

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 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.

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.

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