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

I don't see how this could be a factor. Either method is putting the rocker shaft centre line closer to the camshaft.
He may be thinkin of ohc?

If your motor is already assembled, then won't decking the block run the risk of contaminating the engine with iron?
However, if you do deck the block, to say zero. (or mabe a few thou past) Then the qench/squish will be tighter, and should (at least theoreticly) improve combustion chamber swirl, and reduce dotonation.
You may find that decking the block alone won't raise the CR as far as you would like anyway.
How low is your CR?

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

I know this isn't what you want to hear, but I'd be looking for different pistons. And if they were supplied by the rebuilder, I'd be looking for a credit on the unsuitable pistons they supplied.

or if you really want to use these pistons, you're better off decking the block than the head, because you won't have to machine as much off to get the desired increase in CR. Which in turn, will have less impact on valvetrain geometry. On the block you remove material from the full diametre of the bore, but the head's combustion chamber is rarely as open as that.

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

Saw these on e-bay. Looks like they're close to you. Listed as new standard A14 pistons & rings.
http://cgi.ebay.com/ebaymotors/ws/eBa ... 987431423&sspagename=WDVW
Good luck.

have you tried looking for suitable pistons that are not listed for an A14?

in the back of piston catalogues they have tables of piston data listed by bore size. Find a suitable piston with the right bore, pin dia, and comp height for your application. bear in mind you can also mill .040 to .060" off the top of most cast pistons safely. also remember to check pistons with 1 to 1.5 mm smaller bores than what you want, because their oversizes could be used.

To get the compression where you want it you could do as you have said plus weld up the chambers abit aswell to increase the compression. If you did both you would not have to deck as much either.

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

Anyone? I know lots of you have decked A series engines before. Any input would be appreciated.