ok i have an A15, head is off and im not sure if its been previously milled or not. so how do i find out the CR so i know how much to get it milled to achieve atleast a 10:1?

To check the ratio:
* Measure piston dish
* Measure combustion chamber
* Measure head gasket
Add those three together and divide into 1488 cc

See our tech article: CCing

Sorry this is incorrect.

The correct process is as follows;

Firstly you need to calculate the total combustion volume by adding together the following;
the combustion chamber volume,
+ the volume displaced by the head gasket (Pi (3.141592654) x half the diameter of the head gasket firing ring squared x crushed thickness of head gasket),
+ the dish of the piston crown ( or 0 if it is a flat top piston or minus if it is a domed piston crown)
+ how far the piston sits below (or minus if above) the deck height when at the top of their stroke.

This gives you a calculated total combustion chamber volume.

Next divide the engine capacity by 4 to determine the swept volume of 1 cylinder.

Add to this swept volume of 1 cylinder the calculated total combustion chamber volume.

Divide this by the calculated total combustion chamber volume and the result is the compression ratio.

thanks yes, incorrect. Should have said divide in 1488/4 (372 cc). And that is only if the A15 bore is standard (76mm). Stock pistons have approximately 0 deck height even overbore sizes but as always should be measured

for example:
* 10cc piston dish
* 30cc combustion chamber
* 3cc head gasket
all that = 43cc
372/43 = 8.7

The head gasket can make a big difference. Stock 1.6 mm gasket vs Nismo 0.8 mm gasket vs GX 0.6 mm can make more than 1/2 compression point. Thin gasket is desirable for proper Quench

Use the Compression Ratio Calculator to double-check

Ok so my pistons are the standard dished ones and bore is standard. So should I still measure the piston dish cc or is it 10? If I do need to measure the piston dish how do I do that? Sorry for all the q's. Still learning.

Also what' thickness head gasket should I go for?

you need to measure. There are at least four different A15 piston dishes, all different

To measure the dish, see our tech article: CCing

Ok great. So the same way as you would do the combustion chamber. Thanks for all the info guys. Think I got it now.

So what thickness head gasket do you think will be best?

* The GX or Nismo thicknesses (0.6 and 0.8 mm) are Multi-Layer Steel (MLS) gaskets
* The standard gaskets (1.6 mm) are composite and have various crushed thicknesses, you can measure the used gasket to see what it's thickness was and use as an estimate. These gaskets cannot be reused, but are inexpensive and are adequate. The A15 offset-deep dish is supposed to be better than the full shallow dish

Best head gasket may be one that puts the piston 0.50 to 0.75 mm from the cylinder head face for best power. Tight clearance means more power but any closer runs the risk of touching/breaking the piston at high RPMs. More than 0.75 and the quench effect disappears

10:1 with tight quench - more power
10:1 with loose quench - less power and potential engine run-on (dieseling problems)

Tight quench gains more power than 1/2 point raise in compression

Although dished pistons are inferior even if you up the compression ratio. Dished pistons reduce NOx pollution but also use more fuel and make less power. Even if you mill the head to raise the compression a dished piston still makes less power

When fitting a high-lift camshaft you'll want to measure the piston-to-valve clearance using clay but it shouldn't be a problem with dished pistons

Check this link

http://www.datsuncherry.ch/PDF/Rennteile75.pdf

The engine portion gives you compression ratios based on what combo you are using even down to how many CCs for the gap to the top ring. It's very useful, it also specs how much to mill the head.

On the subject of milling the head, I prefer to use varying thickness of head gasket. The issue with milling the head is it also brings the valves closer to the pistons as well as possibly necessitating shorter push rods.

Tom

Looking at my Pistons they are offset dished. Now these nismo and gx gaskets, where do you get them from. I had a quick look and bprojects has a nismo for $300. Surely they are easier to get and cheaper than that.