Have discovered a .004 deck height difference between clys 1,2 (both .041") and 3,4 (both .045).

All cylinders length within .0005 so no variance there.

All registers are clean and flat. They were not machined. No spacers used.

GA case stock 2l crank and rods.

New AA 96 mm Biral cylinders and pistons. Pistons have 2cc valve relief. Web cam 086 cam.

Searches reveal variances are common from one bank to another.

The .041 on 1,2 gives me 8.5 CR which is what I want.

The .045 on 3,4 gives a CR of 8.4.

All head chambers identical cc volume.

So should I just go with it, or I could have the bases of cyls 3,4 machined off the .004" which would equalize each side.

I'm not at this point going to disassemble to cleanup case registers 3,4.

So comments as to best approach.

Thanks jjs3rd914

Before you go crazy, have you measured the volume of your heads CC at the same precission?
I bet the variance is bigger there.
You should be fine, even more that the 3,4 side gets less cooling so a bit less compression can even thing up.

BTW, my calculations show that 0.005 difference results in 8.5 vs 8.485 compression but I'm before my morning coffee.

You have a magical 45 year old case that has not been machined, yet the case registers are all flat?
Cool!

I am confused, first you say the case registers are all perfect, then you say you will be finishing up disassembly to clean 3,4.

It’s really good insurance to send the case out to be milled flat, they are not flat.

Normal.
They will never be dead nuts on.
T1, T4, 911, 996, all engines, they always have slight variances, not an issue.

It's like balancing rods end for end, why there's a 1g tolerance, try to get then any closer and it becomes an endless game of chasing your tail.

I do agree on the registers they must be flat.

Thanks all

Valy points out the calculators do show a really minor difference in CR if the deck height changes by .005" as compared to even a 1cc difference in chamber volume.

Also the comment on 3,4 getting less cooling is interesting as it relates to the issue which seems to play to my advantage.

Interesting as well is my 1,2 head (new AA 2L castings with 44,36 valves) seems to have just a bit more chamber volume than the 3,4 head for what ever reason. So this too turns out to also be in my advantage it seems. The fraction of the cc should somewhat balance the difference in the deck heights.

Yes i measured the cc as best as i can using the standard techniques with Plexiglas disk and cc syringe.

Thanks again, we believe we are going to continue with what we have and "not go crazy" trying to get everything exact when the bottom line is the accuracy (tolerance) of what gets measured (CCs).

Just want a bit more power than the stock 1.7L engine in the car now (a 1972 model with with working AC). Not looking for a killer motor. Plus its a fun project.

jjs3rd914