Setting deck height for 9:1 compression Options

[Chad911sc]

I have my short block built and I’m ready to set my deck height. It’s 96mm bore with stock 71mm factory crank. This is a 2.0 2056 build. I have a Web 86a cam calling for approx 9:1 compression. I have 60cc heads with 3cc valve reliefs on my flat top pistons. If I plug all this into the calculator, I get 8.3:1 for my compression ratio if I set the deck height to .040 inch
After I set up the piston on the rod and bolt down the cylinder to the block, I get .053 inch from the piston to the top of the cylinder.
My question is, what is the best way to proceed to get the 9:1 ratio with at least .040 inch total deck height.
I am thinking that if I take off 6 total cc’s from the head, that will leave me with 54cc heads with the 3cc valve pocket = total 57cc’s. Bringing me now to the correct 9:1 compression ratio with the .040 inch deck height.

If I am in the right ballpark, this means I need to have my heads fly cut….correct??
If this is correct, how do I go about calculating how many thousands of an inch do I need to have removed by the machine shop to remove 6cc from the head?

I obviously will need to remove at least .013 inch total deck height get to my .040 goal, and that’s with no shims or head gaskets being used.

Thanks for your time, Chad

[Jack Standz]

So I have the case halves back apart and now just need to decide where I’m going to ship it off to. Going to ask them to take .015 thousands off the deck. This should get me into the ballpark of where I need to be. I still need to address the head volume. With all the calculations for compression ratio, I need my head volume to be 57cc instead of the now 63cc that I have with the valve relieved pistons and pocket added together. Along with a .040 total deck height this will put me at the desired 9:1 ration I need for my cam.

I always try to add pictures. This site rejects my posts about 80% of the time. I only try to add one or two pics, but it hardly ever accepts them. After trying 15 times I get pisses and just forget it…lol.

Gonna try one more time on this. Raby says that generally it takes 30 thousandths to clean up the deck on most 1.7/1.8 liter cases, more for 2.0 liter cases. Over the years, most people have come to believe he knows what he's talking about. Not sure why you think your case is so special that you will get away with half that amount? Especially without measuring before and after the machine work. What makes you think your case needs only 15 thousandths removed? Maybe one side needs 30 thousandths and the other side needs 40? Or only 20? How far off parallel does your deck measurements show? Side 1, side2?

Why not have a good and competent machinist cleanup the decks on your case (which, generally will take 30 thousandths or more according to Raby) by removing whatever amount of material necessary to achieve a flat and parallel deck?

After this is done, you will probably need to add a spacer between the case and cylinders to achieve your optimal deck height. Then, after measuring and verifying the machine work, add the right size spacers. Then get the combustion chamber volume right for your desired static compression ratio.

Or, get the case machined, get the heads machined to reduce combustion chamber volume. Then order the spacers based on careful measurement and verifying the machine work, unless somehow the machine work magically comes out to be the perfect combination to meet your CR desires.

Not that I believe a particular CR is all that critical for a street motor (but too high a CR can be a problem). Does 8.8, 9.0, 9.1 mean it's all that problematic? But, good machining is very important, especially for long term performance?