[meborder]

let me try this from a different angle:


I'm not decking the block, so i'll have the 0.020 built in clearance. with the heads i've picked (68cc) my choices are flat top pistons with 6cc valve reliefs which with a 0.039 gasket gets me a SCR of 9.31 and a DCR of 7.59. lower than i would like.

alternatively, i could pick small domes which bumps the SCR to 10.34 and DCR to 8.41, which i'm not sure if that's too high.


In my mind, if the quench distance was good with flat top pistons, the 10.3:1 might be fine, but with the wider clearances I feel like it is probably a little too high.



That leaves me with the flat top pistons and milling the heads to reduce the chamber volume (or just accept it as an 87 octane build). And if I’m going to mill the heads to get the compression up, how far can I go?

or, i could possibly get the domed pistons and have some of the dome milled off in a lathe to get the compression down, but if i decide to do that - how much do i need to take off.


What I don’t think is a good idea, is to use the domes and a 0.060 thick gasket on top of the 0.020 built in clearance to get the compression under 10:1. To me that sounds like all the things that we learned not to do in the late 1970’s.

i really just need some kind of reasonable target, since i lack the real world experince of doing this a dozen times to know what doesn't work.

 

[meborder]

I found some 65cc heads. That gets me 9.6:1 static, which is better. Still a couple tenths lower than I wanted but better.
I’d build it at 10.3, but I’m afraid it would be just on the wrong side of the line.

 

[meborder]

You probably have .025 on your undecked stock block just add .015 gasket for a .040 quench flattops and some 68cc chamber heads should be good to go. 9:8-1 range depending on size of valve reliefs.

if you can find me a 0.015”gasket that is compatible with aluminum heads….

 

[meborder]

Fel pro 1094.

if it is compatible with aluminum heads, I’m all for it. That is better than the .041 I was going to go with.

Cometic C5245-023 is .023. Pretty close if you prefer MLS

i considered those, but they are very expensive

 

[meborder]

Running the math with those shim gaskets and assuming a 0.025 deck clearance, the compression ratio with flat top pistons (4-valve relief) gets me 10:1. Assuming a 0.020 deck clearance bumps me up a tenth.
With the tighter quench clearance, I’m good with that.

Are the 1094’s going to reliable for a street motor? I only ask because the description references racing applications.

 

[meborder]

I’m really liking the coated shim gasket. That gets my quench and compression ratio exactly where i was hoping to get it. I’m thinking that is going to be the plan.

so right now the build is looking like flat top pistons with a ball hone. 65cc aluminum heads with 2.02/1.60 valves and the z28 springs. Taking an earlier suggestion on the cam and found the comp cams high energy 268h cam. 268/268 - 218/[email protected] 110 LSA/106 ICL.
I’m thinking that with the higher compression and the tighter LSA this should be a fun little build.
Have to tear the block down now to see if a simple hone and re-ring with flat top pistons is going to work clearance-wise.