Quench...How tight?

[YellowDuster]

I am about to start building a 360 based 408. I have been figuring compression ratios with different deck heights and quench distances. With a distance of .035 my CR will be right at 10:1. I have heard of clearances this tight but would like to know if anyone has tried it and what experiences they have had.

[sanborn]

A good policy for a SB(or a BB for that matter) is to assemble the short block, install the cylinder head without the head gasket. If the engine turns over without contact with the cylinder head---then you have enough running clearance with the head gasket(.040") installed. This works well with stock rods and the cheaper aftermarket rods. Some use tighter clearance with better aftermarket steel rods---but make sure your wallet is thick before you get this brave.

[YellowDuster]

I'm going to be using a Scat cast crank and I-beam rods with KB pistons (23.5 cc dish with the quench dome milled off to match the closed chamber magnum heads).

[Glen440]

I shoot for .040 with steel rods.

[DartGT66]

.040 is commonly used because it's easy to achieve, the thickness of common composite gaskets, and it works. But you can go with less. I have been at high 0.2" 's with stock steel rods, but haven't opened that engine to see wether you could use even smaller gap or not. That's a 3.5" stroke 4" bore engine, and it works well. In the 4.15" stroke 4.5" bore we had .033" with aftermarket steel rods, and it could have used even a smaller gap, still some carbon build up on the quench pad. So, I hitnk you should have no problem with the .035" gap.

[YellowDuster]

Thanks for the replies. DartGT, that was what I was looking for. I want to deck this block as little as possible and if the piston winds up .005-.010 down I could run the thin MP head gasket to keep the quench tight. At least that is my thinking.

[daredevil]

I used 38 with kb 191s and the heavy duty 360 rods for a cr of 13to1 and had sealing problems till i o-ringed. good to go now and no detonation on a 50 /50 mix of 93 and 105 octane

[djswwg]

You will always be safe at .040". At .035" you should still be fine assuming good quality parts, especially rods, which tend to stretch at high RPM, (as does everthing else.) Anything less than .030" and you are starting to push the limits. I've seen big blocks with .025" that didn't hurt anything but you could read the piston # (Ross pistons) on the quench part of the head. I'd shoot for zero deck and a gasket between .039"-.042" with your combo. A couple thou here or there won't affect your performance much............djs

[YellowDuster]

All good advice, thank you!

[pishta]

If you can read the piston stamps in the chamber, you are close!

[djswwg]

you're right Pishta, it's pretty spooky looking!!!!!!!!djs

[crankshaftkid]

I know some dirt track guys running .025 and they turn some high rpm. Did a 440 once with .019! It was a mistake. It ran a 10.0 first pass off the trailer in a 3500 lb car turning just a tad over 7 and it got 3 more passes on it until the NOS was turned on for the 5th pass......it didn't like that very much lol. If you're not going to turn 7500 with it, I wouldn't worry about .035 at all and would even go for .030