quench vs. zero deck

[Invasivecoyote]

how badly is the quench affected with a zero deck or .005 down the hole? How much stress is then put on the head gasket when that becomes the effective quench.
I'm thinking of .021 compressed thickness. keep in mind i'm ruing a Windsor motor and theirs 4 head bolts per hole.

[KnightEngines]

Zero deck with a .021" gasket is way too close - you'll smack those heads with the pistons guaranteed.

.035" minimum piston to head, preferably around .040-.050", so with zero deck run a .040" gasket, .005" down the hole you can run a .037" gasket.

[user-612937456]

There is negligible improvement in quench effect below .050 so in most cases .035-.050 is idea. I have seen it as tight as .027 in a low rpm engine but you will likely kiss the head with the piston with that low of a clearance.

[In-Tech]

Agreed and after ~.100" it doesn't seem to induce preignition nor detonation that would otherwise happen anyway due to bad tuning.

[novadude]

Agreed and after ~.100" it doesn't seem to induce preignition nor detonation that would otherwise happen anyway due to bad tuning.

That's good to know, as I've always heard that anything over ~.050" would tend to increase the chances of detonation. I'd imagine chamber shape, piston dome, etc also plays a pretty big role in what quench is/isn't OK too.

[panic]

Jenkins wrote 40 years ago that tighter quench than needed to suppress knock = pumping loss.
How to find how much suppression you need is a much more difficult question.

[900HP]

The other thing to think about here is crevice volume.... the head gasket bore is always slightly larger than the cylinder bore. Keeping the piston down .010" and using a thinner gasket reduces crevice volume...... I also feel a thinner gasket doesn't allow the head to move around as much. With your 4-bolts per hole use a Cometic MLS gasket, they work very well.

[dfarr67]

If you zero deck and later do another rebuild, piston, rod, crank combo may push crown out of the hole. Better to give some leeway and there are many gaskets to choose from.

[Dan Timberlake]

"How much stress is then put on the head gasket when that becomes the effective quench."

Just sitting here in my slippers I think the head gasket is always subjected to pretty much the full range of pressure during the compression and power strokes.

[ctk30]

Is there any downfall to having a gasket thats only a few thousandths in diameter larger than the bore?

[pdq67]

Is there any downfall to having a gasket thats only a few thousandths in diameter larger than the bore?

As I see it you need to mock it up and then torque the gasket you want to use to be sure that it doesn't "expand" and overhang the bore is all. And it might be better to do this AFTER you heat cycle the engine several times until everything takes a set...

If it doesn't overhang the bore, the smaller the hole the better. Less creavas(Sp?) volume.

pdq67

[ctk30]

Stock bore is 3.898 and the repalcement gaskets are 3.910 and I'm having it honed out a few thousandths

[Invasivecoyote]

Is there only speculation as to how far (length wise ) the vavle expands with heat? In relation to piston to vavle clearance

[MadBill]

The only information I'm aware of re valve expansion is a valve burning problem GM was having with their road race Corvettes ~ 60 years back. They changed alloys without curing it and finally realized that thermal expansion was reducing the lash to negative numbers, thus preventing the valve from conducting heat into the seat. Maybe one of our historians can say what the old and new lash specs were...

[modok]

Assume .004" for exhaust valve expansion.

Closing lag is a much larger factor.