Can someone explain quench to me? Is it as critical on a forged piston as it is on a hyperutectic? I've had a blown up sbc dropped off to me to dissect and try to figure why it "pinched" the piston tops. It has .0, deck & came with .019 head gaskets My thoughts are replace pistons (wiseco pt003h3) And reassemble with felpro's that measure .041 compressed, should give me the magical .040 quench
But why is .040 "the number"? Thanks in advance
.040" is the number as it is the closest you can get without smacking the piston into the head on every TDC location. It is all the stacked clearance tolerance. Anything really smaller doesn't gain you any additional squeeze on the mixture to get the flame front proprogated. But going larger definitely slows the flame front and creates dirty emissions and a loss of power. Of course, this only applies to non-hemi engines. Those have their own problems.
Oh, and this is just what I remember from some reading I did on this subject wondering the same thing.
In reply to warpedredneck:
I'm not sure about bigger engines, but on my CR125 based kart, we're looking for 0.030 clearance between the piston and head due to "stretching" of the rod @ 13,000rpm. I expect that's why you're looking for .040. If it gets too close then it's easily seen on the piston top via a noncarbonized spot, or worse, marks from the piston and head trying to occupy the same space at the same time(and ussually physics deals with this promptly) Do you have a picture of the piston tops yet?
I'll try to post some pics tonight, I "misted" some powder across the tops of the pistons, and can see an outline on the piston tops, .040 does make more sense now tho!
.019" would be very adventurous above say, idle.
iadr wrote: Tightly fitting pistons (1-3 thou) with small bores this is less a factor. Personally I have a Volvo b20 with a serious offset stroked crank that is right @30 thou IIRC. Cast pistons closely fit, and not a ton of rpm, and 4g63 mistubishi rods.
about .0355"(.9mm) on 8k capable Toyota 4AGE 
I've got a 357sbc with .055" piston/deck + .039 gasket, for a total of .094" "quench"...set up for a 250hp shot of n2o while burning 91octane pump gas.
Uh, to not confuse people, you are talking about squish height, not quench. Quench is the small area along the walls where the flame goes out, hence the name.
.094" is huge, I shoot for .035-.045" on 2 valve single plug. Most I run zero or just under deck and a thin mls gasket.
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