Quench Clearance

[Cobra1]

Was wondering what the TIGHTEST quench clearance any has ever run is for an ALUMINUM block (taking into account expansion). I read on another forum a guy ran .025" with an iron block.

[ScotiaFE]

It's my understanding that a quench of about 0.040" measured cold is good.
As the engine heats up and rpm increases this distance decreases.
It also has a small cooling effect on the squish which helps prevent pre ignition.
A very tight quench will promote detonation as will a very wide quench.

[jayb]

The combined growth of an aluminum block and aluminum heads at temperature is just about enough to change the valve lash from zero to .018"; in fact I will lash my valves on all aluminum wedge FE motors at zero, or just a tad looser, when they are cold, and then at hot they are around .018".  If you figure a 1.75:1 rocker ratio, that means that the head and block together are increasing in height about .010".  Of that, I'd guess that the block is about .006". 

So, if you were to keep the revs down until the engine warmed up, you might be able to cheat the quench distance by .006".  But I agree with Howie, .040" is a good number to go with, and the extra .006" you might be able to reduce it by with an aluminum block won't buy you too much, and poses certain risks.  All the aluminum engines I've run have been set up with .040" for the quench distance.

[CaptCobrajet]

Was wondering what the TIGHTEST quench clearance any has ever run is for an ALUMINUM block (taking into account expansion). I read on another forum a guy ran .025" with an iron block.

That makes me wonder if the deck was really what he thought.  I ran a Cobra-Jet at .034 once.........dead zero with a .034 gasket.  It definitely hit the head.  Ever so slightly, but it did hit.  At .038, and .007 piston-to-wall, it will keep the quench cleaned off at 4.190 bore.  Skirt clearance (rock of the piston) and bore diameter both enter the equation.  I would think at FE bore sizes, .038 is a cut-off.  If they kiss too hard, it will spin the rod bearings.  Super tight clearance, like a cast piston, might allow you to go low .030's, but a cast piston can't kiss the head much before disaster strikes!

[hotrodfeguy]

Off the top of my head the .025 was with 352/360 rods and in a mild build for a Red Galixie, nothing real crazy. 
And on this forum I think that was 65er

[65er]

Off the top of my head the .025 was with 352/360 rods and in a mild build for a Red Galixie, nothing real crazy. 
And on this forum I think that was 65er

Naw, it's not that tight.  I put the pistons .008" above the decks and used a .041 gasket to get .033 clearance.  They're hyper pistons with .002 clearance on the diameters.  I had the heads off once and didn't see any evidence of piston dome contact but when I put it back together I went ahead and used thicker gaskets .  I think they checked at around .058" so probably .055 or so installed,  bumping my quench distance up to .047 as I recall.  I didn't notice a change in performance or behavior so I'll guess being a few thou on either side of .040 typically won't matter a whole lot on a street engine.  My heads and block are all iron though so the thermal expansion aspect kind of gets lost in translation...

[390 Cougar Conv]

Well I just sent my pistons to the machine shop to be turned down and I am aiming for .040 quench. Then I will be checking for piston to valve clearance.  More fun.  Clifford