Piston to Head Clearance Minimum

[Walter R. Malik]

I've generally used .040" as my go to guide for clearance/quench value.
These are your everyday 350/383 SBC's with 7000 RPM peak engine speeds. Current lump has a .014" deck and an .026" gasket with a flat top hypereutectic piston.
What's possible at a minimum for this?
How much is dependant on the piston to bore clearance? (as in piston rock if that's a significant contributor)

YES ... A lot will depend upon the size of the bore, the compression height of that piston, the piston to wall clearance in that bore, the rod material and bearing clearances.
With your example, you'll have plenty.
I have successfully run .025" total with that scenario.

[skinny z]

Hmm

[Geoff2]

Theoretically,

The piston 'rock' should disappear at operating temp. Forged pistons run more clearance, but also expand more.

[hoodeng]

As Walter has said, it depends.

All engines are not the same, .025" fine in some and not so good in others. In what i do we can run as low as .030", but when i get shops to check squish before they send me their cylinders i get them to include the leads with the cylinders, you tell them [depending on the chamber] one piece [or 2] 10mm long x .080" solder placed as near to the center of the cylinders squish shelf,, i have had them returned with criss cross lead, plaited lead, strips 80mm long everything bar what they were told to do.

So when i machine their cylinder bases i set for .040" even though we can safely run less down to .030" and when i return the cylinders i supply two or four pieces the correct length and diameter and get them to do the job again.

After .040" the squish effect diminishes rapidly.

Cheers.

[jeff swisher]

Flat milled? To a 54cc chamber

It was a mistake in the making I handed the heads to a friend at a machine shop and he angle milled .030 off the first head and got to checking things and told me that the head had been flat milled at some time .030" and then he did a .030 angle mill.

He said he would find me another head.
I asked him if there was anything wrong with it and he said no but the chamber is very small and you only wanted .030" off of it.
I said I just wanted a smaller chamber and if it is fine then do the other one the same way.

I had 80 hours each in hand porting those heads and I was going to run them.
I measured the chambers and they were 54cc after i unshrouded the valves a little.

I later got a set of 041 heads and ran them at 64cc and they could not tolerate as much initial timing as the smaller chamber 186 head.

I now run 601 casting heads and have zero issues with them and they make excellent power.
Not hard to go 11's in a daily driver 350"

[70MC]

.041 was cleaning the piston between 9000 and 9500 with H beam rods. RPM is going to determine what you want as far as clearance. It makes a BIG difference after 8500 rpm from what I have seen.
I guess I am using rubber rods!

I seen the same thing with my 434sbc running flat tops at .035 shifting at 7500.
The engine made good power, and I had no idea the piston was kissing the head untill I had it apart and found the piston and head had a perfect clean outline on each other.

[skinny z]

By most accounts, with respect to my combination of parts, it seems that .040" is safe and about as effective as anything possibly narrower. And it's never given me issues.
I'll be looking for other means to give a bump to the CR should I find that necessary.
Every .010" in the gasket is worth about 2/10th's in CR. But that would be .030" clearance.
Every .006" in milling is worth about 1 cc. That's a 1/10th. Heads have .026" off already though.
Might need a different piston spec.

[panic]

A remark I've heard:
"If the block is alloy it will expand faster and taller than the piston, so if it doesn't touch when cold it never will".
Unless you run the engine hard when it's still cold....

[fastblackracing]

Flat milled? To a 54cc chamber

It was a mistake in the making I handed the heads to a friend at a machine shop and he angle milled .030 off the first head and got to checking things and told me that the head had been flat milled at some time .030" and then he did a .030 angle mill.

He said he would find me another head.
I asked him if there was anything wrong with it and he said no but the chamber is very small and you only wanted .030" off of it.
I said I just wanted a smaller chamber and if it is fine then do the other one the same way.

I had 80 hours each in hand porting those heads and I was going to run them.
I measured the chambers and they were 54cc after i unshrouded the valves a little.

I later got a set of 041 heads and ran them at 64cc and they could not tolerate as much initial timing as the smaller chamber 186 head.

I now run 601 casting heads and have zero issues with them and they make excellent power.
Not hard to go 11's in a daily driver 350"

Sounds like you ended up with a nice running combo when all was said and done...
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Is there a build link or threads on it ?
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One I am building right now has the deck clearance at .027".... trying to decicde what I want to go after...

The piston is just a simple big dish, like 25cc's, and the rim at the top is 1/4" wide....sure seems like I could get
by with .025-.028...?
Crower billet rods and crank, 7400 rpm max, hmmmm