Piston to Head Clearance Minimum

[skinny z]

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)

[steve cowan]

The 35"-40" P/H clearance seems to be norm.
I have one i have got 28" runs fine not sure if it helps at all,yes piston 'rock' is a factor.
Hyper pistons run pretty tight.
I have heard rumours of people running as tight as 24" thou piston to head buts that all they were ( rumours)

[skinny z]

Thanks for that Steve.

We've managed .035" (IIRC) with a reworked original ZZZ SBC and cast pistons. Seems that this is about the practical limit.

I'm thinking rather than tightening that spec for the couple of tenths bump in compression it would be worth (and that isn't worth much really) the heads may get another pass on the decks. Thing is they're already milled .026". Not sure how much further I could go before I run into other issues. Or even if that's worth it for the minimal gains.

Wouldn't mind getting to 10:1 from the potential 9.6:1 I'd get from the package on the drawing board at the moment.
Nothing decided yet. Looking at build plans more than anything.

[jeff swisher]

I ran .028" piston to head clearance for 70,000+ miles with 186 casting heads on a 350" the heads were milled .060" and ended up at 54cc after a little unshrouding.
I ran a 270H cam and it had 245 psi cranking pressure.
I had that cam to 7400 rpm.
Later I went with a thicker head gasket and it ran the same but was not as finicky when the temps got to 90 degrees outside.


I love a small chamber that is unshrouded.
I do not get caught up in the tight quench anymore with the small blocks.
I do like a proper shaped chamber though and have found more power there than in quench distance.

[mag2555]

Leave it as is and run a extended tip plug for faster flame travel and or a simdge higher compression and move on to other things like running a Crank scrapper if your not which between 5500 and 7000 rpm is worth a solid 8 hp or more.

[steve cowan]

I ran .028" piston to head clearance for 70,000+ miles with 186 casting heads on a 350" the heads were milled .060" and ended up at 54cc after a little unshrouding.
I ran a 270H cam and it had 245 psi cranking pressure.
I had that cam to 7400 rpm.
Later I went with a thicker head gasket and it ran the same but was not as finicky when the temps got to 90 degrees outside.


I love a small chamber that is unshrouded.
I do not get caught up in the tight quench anymore with the small blocks.
I do like a proper shaped chamber though and have found more power there than in quench distance.

Flat milled? To a 54cc chamber

[PSA]

I have used 0.8mm (0.0315") on I4, +7000rpm.
Some have seen pretty much daily use on the street for years, but it's smaller bore and tight piston clearance.

[steve cowan]

Leave it as is and run a extended tip plug for faster flame travel and or a simdge higher compression and move on to other things like running a Crank scrapper if your not which between 5500 and 7000 rpm is worth a solid 8 hp or more.

How much more power did the engine make with the extended spark plug??

[hysteric]

Leave it as is and run a extended tip plug for faster flame travel and or a simdge higher compression and move on to other things like running a Crank scrapper if your not which between 5500 and 7000 rpm is worth a solid 8 hp or more.

How much more power did the engine make with the extended spark plug??

Yes I'd like to know that too. More importantly how did you come to the conclusion that the flame travel was faster. Did it make more power with less timing?

[skinny z]

I ran .028" piston to head clearance..

That's tight.

[skinny z]

I have used 0.8mm (0.0315") on I4, +7000rpm.
Some have seen pretty much daily use on the street for years, but it's smaller bore and tight piston clearance.

There are those variables mentioned earlier. Could be that I end with a somewhat looser forged piston than the hypereutectic I run now.

[af2]

.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!

[af2]

Leave it as is and run a extended tip plug for faster flame travel and or a simdge higher compression and move on to other things like running a Crank scrapper if your not which between 5500 and 7000 rpm is worth a solid 8 hp or more.

How much more power did the engine make with the extended spark plug??

Yes I'd like to know that too. More importantly how did you come to the conclusion that the flame travel was faster. Did it make more power with less timing?

HMM Like closing the Gap on #7 piston is worth 100 HP? The local guru told my son when we ere building the car?

[Ken_Parkman]

I remember a line in Bill Jenkins Book where he suggested not to run the piston to head clearance too tight as he thought there may be some pumping loss when the operating piston to head clearance approaches zero. The suggestion was there may be gains up to .030" min operating, compression permitting, at least in that time and application.

[tenxal]

I've run as close as .028 with my flat top drag race SBC stuff with no issues to 7300-7400. I'm more comfortable at .030 and anything over .032 is costing you power and torque with the small chamber 461-462-041-291-186 heads. At .034 and more, it's like leaving free money at the table.

Assuming steel rods and piston-to-wall of .004-.007.