piston to head clearance

[Nova4301]

I have short blocked my new stocker project engine and I was curious on people's thoughts on what might be too tight for piston to head clearance. I checked pitson to valve clearance and I have plenty (.150+). But, I figured that I want as thin of a head gasket that is legal and that I can run clearance-wise.

I am probably going with either a GM# 3995633 (which I believe is about .017-.020 compressed) or a Fel-Pro 7733SH1 which is about the same. In another thread I asked what the bore sizes of each were...I believe a person would want the thinnest and smallest bore (respective to the engine bore) size gasket that they could get (that is legal). Any thoughts would be appreciated...I am trying to learn as I build.

[Mike Taylor 3601]

Depends how much pistons rock, RPM,weight,.Have run down as tight .026 with .004 cly. clearance@ 6500 RPM. Check with cometic they have several thickness/bore combo. SCE makes copper that doesn't
use o ringed block. Cheapest way is use fel pro 7733 pt( thicker than min. thickness) Then reduce deck clearance
to adjust for thicker head gasket. You are allowed to reduce deck clearance for thicker than min. head gaskets.
Mike Taylor 3601

[Bobby Zlatkin]

In early 1975 we built a 350 stocker motor for our '69 Nova using the minimum deck height allowed in the NHRA tech bulletin. Put on a pair of RHS prepared cyl. heads using the Chev steel shim gaskets (.017"). Took the car to the Suffolk, VA Div. 2 points meet May 3rd. and set the J/SA record.

Upon tear down for certification we saw that every piston had been hitting the heads. Only way to salvage that motor was to go to thicker head gaskets. Realized then that you didn't want to cut the deck much more than what you needed just to square the block.

[MHoward]

.032-.034 Clearance and the more you hone it the more you will need.

[Mike Taylor 3601]

Around .040 piston/ head clearance is safe for steel rod engine. My 305 dish pure stocker is @.024 with out hitting but I've kept getting closer in small steps I didn't put that close all at once which thats as close
as I'm allowed (with .002 insurance added) I turn 6800 rpm max.
If you go under .040 do it carefully if no contact move .005-.010 closer.
If have .150 piston-valve either don't have aggressive lobe cam or didn't check correctly.You did check
P-V with cam degreed where will be run?You're closest spots were around 10 degrees btdc on ex.and
about 10 degrees atdc on int. right?You used solid lifter? I don't know you just trying make sure you're
headed right direction and make sure are checking P-V correctly.Have people come in my shop all time that think you check P-V @max lift or tdc.they just don't know any better.On stockers the most I've had from.070 to .090 I've had mine is at .070 int.&ex. for a few years.Below.090 you need to be cautious
it's trial and error if they'll touch or not.Int. will live slightly closer than ex. Int. will gain slight clearance ex. will lose slight clearance when running.Load valve train( like its ran springs @all, ready to run)recheck P-V
clearance you'll see different clearances.Like M Howard said more piston rocks in bore more clearance you'll need
Mike Taylor 3601

[Nova4301]

Well, I guess I should have explained a little better on what I am building. I don't pretend to classify myself as an engine builder, but I have built a few small block racing engines. Since this is my first stocker build...I am trying to get everything that I can get out of this power-wise and knowledge-wise.

I am building a 307...just trying to go a couple tenths under...hopefully. This engine will not turn over 6000rpm to speak of, the piston rock is approximately.005 and on the high end it sits about .002 in the hole. I don't have a very aggressive duration cam and therefore am hopeing that is why I seem to have a decent amount of P-V clearance. I hope that I can get away with about .022 to .025 piston to head clearance to increase my compression as much as possible.

[Tony Curcio]

The NHRA Blueprint Specs for 1968 through 1971 200 HP 307's require .002" minimum deck height and .018" min. gasket thickness, or any combination thereof that yields .020.

This is your clearest limitation.

With a 2-barrel, it may not pull to 6000 RPM anyway.

[Nova4301]

Originally Posted by Tony Curcio

The NHRA Blueprint Specs for 1968 through 1971 200 HP 307's require .002" minimum deck height and .018" min. gasket thickness, or any combination thereof that yields .020.

This is your clearest limitation.

With a 2-barrel, it may not pull to 6000 RPM anyway.

Yes, that is why I thought I should ask if .020 piston to head clearance was too close for this combo...should I choose to run the minimums.

[Tony Curcio]

I ran a 2bbl 350 in IHRA Pure Stock about 4 years ago. We ran the .018" shim gasket, but I don't remember the deck height. I'll see if my brother remembers, or if we have any notes on it. It was probably very close to the spec minimum.

This engine had cast pistons, 040 over, and was shifted at 5700 RPM.

[Woodro Josey]

On most all applications, you need .032-.034 total deck including gasket! You can run tighter if you have no more than .003-.004 skirt clearance.