Piston to head clearance.

[Mbowling]

What is the minimum I can get away with on my 487.  Steel rod, street car. Will seldom go past 6K rpm.

[GerryP]

Generally, you don't want to go much less than .040" and for sure not tighter than .035".  The quench zone is so tight that you actually have to build to it.  Less than .035" and you're probably going to see witness marks.  Though there are a lot of examples of just having witness marks at .030" but that's really too tight.  And over .070" and you start losing the quench effect.  You have to carefully measure every cylinder.

[blykins]

Tons of variables:  piston weight, rpm, piston rock, piston/cylinder clearance, dome/flat top, etc.

.040” is gonna be plenty safe unless it's a heavy domed piston.  I wouldn’t run tighter unless you want to experiment or have experience with that combination.

My 352 runs really well at .035”, but it’s a stable 1.920” compression height with no rock, with a flat top, .0035" of piston/wall clearance, and I do a ton of measuring.

[Barry_R]

I have run a pretty well refined 505 (4.350 bore, 4.250 stroke, 700+ HP, 6500 RPM) at .036 and upon teardown it had the machining marks from the head clearly visible in the carbon on top of each piston.  No metal movement that I could see, but it was definitely "right there".

[Mbowling]

Generally, you don't want to go much less than .040" and for sure not tighter than .035".  The quench zone is so tight that you actually have to build to it.  Less than .035" and you're probably going to see witness marks.  Though there are a lot of examples of just having witness marks at .030" but that's really too tight.  And over .070" and you start losing the quench effect.  You have to carefully measure every cylinder.

I have run a pretty well refined 505 (4.350 bore, 4.250 stroke, 700+ HP, 6500 RPM) at .036 and upon teardown it had the machining marks from the head clearly visible in the carbon on top of each piston.  No metal movement that I could see, but it was definitely "right there".

Tons of variables:  piston weight, rpm, piston rock, piston/cylinder clearance, dome/flat top, etc.

.040” is gonna be plenty safe unless it's a heavy domed piston.  I wouldn’t run tighter unless you want to experiment or have experience with that combination.

My 352 runs really well at .035”, but it’s a stable 1.920” compression height with no rock, with a flat top, .0035" of piston/wall clearance, and I do a ton of measuring.

Thanks. It was after a 15 hr workday when I was looking at it last night. So I need to recheck my measurements and check all cylinders top & bottom of the bore. I do have some piston rock so I’m thinking to use the measurements that put the piston closest to the head. 
I’m fairly certain tho that I’ll need to increase my clearance, Are a custom thickness head gasket from cometic a good fix?

[gt350hr]

    .035 minimum is my limit. As Brent said piston weight and RPM are critical especially if a light weight ( read flexible) pin is being used.