Piston to head minimum?

Looking for input on where i should set piston to head clearance. 440-1 heads, molnar 7.1 h beams, Gibtech custom pistons line2line coated, stroke 4.315, bore 4.500. Power should be 925 hp, shift at 7400 max rpm 7600 rpm limit, trap rpm 7500 max. Digital 7 MSD for a rev limiter. What have you run at 4.5 bore and 7500 rpm?

I haven't ran that bore size but I have seen witness marks on my heads with .035 P to head clearances and not with .040 clearances
Those were a set of Ross custom race pistons with around .0080 piston to cylinder wall clearances on my old pump gas stroker motor, those witness marks were on the cam side and motor mount side only so they were close when firing when they reversed directions with combustion pressure on them
I'm of the opinion that with the street alloy you can run them a tiny bit closer than with the looser race alloy
Your choice on this

Cab due to the large bore i am guessing .040 may or may not be safe. I was running .045 with the iron block. I assume i can go .040 cold with the aluminum block? Never run one before.

On my deal I would rock the pistons over both ways to get the piston edges up as high as they would go at TDC use that for making .035 and .040.
I've never warmed up a aluminum block enough to see how much the deck of the block grows in height in relation to the sleeves, rods and pistons at TDC from room temps to above 160 F

For sure .050" clears.
Doug

II run .040 with 4.5 bore Indy block no witness marks. Thru the traps at 7700

JD is that an aluminum block? Steel rods? Stroke?

I think the rule that with steel rods, .040 is ideal and .060 for aluminum rods is pretty much spot on, first from my personal experience. And a few years ago I built a 512, with a world products aluminum block, aluminum rods, and aluminum heads. I knew the .060 rule for those rods, but I pondered about the aluminum block and it's affect on the clearance. WELL, my son is a mechanical engineer and I called him and asked him to calculate the required clearance, he said he needed the aluminum alloy numbers, and some other measurements, after he did the calculations, he came up with .058 for my engine. So I just figure the .060 is a pretty iron clad result.

I've run BME 426 forgings at..055" at 4.500" with no issues.
Doug

The consensus on what is a safe number is going to be .050" for an aluminum rod and .040" for a steel rod. The reason being is you need to have some "wiggle" room. Things many don't account for is skirt design, and piston to wall, as well as ensuring the stroke is equal on all cylinders and the type of material used for the rods etc. So I think you will find those to be the most common numbers thrown around out there.

I have run as little as .040" aluminum block aluminum rod with no issues on my sruff. We run the Vette even tighter at .033" with an aluminum block and rods too at 10,000rpm. But alot of things have to be perfect to get away with that tight of a clearance. When trying to get to 18-1 compression you have little choice but to push things a bit.

Aluminum block Molnar steel rods 4.250 stroke

I have run as low as .028 in a SB with no contact. That being said, it's gonna depend on bore size, and bore clearance, bearing clearance, rod material and RPM. Also remember on an aluminum block the clearance with steel rods will grow as it warms up, aluminum rods should grow at a similar rate and keep clearance about the same.

The lightest contact (-.001") won't even leave witness marks. It may slightly collapse the normal .006" ring to groove vertical clearance and reduce ring seal. If it continues the land will fracture.

There are too many variables to have more than a range.
A 2" long skirt, .001" wall clearance, 2:1 rod ratio, and 3.50" bore with pin offset can run tighter than 1" long skirt, .005" wall clearance, 1.5:1 rod ratio, and 4.60" bore with centered pin.

[i][/i]Greg, I had .042 head gaskets and zero deck clearance on my 528, I over revd it once to 8200. I ended up with witness marks on the heads.

With a .039 head gasket I like to put the piston .003 - .005 in the hole. .000 deck with a .045 gasket.