How far can a piston be run above deck?

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How far can a piston be run above deck?

Post by gregsdart »

I have a mopar 4.5 stroke crank, and current parts will add up to . 032 out of the hole cold .(.022hot) in a aluminum block with an increase to 4.6 stroke. A . 066 cometic gasket would give me . 044 piston to head at 180 running temp. 4.500 bore. I would be using hbeam 7.0 h beam rods, 1.452 c. H. Pistons.
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Re: How far can a piston be run above deck?

Post by PackardV8 »

One unintended consequence of running pistons .032" above deck is the rings spend more time in the solid section of the top of the block. It's hotter up there, because there's no coolant up that high. Sometimes it doesn't make a differerence. Sometimes on max effort/nitrous/supercharged, it's just enough too much.
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Re: How far can a piston be run above deck?

Post by gregsdart »

Jack, 15/1 compression N. A. Methanol motor, at 580 cubes am hoping to make 1050 hp. Indy 572-13 heads.
1965 dodge Dart, 549 cu in wedge, 8.60 at 156 mph best. 2905 lbs, soon, 8.40s!
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Re: How far can a piston be run above deck?

Post by dfarr67 »

Mine are 15 thou above the deck due to error. I run a 35 thou quench.
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Re: How far can a piston be run above deck?

Post by ClassAct »

You are as close as I’d get it. Especially on alcohol.
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Re: How far can a piston be run above deck?

Post by PackardV8 »

gregsdart wrote: Sat Jul 09, 2022 5:01 pm Jack, 15/1 compression N. A. Methanol motor, at 580 cubes am hoping to make 1050 hp. Indy 572-13 heads.
With methanol, the rings being higher should not be a problem.
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Re: How far can a piston be run above deck?

Post by skinny z »

dfarr67 wrote: Sat Jul 09, 2022 8:02 pm Mine are 15 thou above the deck due to error. I run a 35 thou quench.
Interesting. I may be put in that position myself and have concerns with contact. Mine isn't the OP's engine by any stretch but the piston to head clearance is a question in and of itself.
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Re: How far can a piston be run above deck?

Post by gregsdart »

Kevin, compensate with thicker head gasket. Cometica are available for most, in steps from . 027 to . 100 plus.
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Re: How far can a piston be run above deck?

Post by gregsdart »

Kevin, compensate with thicker head gasket. Cometica are available for most, in steps from . 027 to . 100 plus.
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Re: How far can a piston be run above deck?

Post by skinny z »

Crazy bucks for the Cometic I find.
I've an .026" Clevite/Mahle/Victor Reinz that's my go to. But at .008-.009" down and an.026" gasket things are tight.
Still have to do further measurements.
Thanks.
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Re: How far can a piston be run above deck?

Post by FC-Pilot »

skinny z wrote: Sun Jul 10, 2022 3:58 pm Crazy bucks for the Cometic I find.
I've an .026" Clevite/Mahle/Victor Reinz that's my go to. But at .008-.009" down and an.026" gasket things are tight.
Still have to do further measurements.
Thanks.
What is your combo? Piston design, material and clearance? How much rock does your piston do when cold? Rpm, rotating assembly, block material and so forth. Is a standard small block street strip application with basic stuff I like .035 as long as my clearances are not sloppy. If a piston that has short skirts and the clearance was on the loose side then I error on the side of more. I have run some stuff down to .030 without any issues. But again, that was application specific and I knew the clearances were right and the RPM was going to be kept under 5500. Just my thoughts on the matter.

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Re: How far can a piston be run above deck?

Post by Alaskaracer »

Until they hit the head.......
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Re: How far can a piston be run above deck?

Post by skinny z »

FC-Pilot wrote: Sun Jul 10, 2022 5:24 pm
skinny z wrote: Sun Jul 10, 2022 3:58 pm Crazy bucks for the Cometic I find.
I've an .026" Clevite/Mahle/Victor Reinz that's my go to. But at .008-.009" down and an.026" gasket things are tight.
Still have to do further measurements.
Thanks.
What is your combo? Piston design, material and clearance? How much rock does your piston do when cold? Rpm, rotating assembly, block material and so forth. Is a standard small block street strip application with basic stuff I like .035 as long as my clearances are not sloppy. If a piston that has short skirts and the clearance was on the loose side then I error on the side of more. I have run some stuff down to .030 without any issues. But again, that was application specific and I knew the clearances were right and the RPM was going to be kept under 5500. Just my thoughts on the matter.

Paul
It seems I may have executed the perfect thread hijack.
The OP's combination is far different from mine so taking all of the above criteria into account, the safe limit values I suspect will be very different.

I'll have to post up in a thread of my own once I've got further measurements in the books.
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Re: How far can a piston be run above deck?

Post by FC-Pilot »

gregsdart wrote: Sat Jul 09, 2022 4:42 pm I have a mopar 4.5 stroke crank, and current parts will add up to . 032 out of the hole cold .(.022hot) in a aluminum block with an increase to 4.6 stroke. A . 066 cometic gasket would give me . 044 piston to head at 180 running temp. 4.500 bore. I would be using hbeam 7.0 h beam rods, 1.452 c. H. Pistons.
If I were in your shoes I would be looking for a gasket as reasonably close to the bore size. If I can get a gasket I feel good about at that thickness I would run it in a heart beat. I did the same on an alky BBC as you did with no issues. The one thing we did not do a good job of checking was if there was any gasket overhang. On one hole our copper head gasket hung into the bore just enough for the piston to clearance it all by itself. We had no idea until we pulled the engine down after 300 runs. Thank the heavens it was just soft copper.

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Re: How far can a piston be run above deck?

Post by gregsdart »

Thanks for the input everybody. I may go with a more conventional setup and use a thinner gasket. At some point i have to take stock of what i have on hand, like very expensive pushrods, gaskets, and rods, and ad up the costs of my choices.
1965 dodge Dart, 549 cu in wedge, 8.60 at 156 mph best. 2905 lbs, soon, 8.40s!
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