Pistons hitting heads
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1st Gear
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Pistons hitting heads
Just built a 355 with 10.5:1 pistons and close chamber aluminum heads. When I torqued the heads and went to turn the motor the pistons were hitting the heads. Anyone know a simple fix. Adding a one hundred thousandths head gasket is still going to be about .040 short of safe operating clearances. It clears, but under running heat and stress the rods stretch. HELP!!!
Melting Slicks
Joined: Apr 2021
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From: Minnesota
Welcome to the forum!
That definitely doesn’t sound good. Do you have the piston part number? Also has the block been decked? You may need a thicker head gasket regardless to get adequate quench.
If that doesn’t solve it I think your next step would be changing out either your pistons or your cylinder heads. Heads would be easier, but pistons would be cheaper and probably the better move in the long run.
That definitely doesn’t sound good. Do you have the piston part number? Also has the block been decked? You may need a thicker head gasket regardless to get adequate quench.
If that doesn’t solve it I think your next step would be changing out either your pistons or your cylinder heads. Heads would be easier, but pistons would be cheaper and probably the better move in the long run.
Safety Car
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From: Fairfax Virginia
Welcome to the Corvette Forum DaveGee67!
We are glad you have chosen to join us here on the Corvette Forum as this is where issues are solved and questions answered.
Whenever you are working with an unknown you should try and put a bit of play dough on the pistons to be sure you have the room you need.
Using a spare set of the head gaskets that I was planning on using, I assembled the heads on the engine like I was doing final assembly. I used a set of high compression closed chamber heads with matching pistons and with the play dough on top we rotated the assembly around a few revolutions and then pulled the heads off to measure. I had over .125" of room between the valves and the pistons which was more than enough explained the engine builder I used.
The closed chamber heads are from 1968 and older engines. It is likely you have the wrong heads for the pistons being used. I personally love the closed chamber heads and use them on my 427 BB.
We are glad you have chosen to join us here on the Corvette Forum as this is where issues are solved and questions answered.
Whenever you are working with an unknown you should try and put a bit of play dough on the pistons to be sure you have the room you need.
Using a spare set of the head gaskets that I was planning on using, I assembled the heads on the engine like I was doing final assembly. I used a set of high compression closed chamber heads with matching pistons and with the play dough on top we rotated the assembly around a few revolutions and then pulled the heads off to measure. I had over .125" of room between the valves and the pistons which was more than enough explained the engine builder I used.
The closed chamber heads are from 1968 and older engines. It is likely you have the wrong heads for the pistons being used. I personally love the closed chamber heads and use them on my 427 BB.
Drifting
Joined: Jun 2025
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^^^THIS^^^
There's no simple fix.
And, while extra-thick gaskets might jack the heads far enough to obtain adequate P-V clearance, by doing so completely destroys any quench (anti-detonation) benefit that might have existed.
Select different piston and/or select different cam profile.
There's no simple fix.
And, while extra-thick gaskets might jack the heads far enough to obtain adequate P-V clearance, by doing so completely destroys any quench (anti-detonation) benefit that might have existed.
Select different piston and/or select different cam profile.
Burning Brakes
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I’m curious why there’s clearance issue?
how decked is the block? What valve lift is there?
Without a head on, how far past does the piston extend from the block?
10.5:1 is a pretty common build that’s why I’m surprised
do you mind sharing what crank rod piston combo you used?
how decked is the block? What valve lift is there?
Without a head on, how far past does the piston extend from the block?
10.5:1 is a pretty common build that’s why I’m surprised
do you mind sharing what crank rod piston combo you used?
Last edited by randallsteel; Oct 10, 2025 at 10:56 AM.
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Yep..some pics and measurements of how far the pistons are out of the hole would be good. And what part number rods and pistons you have.
Not a common problem..somethings is out of whack with the parts combo or machining.
My engine has the pistons .009" out of the hole, but I make it up with gaskets and have run it with .028" total quench clearance at 8K RPM. Yes..the pistons rub the heads a little but no issues.
JIM
Not a common problem..somethings is out of whack with the parts combo or machining.
My engine has the pistons .009" out of the hole, but I make it up with gaskets and have run it with .028" total quench clearance at 8K RPM. Yes..the pistons rub the heads a little but no issues.
JIM
Pro
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As suggested and to summarize, you have to pull a head and verify where the mechanical interference occurs. Is it piston to head or piston to valve(s) or both. If it’s just valve(s) then a cam with decreased lift may resolve the issue. If you don’t want to sacrifice lift then you can remove the pistons and have valve reliefs fly cut in them. That would contain costs to labor only although that cost may be higher than replacing the pistons. Take it apart, weigh the options and associated cost, make a decision and move on. How about some pics and PNs?
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Is my vette stock?? HAHA
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The problem is closed chamber heads. Are you sure they are closed chamber. I know big blocks run them and you can run a closed chamber piston in an open chamber head but you can not run an open chamber piston in a closed chamber head. Where did you find closed chamber aluminum heads for a small block ? Why would you use them, they have horrible flow characteristics for a non hemi head
Last edited by Rescue Rogers; Oct 12, 2025 at 08:26 AM.
Drifting
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The problem is closed chamber heads. Are you sure they are closed chamber. I know big blocks run them and you can run a closed chamber piston in an open chamber head but you can not run an open chamber piston in a closed chamber head. Where did you find closed chamber aluminum heads for a small block ? Why would you use them, they have horrible flow characteristics for a non hemi head
I'm accustomed to thinking only BBC when I hear the closed or open chamber monikers.
However, as far as good flow + good port velocity + broad power band is concerned, many small comb chamber non-hemi sbc heads perform quite admirably.
Is my vette stock?? HAHA
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thats a small combustion heart shaped chamber, not a closed chamber. Like you said, a big block closed chamber is totally different and they are garbage. I had a 396 BBC that I cammed up and used some 100cc closed chambered heads on which was a complete dog. I was so disappointed I threw my 115cc 270 oval port open chamber heads on it that lost a point and half of compression. I hoped it would be better but expected to be disappointed again but those heads flowed so much better that it woke that motor up and it revved like a small block. It was very impressive after that even with 8.5:1 compression. If I build that motor up its getting a domed piston to get it up to 11.5 to 12:1. It will be a high revving beast
Old Pro Solo Guy
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Let's see a picture of your piston "poking" thru the top of the block. At TDC.
The piston to deck measurement is monumentally critical.
Stock is usually around .015" down in the hole.
Taking them up to "0" deck when milling the block is somewhat common.
Going "above" deck is extremely rare, only done by a few thou, and harder to deal with.
It's just not necessary on street motors.
10:1 CR 350 SBC pistons are usually flat-tops.
I believe this is a deck height issue, unrelated to the heads.
IF it is the piston that hits the head, and not the valve.
Deck height corrections are usually done by rod length, piston compression height, and block decking.
If you need another .100 to .140" as stated, one of the parts selected is incorrect.
My guess is rod length or piston combo.
I have some .125" longer rods in my BBC, and they had to be matched to the correct pistons.
SBC rods are available in both 5.700 and 5.850" rod lengths.
That is .150" longer and seems to match your comments about needing another .100 or so.
You need the correct piston (pin height) for each rod length choice.
Pictures please!
The piston to deck measurement is monumentally critical.
Stock is usually around .015" down in the hole.
Taking them up to "0" deck when milling the block is somewhat common.
Going "above" deck is extremely rare, only done by a few thou, and harder to deal with.
It's just not necessary on street motors.
10:1 CR 350 SBC pistons are usually flat-tops.
I believe this is a deck height issue, unrelated to the heads.
IF it is the piston that hits the head, and not the valve.
Deck height corrections are usually done by rod length, piston compression height, and block decking.
If you need another .100 to .140" as stated, one of the parts selected is incorrect.
My guess is rod length or piston combo.
I have some .125" longer rods in my BBC, and they had to be matched to the correct pistons.
SBC rods are available in both 5.700 and 5.850" rod lengths.
That is .150" longer and seems to match your comments about needing another .100 or so.
You need the correct piston (pin height) for each rod length choice.
Pictures please!
Last edited by leigh1322; Oct 18, 2025 at 10:22 AM.
Drifting
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Seems OP may be a one-and-done; or perhaps has more pressing issues.
Also, most OE aka stock sbc pistons fit about 0.025" down in the hole (below deck); a spec reflected in Many year-specific docs at GM heritage center archives.
https://www.gm.com/heritage/archive/...formation-kits
Also, most OE aka stock sbc pistons fit about 0.025" down in the hole (below deck); a spec reflected in Many year-specific docs at GM heritage center archives.
https://www.gm.com/heritage/archive/...formation-kits
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