Piston-head clearance...how close?
#2657436
05/21/19 08:28 AM
05/21/19 08:28 AM
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Joined: Jan 2003
Posts: 13,354
Marion, South Carolina [><]
an8sec70cuda
OP
I Live Here
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OP
I Live Here
Joined: Jan 2003
Posts: 13,354
Marion, South Carolina [><]
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How close have you run piston to head clearance? Application is steel rods, flattop pistons, and less than 6500 RPM.
Seems most like to see .035-.040" minimum. I've run one at .035" and never had an issue.
Just curious to see if anyone has run them tighter?
CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax
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Re: Piston-head clearance...how close?
[Re: an8sec70cuda]
#2657483
05/21/19 10:25 AM
05/21/19 10:25 AM
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Joined: Oct 2007
Posts: 4,206
New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,206
New York
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Being slightly too close (.030") causes damage not easily detected: closes up the top ring groove and inhibits ring seal.
Boffin Emeritus
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Re: Piston-head clearance...how close?
[Re: an8sec70cuda]
#2657557
05/21/19 12:52 PM
05/21/19 12:52 PM
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Joined: Aug 2003
Posts: 43,115
Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,115
Bend,OR USA
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I saw witness marks on all the outer edges of my old pump gas motor pistons at .035, none after changing it to .040+  The ring lands where fine, no damage but the edges where shiny, no carbon and the same marks on the heads  that motor got shifted at or above 7000 RPM on every run also so 
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Piston-head clearance...how close?
[Re: Cab_Burge]
#2657659
05/21/19 05:48 PM
05/21/19 05:48 PM
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Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
BradH
Taking time off to work on my car
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Taking time off to work on my car
Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
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I saw witness marks on all the outer edges of my old pump gas motor pistons at .035, none after changing it to .040+ The ring lands where fine, no damage but the edges where shiny, no carbon and the same marks on the heads that motor got shifted at or above 7000 RPM on every run also so Mine's been good with .040", even with some piston-to-bore clearances looser than I'd prefer. I wouldn't feel comfortable with much less, having seem other engines with witness marks like Cab mentioned when running a tad too close.
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Re: Piston-head clearance...how close?
[Re: Cab_Burge]
#2657714
05/21/19 07:28 PM
05/21/19 07:28 PM
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Joined: Oct 2007
Posts: 4,206
New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,206
New York
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Compression height, skirt length, and rod ratio all affect how much extra P-H you need, but .040" is pretty much safe in everything.
Jenkins said 40 years ago don't run closer than you need for knock suppression - any closer = pumping loss. Some engines with small bores and excellent swirl can be knock-protected at .060".
Boffin Emeritus
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Re: Piston-head clearance...how close?
[Re: polyspheric]
#2657922
05/22/19 09:50 AM
05/22/19 09:50 AM
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Joined: Dec 2006
Posts: 1,178
Indy
FlyFish
super stock
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super stock
Joined: Dec 2006
Posts: 1,178
Indy
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The 408 motor I just built for my Barracuda is 0.033 (piston is 0.005 out of the hole). Just got it running on Saturday, drove straight to the drag strip. Only made 5 passes on it (I lost in rd3). It held together and got me back home....I call that a win, lol. Ran 6.59 in bad air, on the brakes (would have been a 6.58). The car kept getting faster on every pass...even as the air got worse.
Edit: I shift at about 6500.
Last edited by FlyFish; 05/22/19 09:52 AM.
67 Barracuda street car, 408, e85, 1.38 60', 6.44 @105.9 in the 1/8 mile, 10.19 @130.5 in the 1/4...so far....
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Re: Piston-head clearance...how close?
[Re: FlyFish]
#2658024
05/22/19 02:04 PM
05/22/19 02:04 PM
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Joined: Aug 2003
Posts: 43,115
Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,115
Bend,OR USA
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All the SB Mopar I've raced and help drag race always made more power after 7 to 10 runs or pulls on a engine dyno to seat the rings fully Once they level off we start to tune for max power 
Last edited by Cab_Burge; 05/22/19 02:05 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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