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Old 06-08-2008, 01:18 AM   #1
enkeivetteenkeivette is offline
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Default Piston rock.
So I'm measuring my piston to deck height today so I can figure out exactly what my static CR is. I hold the straight edge over the center of the piston, and I can barely fit a 25/1000 feeler guage in there, move the straight edge to the top of the piston, and the feeler guages doesn't fit!

So I look closely and I notice that the piston is actually tilting forward! Come to find out that this is actually normal. Pistons (especially forged pistons) have enough clearance to pivot off the rod and rock back and forth in the bore. So I push on the top of the piston, and it tlits back!

Sorry, I just found that really surprising.
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Old 06-08-2008, 10:29 AM   #2
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Originally Posted by enkeivette View Post
So I'm measuring my piston to deck height today so I can figure out exactly what my static CR is. I hold the straight edge over the center of the piston, and I can barely fit a 25/1000 feeler guage in there, move the straight edge to the top of the piston, and the feeler guages doesn't fit!

So I look closely and I notice that the piston is actually tilting forward! Come to find out that this is actually normal. Pistons (especially forged pistons) have enough clearance to pivot off the rod and rock back and forth in the bore. So I push on the top of the piston, and it tlits back!

Sorry, I just found that really surprising.
This is only when it's cold. Forged pistons expand much more than cast under heat, that's why that "cold" tolerance is built into them. It's also why when putting forged pistons into a motor with knock sensors tuned for cast (like a stock LS1), you have to change their knock sensitivity or some such thing or the computer will unnecessarily pull timing and dump full. It's also why you don't want to jump all over a forged motor until it is thoroughly warmed up.
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Old 06-08-2008, 11:48 AM   #3
enkeivetteenkeivette is offline
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Very true, but still I had no idea there was that kind of tolerance.
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Old 06-09-2008, 10:25 AM   #4
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Very true, but still I had no idea there was that kind of tolerance.
Clearence is .004 to .0065. Yes, the piston does rock.
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Old 06-09-2008, 11:40 AM   #5
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Clearence is .004 to .0065. Yes, the piston does rock.
What he describes "sounds" like a lot more. Wonder if he's got issues. Well, I know he's got issues but you know what I mean.
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Old 06-10-2008, 08:03 AM   #6
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What he describes "sounds" like a lot more. Wonder if he's got issues. Well, I know he's got issues but you know what I mean.
The .004 to .0065 is at 90* of wrist pin. The rocking of the piston is normal. Weird, but normal.

Weird is relative to what is normal.
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