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Whats the deal with "Piston Slap"?

This is a discussion on Whats the deal with "Piston Slap"? within the Internal Engine forums, part of the LSx Technical Help Section category; had the motor back together for a few months now and on a cold start there is a knocking sound. ...

  1. #1
    T/A Everyday oreowizard's Avatar
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    Whats the deal with "Piston Slap"?

    had the motor back together for a few months now and on a cold start there is a knocking sound. When the temp comes up the sound goes away? Sometimes under hard acceleration I can hear what seems to be the same sound. Any help?

  2. #2
    Senior Member OnEbAdReDSS's Avatar
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    just how it is man... its because they heat the block up to a certain degree soo when the motor is cold the piston isn't fully seated

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    King 0f n00bz shady milkman's Avatar
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    x2 the problem was solved with ls2+ engines..they switched to a floating pin design for the heads. that is why its pivotal for ls1 owners to let their car's warm up to op. temp before they drive them

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    T/A Everyday oreowizard's Avatar
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    Will it hurt them if we dont let them warm up. No jumping in them and going?

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    King 0f n00bz shady milkman's Avatar
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    Quote Originally Posted by oreowizard View Post
    Will it hurt them if we dont let them warm up. No jumping in them and going?
    well it creates extra wear.

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    The Demon Demon_Z28's Avatar
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    ive never had the problem, I know I have a sticky vavle, buts its been like that since day 1.

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    Senior Member Danger731's Avatar
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    welcome to the slapper

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    Member 2-bowties&abird's Avatar
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    5.3's did it too. i wouldnt worry, almost 140,000 on my 99 z71 with no problems what so ever.

    demonz28 is one of the lucky ones, i have heard a few people say their's doesn't do it, but 9 out of 10 do.

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    T/A Everyday oreowizard's Avatar
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    well then Im gonna drive on!!! Still looking for a '02 SS M6 to make a monster out of or a WS6.

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    We'll be back... GatorSS's Avatar
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    Yes, it's quite common on many LS1s. Fortunately, mine has never had it.

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    Mine doesnt do it often. Just when its really cold or i havent started the car in a couple of days...

    Sounds like a exhaust leak

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    Ebaaaaaaaaa Speedy_Gonzales's Avatar
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    mine does it but with a new oil change its pretty quiet

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    Grand Imperial Wizard Sarge's Avatar
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    Piston Slap is caused by the piston skirts being too short and allowing the piston to rock in the cylinder. Doesn't hurt anything but sounds like hell. It goes away not when the engine warms up but when the oil film builds up enough to cushion the "slap" of the piston skirt against the side of the cylinder wall.

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    King 0f n00bz shady milkman's Avatar
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    Quote Originally Posted by Sarge View Post
    Piston Slap is caused by the piston skirts being too short and allowing the piston to rock in the cylinder. Doesn't hurt anything but sounds like hell. It goes away not when the engine warms up but when the oil film builds up enough to cushion the "slap" of the piston skirt against the side of the cylinder wall.
    ahh see i heard that too. i thought it was the pin design on the ls1 heads that caused the issue

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    Member danziger's Avatar
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    Quote Originally Posted by Sarge View Post
    Piston Slap is caused by the piston skirts being too short and allowing the piston to rock in the cylinder. Doesn't hurt anything but sounds like hell. It goes away not when the engine warms up but when the oil film builds up enough to cushion the "slap" of the piston skirt against the side of the cylinder wall.
    Sarge is correct. The short skirt and pin placement cause the piston to rock, "slapping" against the cylinder wall. As the piston warms it expands (as does the cylinder) and stabilizes. Now if your engine makes noises ouside of the normal noisy valvetrain when warmed up, there may be an issue.

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    Senior Member INMY01TA's Avatar
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    Quote Originally Posted by shady milkman View Post
    well it creates extra wear.
    This is a myth.

  17. #17
    Quote Originally Posted by INMY01TA View Post
    This is a myth.
    You're in Crofton right? If you feel like stopping by the Severn area sometime I'll disprove your comment You can actually see where the #1 piston was rocking in the bore, wearing the liner. Not coincidentally, all my piston slap noise in that engine was traced to the #1 or #3 cylinder with a stethoscope.

    To the OP, if you still hear the same noise when warmed up during acceleration, you may have more than just piston slap. A damaged oil pump pickup o-ring will cause oil aeration and cause a VERY similar noise. GM issued a TSB for this.

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    Member 2-bowties&abird's Avatar
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    Quote Originally Posted by Marc 85Z28 View Post
    You're in Crofton right? If you feel like stopping by the Severn area sometime I'll disprove your comment You can actually see where the #1 piston was rocking in the bore, wearing the liner. Not coincidentally, all my piston slap noise in that engine was traced to the #1 or #3 cylinder with a stethoscope.

    To the OP, if you still hear the same noise when warmed up during acceleration, you may have more than just piston slap. A damaged oil pump pickup o-ring will cause oil aeration and cause a VERY similar noise. GM issued a TSB for this.
    with how many miles on the motor?

  19. #19
    Quote Originally Posted by 2-bowties&abird View Post
    with how many miles on the motor?
    It was pulled at 38K. It had one of the worst cases of piston slap I've ever heard. I put in another shortblock (60K miles) and it has absolutely no piston slap at all. Oddly enough, the cylinder liners looked perfect.

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    Member 2-bowties&abird's Avatar
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    Quote Originally Posted by Marc 85Z28 View Post
    It was pulled at 38K. It had one of the worst cases of piston slap I've ever heard. I put in another shortblock (60K miles) and it has absolutely no piston slap at all. Oddly enough, the cylinder liners looked perfect.
    wow, i was expecting you to say like close to 200,000 miles or something.

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