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#2522649 - 02/02/12 04:58 AM What is piston slap?
Russell Offline


Registered: 12/04/04
Posts: 2136
Loc: Lexington, KY
Also what kind of sound does it make? Does it go away as the engine warms up. I have been told my BMW has some piston slap at 210,000 miles and I should not worry about it.
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1995 BMW 525i M50TU 2.5L-6, abt 235K miles. Rebuilt trany.
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#2522667 - 02/02/12 05:30 AM Re: What is piston slap? [Re: Russell]
SteveSRT8 Offline


Registered: 10/10/08
Posts: 15403
Loc: Sunny Florida
Typical slap starts when the car is started, then quickly fades. One of our trucks with 400k miles on it has done it since new.

We own a lot of 'slappers', GM V8's, and they have proven to be very reliable for many hundreds of thousands of miles.
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#2522686 - 02/02/12 06:15 AM Re: What is piston slap? [Re: Russell]
Tegger Offline


Registered: 03/20/06
Posts: 1489
Loc: Canada
Originally Posted By: Russell
Also what kind of sound does it make?

A knocking noise.


Originally Posted By: Russell
Does it go away as the engine warms up.

Yes.

Originally Posted By: Russell
I have been told my BMW has some piston slap at 210,000 miles and I should not worry about it.

That's correct; it's no cause for concern, just annoying.

My understanding is that piston-slap incidents have increased in recent years as engine designs have changed to suit emissions and gas-mileage laws.
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#2522704 - 02/02/12 07:03 AM Re: What is piston slap? [Re: Tegger]
Turk Offline


Registered: 02/03/06
Posts: 8172
Loc: MN
Oh man, my 2000 Sierra 5.3L has Piston Slap so bad.....

I traded it in for a 2003 and during the eval they did I made sure it was quite warmed up.
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#2522744 - 02/02/12 07:59 AM Re: What is piston slap? [Re: Russell]
Corvette Owner Offline


Registered: 01/22/03
Posts: 1629
Loc: Waldorf, Maryland
Here is a great website about GM's piston slap.

GM's Piston Slap

Here is their definition:

Piston slap is nothing new to piston driven internal combustion engines and compressors. It is the secondary (sideways or perpendicular) movement of a piston against the side of a cylinder bore where the primary movement of a piston is intended to be parallel (up and down) to the cylinder bore. All piston driven internal combustion engines and compressors have a certain amount of piston slap.

Excessive piston slap occurs when the clearance between the piston and the cylinder bore is too great. The piston to cylinder bore clearance becomes too great either through wear, mismatched pistons and cylinder bores at manufacturing or, a combination of both. The audible noise associated with excessive piston slap is due to the perpendicular impact of the piston against the wall of the cylinder bore. Audible piston slap is typically loudest when the engine is first started up. The pistons then expand with heat reducing the piston to cylinder bore clearance thus, reducing the perpendicular impact of the piston against the cylinder wall and its resulting noise.



Edited by Corvette Owner (02/02/12 08:01 AM)
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#2522798 - 02/02/12 08:53 AM Re: What is piston slap? [Re: Russell]
daman Offline


Registered: 09/23/06
Posts: 10517
Loc: Bad Axe, MI
Basically it's your piston(skirts,very bottom edge)rocking back and forth in the bore when the piston is traveling up and down, the piston to bore clearance is too great. Usually once up to operating temp piston swells takes up that loose tolerance and the noise stops.

it's nothing to be overly concerned about.
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#2522810 - 02/02/12 09:10 AM Re: What is piston slap? [Re: Russell]
Smoky14 Offline


Registered: 04/23/03
Posts: 1002
Loc: Nowhere NM
Some Honda engines get piston slap when they get hot. The clearance cold is .0008 and the piston shrinks as it warms to provide adequate gap between pistion and cyl wall. Most pistons expand with heat and thus the noise at start up goes away as the engine warms.
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#2522847 - 02/02/12 10:05 AM Re: What is piston slap? [Re: Smoky14]
scurvy Offline


Registered: 02/19/07
Posts: 2277
Loc: Chicago IL USA
Originally Posted By: Smoky14
Some Honda engines get piston slap when they get hot. The clearance cold is .0008 and the piston shrinks as it warms to provide adequate gap between pistion and cyl wall.


What magical material is Honda making pistons out of that shrink as they heat up? Ice, quartz, cubic zirconium tungstate?

wink2

Hypereutectic pistons (AA 4032 or similar) still expand as they warm up.
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#2522858 - 02/02/12 10:15 AM Re: What is piston slap? [Re: Russell]
Vikas Offline


Registered: 07/22/05
Posts: 8444
Loc: NorthEast
But if you have piston slap because your piston to bore clearance is high then that implies your compression is being lost; correct???

Why would that not be a problem???

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#2522882 - 02/02/12 10:31 AM Re: What is piston slap? [Re: Vikas]
OVERKILL Offline


Registered: 04/28/08
Posts: 26820
Loc: Ontario, Canada
Originally Posted By: Vikas
But if you have piston slap because your piston to bore clearance is high then that implies your compression is being lost; correct???

Why would that not be a problem???


Because it quickly goes away?

The trend for it happening in more recent years appears to be because of the increasing short piston skirts. Compare an old SBC or SBF piston to one from an LSx or Modular and the height difference is HUGE.
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#2522904 - 02/02/12 10:52 AM Re: What is piston slap? [Re: Russell]
Russell Offline


Registered: 12/04/04
Posts: 2136
Loc: Lexington, KY
Thanks for the info. apparently not a real issue. Possibly just a sign of wear?


Edited by Russell (02/02/12 10:53 AM)
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1995 BMW 525i M50TU 2.5L-6, abt 235K miles. Rebuilt trany.
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#2522934 - 02/02/12 11:29 AM Re: What is piston slap? [Re: Russell]
SteveSRT8 Offline


Registered: 10/10/08
Posts: 15403
Loc: Sunny Florida
It can be a sign of wear. But most of it is coming from piston design and clearances.

Note that just because the engine slaps a bit does not mean any compression is lost. The rings do the sealing and oil control, our vans that slap use no oil and do not smoke... even with 400,000 miles.
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#2523229 - 02/02/12 04:25 PM Re: What is piston slap? [Re: Smoky14]
mechtech2 Offline


Registered: 09/05/06
Posts: 19479
Loc: Chicago Area
Originally Posted By: Smoky14
Some Honda engines get piston slap when they get hot. The clearance cold is .0008 and the piston shrinks as it warms to provide adequate gap between pistion and cyl wall. Most pistons expand with heat and thus the noise at start up goes away as the engine warms.


Huh??
No.

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#2523238 - 02/02/12 04:34 PM Re: What is piston slap? [Re: Russell]
SteveSRT8 Offline


Registered: 10/10/08
Posts: 15403
Loc: Sunny Florida
What? You didn't know that Honda has their own proprietary aluminum alloy that SHRINKS when heated?

In the frozen North these engines can't even be started!
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Best ET-12.79 @ 111 mph
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#2523324 - 02/02/12 05:59 PM Re: What is piston slap? [Re: Russell]
artificialist Offline


Registered: 09/23/07
Posts: 7043
Loc: Florida
Either that, or the bore expands faster than the piston expands in certain Honda engines.
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