When does piston slap become a problem?

[c good]

200 k miles on my 2002 Envoy. It's had a slight amount of piston slap since I bought it 13 years ago. It settles down when warmed up but is still slightly present.

How long can one run with piston slap before it becomes a problem? What would be signs of an increasing problem other than a little more noise?

 

[mrrsm]

Some additional Piston Slap Threads:

gmtnation piston slap at DuckDuckGo

DuckDuckGo. Privacy, Simplified.

duckduckgo.com

 

[c good]

Thanks Mrrsm!

 

[Mooseman]

And be sure it's actual piston slap. Although not on a 4.2, I had what I thought was piston slap in my Saab with the 5.3, which turned into rod knock. Mine was the other way around where it would get noisier when it was warmer. It did it the whole time I owned it for 6 years (bought used) and eventually blew the rod while towing when huffing uphill in 2nd gear.

You could get an oil analysis done at your next oil change. That should reveal any damage going on.

 

[Schurkey]

And be sure it's actual piston slap...

...You could get an oil analysis done at your next oil change. That should reveal any damage going on.

^^^ Yes. BIG fan of oil analysis.

GM has a long, sad history with piston slap. Theoretically--according to GM--piston slap is harmless and can go on forever.

In reality, it may make the knock sensor unhappy, which retards timing, which reduces fuel economy and increases emissions. But it'll take the EPA to do something about it, because if GM acknowledges the harm of piston slap, they're gonna be sued for a bazillion dollars for ignoring it for decades.

Good luck.

 

[Dr.Fiero]

I've had slap in my 4.2 since I bought it (cold... goes away...).

Then I thought it was getting bad. Worse. Worser (not a word)!
Well, it turned out that a water pump dying sounds like slap getting really bad!! Haha.

Turns out you can partially cure 'slap' by replacing the water pump.

 

[mrrsm]

The important thing to understand is that "Piston Slap" can occur in all manner of combustion engines...However, it's Origins begin with The First, Naturally Balanced Engine ever made: The In-Line Six Cylinder Engine.

The problem at the heart of this issue ...is the need to have the Wrist Pins of the Pistons.. Moved Ever So SLIGHTLY -=Off Center=- in order to encourage the engine crankshaft to maintain its ONE Rotational Direction... and continue moving Downwards on the Power Strokes with no risk of it going against the Crankshaft Journals in the Opposite Direction. This condition persists even to this day within the design of the GM 4200 Vortec In-Line LL8 Motor. lead by Mr. Ron Kociba back in the Late 1990s:

GM's Vortec 4200 I-6 Named 2005 Ward's 10 Best Engine

Popular SUV Engine Honored Each Year Since Inception In 2001 PONTIAC, Mich. December 15, 2004; For the fourth consecutive year, General Motors' Vortec 4200 inline six-cylinder engine has been recognized as a Ward's Communications "10 Best Engine." When it was introduced in 2002...

www.theautochannel.com

Consequently, when the Piston crests the "Top Dead Center Hill Point" , its Off-Center Design will cause (coax) the Piston Head to *Bobble* slightly from One Side of the Upper Combustion Chamber to the Other Side, very much like a "Slight Piston Slap" that dominates inside of ALL In-Line Six Cylinders as the Six Pistons make this dramatic transition in rotation through the 720 Degrees of Four Stroke Combustion.

Over Time, Trapped Upper Compression Rings and Worn In Cylinders will accentuate this *Slight Bobble* into a more decisive "SLAP!" Sound that was encouraged to occur by the very nature of the necessity of having the Off-Set Wrist Pin Position built into its design.

Learn about every Engine Layout in just one video | V-W-X-U-H Engines : The Engineers Post : Free Download, Borrow, and Streaming : Internet Archive

archive.org

This Video will show some of the earlier Chevrolet In-Line Six Cylinder Engine "innards" and help to illustrate the commonality of designs shared by ALL of these remarkable motors:

 

[Schurkey]

The important thing to understand is that "Piston Slap" can occur in all manner of combustion engines...However, it's Origins begin with The First, Naturally Balanced Engine ever made: The In-Line Six Cylinder Engine...

Balance has nothing to do with piston slap. An un-balanced engine may have a shorter service life due to other factors.

Piston slap is due to an ill-fitting piston in the bore. This ill-fitment can be because of bore wear, but is more likely due to the piston itself. Offsetting the piston pin can reduce the force of transition from minor to major thrust surface; i.e., from one side of the cylinder to the other as the rod passes TDC and BDC, except BDC doesn't have combustion pressure so the forces are inherently less and therefore the transition is more quiet.

In terms of GM piston slap, it's because the pistons are not properly supported by the piston skirts--the skirts have been minimized in an effort to reduce weight and friction.

Offsetting the piston pin has been done in cast pistons since Fido's great-great-great Granddoggie was a pup. Forged pistons are rarely, if ever designed with piston pin offset; it's accepted that the greater hot expansion of the piston (and therefore greater cold clearance) will inherently cause piston slap until the piston expands from heat, and therefore an offset pin is not enough of a band-aid to help. Engines with forged pistons tended to be of higher output than cast-piston engines, and the more-pure geometry of an on-center wrist pin is seen as a performance advantage.

Worst piston-slap I ever heard? I worked for a school-bus company once upon a time. They had a piece-of-crap van that used a Ford 302 engine. When it wore-out, they bored the cylinders .020, and then had the original pistons "Knurled" to take up the extra clearance. "Knurling" pistons (and valve guides) was old-technology in the '60s; (and ENTIRELY un-acceptable after that) a half-assed method of tightening small deviations in clearance, never intended to take up that much clearance. It was used with only moderate success to take up a few thousandths, not twenty thousandths. That poor engine knocked like the rod bearings were gone.

 

[c good]

So far I'm OK with it the way it is. It slaps when cold, then when it warms up to operating temperature I can barely hear it. I'm going to send in an oil sample on my next oil change to see if it's anything possibly worse, like a rod bearing or something. I don't think it is as it mostly goes away when warmed up. I can live with it for now.