It's the lower portion of the piston (skirt) rocking against the cylinder wall. The sound is magnified in aluminum blocks. A piston has some of the same acoustics as that of a bell. Pistons with longer skirts are less prone to rock on the wrist pin under load. The LS1 piston has almost no skirt. Hence, it is a short piston and prone to rocking if there is much clearance. A near perfect tolerance will not allow the piston to rock enough to make the noise. Unfortunately the tolerances were too open on many engines.

 

On aluminum engines the tolerances have to be left more open because of the rate of expansion of the aluminum. If the tolerances were to tight then the minute that the motor warmed up the pistons would swell up and it would seize up.

 

Aren't there iron sleeves in the cylinders to combat that expansion in LSx engines?

 

The expansion is the piston itself. A miniscule difference between perfect clearance and enough to make the slap.

 

so then does the Z06 experience slap? The cutaway makes it look like there is no skirt at all?

 

You will also get piston slap from forged pistons because they take longer to warm up and expand in the bore. I had slap on both my 5.0s with no issues and don't really have any on my 2000 Z28. As stated above, with the LSx it is the due to wrist-pin location on a short-skirted piston. IMO, it is a non-issue if it goes away within a couple of minutes. Now if you have a knocking sound well after warm-up, that's a different story

 

Yup, my '99 with 78k on it does the same thing, but only when it's very cold outside (say below 30 degrees). You hear it when first starting it and it goes away after maybe 15 seconds or so. Car uses no oil. I'm told it's pretty normal. You'd have thought GM engineers would have figured this one out but hey, as long as it runs fine.

 

The 2001 Z06 did have piston slap issues. In 2002 GM changed the piston design slightly in the Z06 and the piston has a teflon coating.

My 02 seems to have some slap at cold start, but it's hard to tell if that's really it. A stethiscope doesn't isolate it to a cylinder. My noise is actually more at the front of the engine like maybe it's an idler pulley. I have no oil consumption issues between changes.

GM has a tech service bulletin in response to the complaint of engine knock or piston slap that indicates that a bad pickup tube o-ring can cause oil foaming that causes the indications. There is also a tech service bulletin regarding excessive oil consumption that indicates replacement of one ring on each piston.

 

I'm so glad I don't have any of these issues. I must have "the best Camaro Ever"

 

Mine also knocks when it is cold out. Like someone else said, only below about 40 I would say. Goes away in a couple minutes. I never let it just idle but I also don't gas it while driving until it goes away. I am going to have to monitor the oil a little more closely now though after reading this thread.

 

I have piston slap when the motor is cold and so do all of my friends and most of us are running nitrous and we have no problems, i have been spraying for about 35000K miles and it still runs like a champ, so i would buy the car if its a good deal, and dont even wory about a little piston slap its normal in the LS1

 

Hey, maybe my Z28 does have piston slap. I've never driven it in <40 degree weather.

It was in the 40's yesterday -- if the parking lot at my apartment and at work weren't completely caked with road salt, I might have gotten her out...

Time for spring! Get on with it!