I've read that LS1s are notorious for piston slap, due to too much tolerance in the rings. My 99 Z has mods, and was probably run pretty damn hard beforee I bought it. The noise is prominent on start up. I always let the car warm up to full temp, and the noise quiets down, but it's still there. It's hard to tell if the noise gets worse under load, as I have a Borla muffler, and it's pretty loud. Also, one of the pipes for the exhaust is sitting on a crossmember, and vibrates. (Gonna fix that).

I've heard of "gapless" rings, and wonder if this might solve the problem, or if I'm going to need to bore the cylinders due to the piston slapping around. Anyone who has experience with this, your input would be great. Also wondering if I would need to completely pull the motor, or if the work can be in-car.

 

im not to familiar with piston slap (just know what it is) but as for fixing id pull the engine it will make it alot easier than getting under the car then jumping up to get the piston out then reaching to get a hold of the crank to spin it then getting back under the car and so on also if your gunna get all in her id check out the cly walls and if there not to bad at least hone them to a nice mirror finish maybe replace all bearings... how many miles on this motor... maybe some new pistons if you have the funds now would be a perfect time for a rebuild maybe a few mods too

 

Piston slap is just that, the piston rocks a little in the bore and the skirt hits the cylinder wall. It's not unique to any specific engine, but here's what I understand about the LS1:
The pistons are hyperutectic, slightly out of round when cold, and when the engine heats up, they round themselves out (which can explain why your slap is less pronounced when it warms up). The rod pin is more centered up and down on the piston, so there is less side thrust which allows the piston more to float in the bore. Plus the piston skirt is short, and that allows the piston to rock more than a long skirt piston would. Why some LS1's slap and others don't (with all other things being equal) could possibly be due to a carbon buildup on top of the piston, more to one side than the other, which can throw it off balance and cause it to rock more. An internal engine cleaner, or a water injection treatment to remove carbon buildup might lessen the slap noise, if that's the cause.

 

http://pistonslap.com/ -- do a search, this keeps coming back up...

 

Another factor that is not mentioned above is the tallerances for the rings seem a little loose. They are not when the engine warms up Aluminum will expand more than iron will and the bores will actually get smaller at operating temp. (all engines do this but aluminum expands more) hyperutectic pistons are made of an aluminum alloy usually with silicone. There is more silicone mixed in with the aluminum than will naturally be allowed to mix together. How they do this is beyond me but it causes the piston to be extremely hard and to have a high strength to weight ratio and more predictable expansion qualities. This alloy allows for a smaller and lighter piston with higher strength than a forged piston.

GM recieved alot of complaints from LS1 owners about the piston slap noise. The 4.8L engines also experienced this problem as well. In 2001 GM redesigned the pistons in the LS1 and other 3rd gen engines with a longer skirt that was not as likely to make as much noise. The weight was only a couple of extra grams and really didnt hurt performance but a quiet engine makes for a quiet customer.

Massey

 

Flip of the coin man, my LS1 did it from day one right off the showroom floor. Same thing, would quiet down after the motor warmed up.

 

To the OP, has the car thrown any codes ? It seems like the knock sensors would pick up something like that.

 

No knock sensor errors. Never thought of that though. I suppose someone would have mentioned that also.

 

My knock sensors keep throwing up a code about every 500 miles or so and I never hear a damn thing. I've even had them replaced and it quit throwing them for a couple thousand miles but now it's back.

 

Engine knock has a specific harmonic that the sensors are calibrated to detect. The sensor can detect a knock well before we can hear it. Actually if we can hear the knock then it is strong enough to cause damage to the engine. Try running better fuel, fuel additives (safe for Cats). If you are burning any oil that will lower your ignition point of the fuel and cause knocking no matter what grade of fuel you use. It dont take much oil do cause this.

Massey