Copied from ls1.com . . . check it out .

Very interesting. I received a memo from GM explaining this after I took my 2001 Silverado for this problem. I purchased it before the SS. It has the 5.3L and it has piston slap, just like the SS. It has not affected performance at all, it now has 58000 miles. The memo read:

(GM MEMO START)

" COLD START ENGINE KNOCK
GEN III Engines 1998-present

It is characteristic to hear a knocking noise after starting a cold engine. Thiss noise will be present until the engine reaches full operating temperature. The lenght of this time and the degree of loudness will depend on: ambient temperature, engine temperature, interior selected and underhood sound deadening level of content.

Engineering has made changes to these engines that reduce rotating mass and internal friction. Specifically, this was done by a significant reduction in the lenght of the piston skirts, piston weight and wall thickness. These changes were done to improve fuel economy, meet customer performance expectations and adhere to government's environmental (EPA) and Cafe regulations.

Due to the thermal dynamics and the expansion rates of different metals and alloys, the pistons are shaped in a way that allows it to expand into a full circle. When cold the piston is not round. It is slightly oblong, its general shape is that of a figure eight "8". the lightweight materials used in the pistons expand at a different rate than the materials that make up the block and cylinder walls.

The factors result in internal clearances when the engine is cold that allow the piston to have a rocking motion in the cylinder, which is the cause of the sound(knock). When the engine reaches full operating temperature, these parts expand to their designed clearances and the knocking sound goes away. This typically will take 2-15 minutes depending on conditions. The clearances are designed to allow engines to operate in extreme temperatures from arctic to desert conditions and perform as required by their owners.

This noise is NOT a defect and does NOT affect the long term durability, function, or performance of the engine. "

(GM MEMO END)

 

I'm sure most of us have heard about the famous LS1 piston slap. Out of close to a thousand starts of my LS1 I might have heard it a few times. It went away after a couple of minutes, no problem. I only drive my LS1 through the summer months when the temp is warmer therefore I believe less of a temp change in my engine. Starting it from a cold night would present more of this piston slap I believe. I could be wrong, all I know is I really haven't noticed much of a difference in sound from start-up.

 

I think someone at GM has ripped me off! That memo was almost word for word the same as the post I put out on this 'so called' problem about 2 years ago on the SLP board...

Dave and Steve, I have had 2 starts since 2001 that I could hear any slapping at all. The 78 'vette L82 I raced (from new) had this forged piston slapping each and every cold start of its long life! And it ran strong for more than 4 years of continuous abuse. Its a non issue as far as I'm concerned.

 

I concur.

 

I just figured Id post it because there was alot of hype about piston slap spread around . Its pretty good info imo . . . . thanks larry

 

You know, I was just going to say that I have seen that exact statement (verbatum) released from GM a couple of times in the past three year Larry.

I have 4 GM vehicles right now. The LS1 5.7 V8, 8.1 Litre V8, and two 3.8 litre V6s. I also had two more V6s that I turned in 2 years ago. All all of them do have (or had) knock of some kind on start up. They were all 99+ vehicles too. Though its tough to seperate the knock from valvetrain noise too, it seems to be a trait in recent GM vehicles.

I'm not thirlled about the noise, but I don't think its a concern to me.

Thanks for the post though twitch. I like mechanical related posts.

 

The funny part? If you put Gen-II motor where they have Gen-III you have the exact same memo they released for the LT1 motor

 

How loud is piston slap?
I've never heard it before.........

 

Sounds to me like someone tapping a big wooden hammer againsed the block low down ( listening from about ten feet back ) . . . kinda muffled and follows a distinct pattern . I can hear it the best standing back a ways and to the side . . . so its not covered by the valve train or intake sucking sound .

 

I have a 99 Z28 with 50k miles on it with a good amount of abuse in its life, and no knock..

I just got a '04 Avalanche, so I guess we'll have to see if it develops knock.

 

Ken, I got it in my Avalanche. What engine did you get? I have the 2500 HD with the 8 litre. Good for about 8 mpg...

 

bah, don't want to hear it? plug your car into the wall everyday & get that block heater going.

 

hmm.. i dont think ive heard piston slap on my car.

maybe once or twice.

godbless LS1

 

Consider where the knock comes from: cam-ground pistons...
If you heat the block but not the pistons it will actually be a worse fit for start up than if they were all cold-soaked to the same temp! The piston needs to grow round from direct heating while running for it to change to its round shape. Ref: Level 2 Basic Engines, ala the late B. Comis...

 

I just have the 5.3L.. It seems to be getting a decent 15 mpg.. base 4x4 model with no cladding..

I'll actually be highly annoyed if it knocks.