tj65

I have heard this term used with LS engines. what exactly is it and is it anything to be concerned about.

redbird555

Ls engines are assembled with somewhat loose piston tolerances. The piton skirts are also very short to cut down on weight, the result of this is that when the engine is cold the piston skirts will rock in the bore causing a sound like a diesel engine. Sometimes it goes away when warm as the metal expands other times its still there but to a minimum effect. Cars do this new and continue to run to 300k so no i wouldnt say its damaging

bjones7131

In other engines pistons are nearled (spelling maybe wrong). I built an engine and didn't nearl them and took it to a really knowledgable guy and he listened to the engine run and that is the first thing he asked, did you nearl the pistons and obviousley I didn't.
They make a tool that runs around the piston and it does the nearling or makes an impression on the piston. Just food for thought, I know it isn't a vette motor I am describing so don't torch me.

Mike Beck

Some do it, some don't. My Silverado did it from day one, now has 120K on it, burns no oil. My parents 2000 Silverado never did it. My 2002 Corvette doesn't do it. Just seems to be random luck (or lack-of). It isn't a problem, but it is annoying!

SG Lou

Since purcheased new in 03, my Silverado has been and is still doing it.
My 04 Coupe is doing it as well...neither have a problem !

bjones7131

My 01 vette and 04 tundra does it, no problems

QCVette

My '99 targa does it, but my '99 FRC doesn't. The targa quiets down within a minute or so. Neither car uses any oil and both run great. As you said, It isn't a problem, but it is annoying!

billla

Cast and hypereutectic pistons are fit very tight, with .001- piston-to bore clearance.

Forged pistons are fit .003 - .005 or even looser in some cases.

It just means at startup the bore clearance is pretty broad and you'll hear some ticking until the pistons come up to temp. It's not a big deal.

tj65

Thanks to everyone for the replies, really appreciate them all.

onlyavette

Knurled is what you talking about. The only reason to knurl pistons is to spare the expense of boring the block and replacing the pistons. As engines wear the bore can become tapered or larger in diameter. If an engine is right at the maximum wear limit or just a little under, it used to be common practice to knurl the pistons. Knurling a piston in effect increases the diameter of the piston in the skirt area which will help keep it from rocking and make it quieter. It isn't something that one would do to a high performance engine and if you have to do it to an engine that you had someone bore and replace the piston then someone screwed up and didn't fit the pistons properly. Good engine builders use inside and outside micrometers to fit every piston to make sure the pistons are at the exact clearance required for the type of piston that is being used.

I seriously doubt that you will hear of anyone knurling the pistons on their ls1 around here.

RonSSNova

Mine (2000) slaps bad enough it's embarrassing. Needs to have fully warm oil before it quits.
Ran a high ten though!

I'd like to replace the pistons and rods at some point.

The alum 5.3 in my Nova on the other hand is silent.

Ron

Bill Dearborn

The tapping goes away once the OIL temp (not coolant) gets in the range of 100 to 120 degrees. When there were a lot of C5s at track events you could hear the engines idling/knocking on cold mornings as the owners warmed them up before hitting the track. Some people say this isn't piston slap but it is what my LS1 and LS6 engines sounded like and what all the other C5s I knew about sounded like. They all did it when cold right from day 1. I took this video about 3 years ago and the 02Z in it is still running strong at track events.



Bill

onlyavette

It is normal and it is because of the piston design. Very short skirts. It is common to a lot of modern engines. My 08 Nissan Altima has made the same sound since it was new. My ls1 does it until it stats to warm up. Even after these engines are at full operating temperature the are pretty noisy when compared to the older cast iron small block Chevys and Fords. My wife's Shelby Gt has the Supercharged 5.4 and it has to be the noisiest engine I have ever heard. ( I've heard a few). It's the same thing with the 5.4. Very short piston skirts.

RonSSNova

I don't hear slap in Bills vid. Just valves......

onlyavette

Yes. I have to turn the sound way up to hear the audio in the video. I hear what you are talking about and mine is the same. I just thought I wasn't able to hear the piston noise that everyone calls "piston slap" which is a different sound than what I hear on the video. I have always thought that the lifters were making a lot of noise along with the pistons. They both go away by the time mine reaches full temperature. A lot of people aren't used to hearing it and worry until they hear more about it. It does take some getting used to.

DWC4

My new to me 2003 makes what I would describe as a knocking noise, not like valves or injectors at all, when cold. And as said above, once the oil comes to temp - and never again on a hot day even if it sits for many houses - does it do it again. I mentioned this on another thread here some months back and was told 'piston slap' and the explanations above are terrific and make sense.

But I don't think that's what mine is so I'll take a vid in the a.m. and see what your opinions are. Runs great, good mileage but that sound in the morning unnerves me no matter what.

Mine's more like a 'chock chock chock' sound. (I'm sure you found THAT helpful).