Piston Slap
Drifting
Joined: Nov 2009
Posts: 1,385
Likes: 102
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
Safety Car
Joined: Jan 2014
Posts: 4,496
Likes: 856
From: Walhalla South Carolina
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.
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.
Racer
Joined: Feb 2014
Posts: 328
Likes: 0
From: South River NJ
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!
Le Mans Master
Joined: Jun 2009
Posts: 6,528
Likes: 752
From: South Dakota
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!
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
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.
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.
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Thread Starter
8th Gear
Joined: Mar 2014
Posts: 8
Likes: 0
From: Bay Area California
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.
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.
Pro
Joined: Aug 2012
Posts: 568
Likes: 1
From: Fresno California
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.
I seriously doubt that you will hear of anyone knurling the pistons on their ls1 around here.
Safety Car
Joined: Aug 2011
Posts: 4,449
Likes: 409
From: Portland OR
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
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
Tech Contributor
Joined: Oct 1999
Posts: 41,058
Likes: 9,821
From: Charlotte, NC (formerly Endicott, NY)
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
Bill
Pro
Joined: Aug 2012
Posts: 568
Likes: 1
From: Fresno California
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
Bill
Pro
Joined: Aug 2012
Posts: 568
Likes: 1
From: Fresno California
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.
Last edited by onlyavette; Jun 2, 2014 at 10:04 PM.
Pro
Joined: Mar 2006
Posts: 521
Likes: 23
From: Golden CO
Piston slap or?
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).
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).