Topic: L82 Piston 'slap'
in Forum: C3 Engines
Already a Member?
Click Here to Login
Not yet a Member?
Click Here to Register for Free!
I would like some input. I recently purchased a 1975 L82. It runs great,..but has the "forged piston slap". While most of the engines with forged pistons usually have a slight "slap" effect,..this one is a little louder than most.
It has great compression,
It has great compression,
Runs very smooth,
Does not burn oil,
No "blow-by",
Runs cool.
However, the "piston slap" is a little concerning.
What is everyones take on this?
How can it be remedied?
What is everyones take on this?
How can it be remedied?
SPONSOR AD:: (Our Sponsors help support C3VR)
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
Piston "slap" is not exclusive to forged pistons...any material can do it.
Does it make this noise mainly when you first start it?
Does it get better as it warms up?
Can you hear it while your driving?
A certain amount of piston slap noise is exceptable in some cases. It really depends on the piston composition(forged, hypereutectic, etc), AND the amount of piston skirt to bore clearance.
My '85 has had a slap since I've had it...it's never gotten any worse, and it pretty much goes away when it gets to operating temp. Very common on the L89 engine. Not so much on an L-82, or L-48.
Joel Adams
C3VR Lifetime Member #56 
My Link 
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
It makes the noise all the time. Much worse when cold, but never completely goes away. I had a local performance engine builder listen to it, and he says it is normal for a forged crank / forged piston engine.
I don't like the diesel sound. Almost everyone asks, "what's wrong with it, why is it knocking".
Anything that can help the noise,.... Short of a rebuild and standard pistons?
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
The crank will have nothing to do with piston slap. The noise, if it is truly piston slap, is from the clearance between the piston skirt and the cylinder bore. Too much clearance allows the piston to rock back/forth in the bore, and the skirts will bang up against the sides of the cylinder bore.
While it's true that forged pistons will sometimes make more of a slap noise than other types of pistons, it is not exclusive to them.
High compression pistons will make the same noise...if the engine has an 11:1 or higher ratio, it will always make that noise....which is why they sound sorta like a sneezle.
Since you just bought the car, did you get any info on the engine components? A normal L-82 would not have a piston slap, unless the pistons and cylinder bore are worn. If the engine has been modified, it might be a good idea to have it disassembled and checked. Since we can't hear the noise over the innernet, it's hard to determine how bad it is, or even what it is. My suggestion is, if it bothers you, you should prolly tear the engine down and make sure it is not about to explode. 
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
Adams' Apple said: The crank will have nothing to do with piston slap. The noise, if it is truly piston slap, is from the clearance between the piston skirt and the cylinder bore. Too much clearance allows the piston to rock back/forth in the bore, and the skirts will bang up against the sides of the cylinder bore.
While it's true that forged pistons will sometimes make more of a slap noise than other types of pistons, it is not exclusive to them.
High compression pistons will make the same noise...if the engine has an 11:1 or higher ratio, it will always make that noise....which is why they sound sorta like a sneezle.
Since you just bought the car, did you get any info on the engine components? A normal L-82 would not have a piston slap, unless the pistons and cylinder bore are worn. If the engine has been modified, it might be a good idea to have it disassembled and checked. Since we can't hear the noise over the innernet, it's hard to determine how bad it is, or even what it is. My suggestion is, if it bothers you, you should prolly tear the engine down and make sure it is not about to explode.
I did not get any info on the engine. It was purchased through an auction, so I have very little back ground on the car. When I purchased the car, I assumed the noise was the normal sound you get from a forged piston engine, (high performance). However, I thought the noise should go away once the engine warmed up,...as had been my prior experience. This one does not ever quiten completely down. It sounds like a diesel all the time.
It does not sound like anything abut to come loose,...not like a rod or main bearing. It just sounds like piston slap.
I was just curious if anyone else has experienced this with the L-82 engines and if there was anything that could be done to lessen the sound,..i.e., a different oil viscosity or something.
in Forum: C3 Engines
SPONSOR AD: (Our Sponsors help support C3VR)