My 78 has an annnoying knock. Exhaust is new....manifolds and back. Ive been told L82s that yr were known for piston slap.....any input on this.?

I've never heard that about the L-82 of any year, but I s'pose it's possible. The C4 model L-98s are BAD slappers when cold...just a fact. Doesn't hurt anything on them, tho.
Does yours only do it when cold, or does it have this noise all the time?
IF all the time, does the noise change when it warms up, or stay the same?
Does it sound like only one piston making the noise?

Piston "slap" is what happens if there is a little too much clearance between the piston and the cylinder bore. When the piston reaches TDC(top dead center) OR BTC(bottom dead center), it will rock in the bore, and create a noise as the piston barrel  hits the bore wall...kinda like a diesel engine running, but not as loud, or constant. Most of the time, this doesn't hurt anything. If it is really bad, it could eventually break the piston.


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Joel Adams
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My wife got piston slap me in da face!!

Are you sure it is piston slap and not a mechanical fuel pump noise??  That Luke guy up there ^ !!  He stole my answer!!

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actually.....just changed the fuel pump and its still there.Dosnt seem to be anything serious....just more anoying than anything....

 Put a bottle of S.T.P. or equivilent in the oil. Not to good for pistion skirts or the jug openings.

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Even with a new fuel pump, it could be the fuel pump push rod.  A worn rod will do this.  Pinch the fuel pump suction line for a minute or two.  If the noise stops before you run out of fuel, replace the rod.    Still could be a piston.

I just thoughted of sumptin else....it could be the dadgum heat riser valve. Those things can flutter really bad, and sound like an engine noise, especially the vacuum controlled ones.

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Joel Adams
C3VR Lifetime Member #56    
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

will try pinching the fuel line at the pump.....and the heat riser isnt coming into play i dont think...it was disconected when i bougt the car.....and its frozen open.....kinda stumped with this....i work in a new car dealership and have my techs do the work on my vette....one of them was with chevy in the late 70s....he says just drive the car....its nothing to worry about...oil pressure is good....car has plenty of power....only has 32 thousand miles on it

Rut roh sounds like carbon knock to me ....but there is a good remedy for this.Pull all the plugs and get some EOS at the gm dealer,fill all the spark plug holes with it and let it sit for 72 hours and then turn the motor over with the plugs out(it will be messy) put an old set of plugs in and let the motor warm up and then go hammer on it for a while put the fresh plugs back in and you should be in good shape