Vet76te

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, runs very smooth, does not burn oil and no "blow-by". However, the "piston slap" is a little concrning.

What is everyones take on this?
How can it be remedied?

roadblock69

I ran an L-82 with piston slap for nearly 40,000 miles without issues. I certainly do not like listening to it though. Forged pistons are installed with a little more clearance than cast pistons as they are not as thermodynamically stable. The slap should go away after a short warm up period. If it does not go away, you should address the issue. Forged pistons can take that abuse but, there are limits.

bashcraft

If it gets bad enough it'll collapse the piston skirts and eventually break them off, or maybe it already has.

Only fix is bore the block and new pistons.

MelWff

What temperature does the engine run at when fully warmed up? What viscosity oil are you using? What oil pressure do you have when fully warmed up?

jb78L-82

My L-82 has had piston slap when cold since I bought the car in 1983 with 25,000-Now has 66,000 miles. The slap always goes away once the car warms up and is very normal for these and many other GM engines as long as it goes away after warm up. My 2001 Pontiac Grand Prix 3.1 V6 also piston slap when cold and ALWAYS has since day 1 new-140,000 miles on the engine now and runs like the engine was new.

mds3013

Mine is the same. Slap goes away after warm up. You could drop the oil pan and visually check the skirts and see which pistons you really have. mike...

Solid LT1

Noise should go away on full warm up otherwise you probably have worn pistons/bores and will have an oil gulper.

Vet76te

She runs right around 190 when full warm. Never gets any hotter. Oil pressure is slightly over 40lbs, (half way point on the gauge), when full warm abd idling. The rattle, (slap), never goes completely away.

Vet76te

The noise never goes completely away. But,..it does not use ANY oil. I have driven it a few hundred miles so far with no oil consumption yet.

No blow by, no leaks, no smoke and no oil consumption yet. It holds very good oil pressure and runs out oerfectly. Just sounds like a diesel running.

Vet76te

It runs just over 190 when fully warmed up.
I am running 20w50 oil right now.
It holds just over 40lbs oil pressure wen hot idiling.

540 vette

You should have heard the slap in my C5. It sounded like marbles in the engine until it warmed up.

Vet76te

What did you do about the noise?

cv67

Dont worry about it.

My 02 Silverado sounds like a diesel the slap is bad it doesnt hurt a thing.

There isnothing you can do about it except rebuild and use a piston with a tigheter piston to wall clearance.

These are performance motors youre going to hear a little something.

Vet76te

Does a particular oil viscosity help any? Thicker (20w50) or thinner (5w30)?

540 vette

Nothing it was normal for the LS 1. I eventually sold the car.

mds3013

From my experience that oil pressure is a little high for a stock engine. Nothing wrong with it though. Mine is only about 25lbs. at warm idle. Original L82 with 40,000 miles. Perhaps someone has changed the oil pump in the past? I would stick to 10w-30 oil. This of course has nothing to do with the noise. Just wondering if the engine is stock. I have verified that I have TRW pistons by removing the pan in the past but mainly to change the pan gasket. What is the history of the cars engine? mike...

Vet76te

I don't know any of the engine's history other than it is a matching numbers car and the engine number matches the VIN number,...so it is the original L-82 block at least.

It is VERY peppy,...runs out very strong.

I came from a classic car auction,...so I have no prior history on the car.

74 LS4-454

My 75 L82 came from the factory with the slap built in...
It ran fine for many years, never had an issue.....

cardo0

Try and google that for yourself. But i read a lot of posts saying it could be carbon on the pistons and a good fuel cleaner like SeaFoam will help. Others say stay away from SeaFoam. And others say use different oil additives or oils or even synthetic.

I leave it up to u to straighten it all out.
Please report back what works and what doesn't,
cardo

63split63

Change the oil and put in some 10 x 30 Ritella T or similar diesel type ans see what the results are .
Since you purchased it at an auction the seller may have put heavy oil in to reduce engine noise .
The idol oil pressure is high for a used engine and leads me to suspect heavy oil in the crankcase to reduce piston slap . You may not have forged pistons just collapsed skirts .

Bill