Any one with forged pistons have a problem with pistom slap at start up? i'm installing 8.8:1 forged and eagle rods in the next week and am haveing some second thoughts. If you have the same setup please post any problems with motor after rebuild (beside improper instalation)i know that has been asked hundreds of times before, sorry

Piston slap is caused by too much cylinder wall-to-piston clearance. The best way to eliminate this problem is taking the block and the pistons to the machine shop. They will measure each piston and will bore the block to each given piston. They can even match the piston with each cylinder. You can ask this procedure, but it maybe cost a little extra money.
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1995 Eclipse RS-T
GT35R, SPEC Stage 5 clutch
E85 Fuel(Helping the Ecosystem)
MS, v3.0, 029t
10.87 @ 135

Piston slap is normal and even with manufacturer spec clearances your motor will still be a little noisy when cold. There is nothing wrong with it. Piston slap gives me wood.

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I had really bad piston slap once.



It turns out that if you melt a piston, it actually becomes a problem.

well that would be sutoble for hypertunic cast pistons , but forgings by nature need a large bore wall clearance becuase at tempatures reached by boosting and or high compresion the pistons swell to meet the bore size , that is the cause of the piston slap ant low temps like startup. But I may be wrong about that this was the fact of forgings of the past as outlined in "Maximum Boost" by corky bell

The piston slap will eventullay cause the piston to fail makeing forged pistons undesirible for a car that is driven daily. but i may be wrong about newer forged pistons.

I think different. You shouldn't have piston slap at all. This will eventually damage the piston and cylinder. A good machine shop will know how much to bore. Check with your machine shop, ask them, get information.

1995 Eclipse RS-T
GT35R, SPEC Stage 5 clutch
E85 Fuel(Helping the Ecosystem)
MS, v3.0, 029t
10.87 @ 135

Forged pistons swell when hot ad if bore is insufficent the piston rings will act like a brake . this will most deffently cause premature failure. has any one ever heard this?

Yes. That will happen to pistons with insufficient bore clearance.

Corky Bell is not wrong per se, but he exagerates the repercusions of running forged pistons in a street car. There is, in all actuality, very little issue with longevity when running properly installed forged pistons. I have yet to see minor cold-start piston slap cause any damage in a 420A bore. Case in point - my first motor: 30,000+ miles, cold start piston slap, when i tore it down there was very minimal scuffing of the skirt, no scuffing of the bore, crosshatch still showing. You have nothing to worry about.

And YES, even with JE SPECS, you will have piston slap when the engine is cold. If you don't, your clearances are probably too tight, or you're running alloy pistons with a lot of silicon content, which makes them brittle and not much better than cast anyway.

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Thank you,that makes me feel much better.thats the answer i was looking for.

besides theres plenty of diesel trucks out there with piston slap running for LOOONG hours at a time.

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the slap alone isn't the problem, diesel trucks are engineered to run under those conditions, the dodge/crysl 420a 's designers most likley didn't take into account that later on down the line a few weekend speed tuners would rip out thier rotating assembly and stuff in low compression forged alloy pistons, but you do have a point .


my only concern is with the long term durability of the forgeings

and no one seems to have had too many problems. But then again how many miles are on your motors after your rebuilds?

I have over 50,000 miles on my rebuild and so far so good.

Jason
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Yup, I have the loudest piston slap anyone has ever heard. Quite loud when the engine is cold and still audible when it's warmed up. I was so convinced that it was lifters so I replaced them, but it never went away. But I have 32k miles on my built engine with no problems, and I had the head off 2 weeks ago and the pistons and cylinder walls still looked great!

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01 Audi A6 2.7T
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yeah mine slaps all the time, I wish it would pure like a normal rice rocket but then again its not a normal car hehe. think of it this way if you bore the block to be perfectly the same size as the piston and now we have these highspeed forged aluminum pistons that when heated have a faster thermal expansion rate than that of cast pistons... hmm put two and two together you have a seized engine! That clearance may very well save your engine if lets say your engine gets too hot or something.

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Yea Mine sounds like a diesel truck when Its idleing. Cold or hot does not make a diffrence only time its quiet is when i drive the hell out of it. I pulled the stock slugs out and put wiseco and eagle rods in. That was 3 years ago and the Motor is still strong as ever was.

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i have slight piston slap when cold. and only when im driving it cold. if im idling i cant hear it. cp pistons