symptoms of piston slap

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99sidude

99sidude

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So my roommate is trying to diagnose what is wrong with his built ka24 240sx. Since the build, the engine always sounded like it had excessive piston slap. The last time he had the head off, there was visible wear along the thrust sides of the cylinder liners. You couldn't feel any difference in texture, but visibly it looks a little worn. He drove with the slapping built motor for a year N/A with no issues. This past summer he finally turbo'd the thing, and got it tuned professionally on aem ems to 408whp. Drove the thing for no more than 50 miles and the HG blew. He maybe did 5 or 6 hard high speed pulls with the thing. After going through multiple head gaskets, and finally getting the head milled (again), it finally seals. Now it's still smoking white/blue but not burning coolant. It's gotta be oil. Would piston slap cause severe oil burning? Keep in mind it ran perfect for a few thousand miles N/A with the slap noise. Just started when boosted.
 
Did he get the block decked? Are the cylinders still round?

Yes, excessive piston slap would insinuate loose clearances. Did it get quieter when the motor got warm?
 
Piston slap is fairly common in performance builds. If the engine already has already been broken in, one of two things happened:

a) ring gaps weren't set properly
b) the engine wasn't broken in properly

Either way, new rings are just about the only way to fix it that I know of.
 
It gets quieter when it warms up, but it still sounds like an early 90's gm v6 when you rev it up. I drove behind him today and it's putting out lots of blue and white smoke. It smokes the worst upon deceleration which makes me thing the oil control ring is toast. Upon a compression test we just did, it was 130 across the board when it should be 150. Adding a cap of oil down the spark plug hole brought it up to 150.

When the engine was built, I remember that he had the rings "gapped for a forced induction build". I'm guessing that they either screwed that up, or the motor was bored too large. He's running CP pistons btw, which I always thought were the best.
 
Who did the work? I'm going to guess the piston to wall clearances are too loose.

Even on performance builds, slap should go away when the engine has warmed up.
 
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CP's don't swell as much as JE's do, but that honestly shouldn't have anything to do with the build.

x2 on the piston to wall clearances being too close. It sounds like as soon as the rings get some heat in them and expand it quiets down, which makes sense, because that'd push the piston away from the cylinder wall..
 
K2e2vin said:
Who did the work? I'm going to guess the piston to wall clearances are too loose.

Even on performance builds, slap should go away when the engine has warmed up.
Click to expand...

Agreed. I have heard built bottom ends that are noisy from too much clearance. Especially the builds were people stick with factory size pistons and hone the cylinders. The sloppy clearance wear on the cylinders and will definetly create more blow by. Did he go with factory size pistons are they oversize? My built bottom end is super quite, even when cold. I went .10 over to clean the cylinders up all of the way.
 
He went like .30 over for the build if I'm not mistaken. What is weird is that when it was N/A, it was noisy, yet still had perfect compression and used no oil. Less than 100 miles into boost, the thing is totally screwed. It wasn't smoking right after he got it tuned either. I drove behind him on the way home.
 
K2e2vin said:
Who did the work? I'm going to guess the piston to wall clearances are too loose.

Even on performance builds, slap should go away when the engine has warmed up.
Click to expand...

:werd: :werd:

99sidude said:
He went like .30 over for the build if I'm not mistaken. What is weird is that when it was N/A, it was noisy, yet still had perfect compression and used no oil. Less than 100 miles into boost, the thing is totally screwed. It wasn't smoking right after he got it tuned either. I drove behind him on the way home.
Click to expand...

Honestly- sounds like the clearances were set too loose to begin with- and once the engine was under more pressure (literally), the flaws in workmanship began to show more. CP pistons are good- I doubt the pistons themselves are at fault- more likely the installation of those pistons.
 
Agreed. The good news is that it's not really my problem (not my car). The bad news is that the car gets to junk up my garage for another 6 months
 
If he didn't 'gap' the rings correctly they could've bound and cracked 1 or more ring land(s) causing a good scuff or 4.
Rings could've been installed upsidedown too.
I've even seen peeps hit the piston too hard when installing (and not enough of a chamfer on the bore) that it had chipped the molly off portions of the top ring, causing a very sharp edge and loose chips flying around.
Did they use a torque-plate when it was bored?
Were the pistons 'slightly used' from a buddy?

Anyhow, BLUE= oil, WHITE=water/coolant, BLACK=fuel on the color of the smoke.
It could be a few things wrong. BUT white is most definately coolant.
Cracked head?
Head-bolts stretched?

E
 
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