Trying to determine skirt scuff failure

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crazycuda
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Trying to determine skirt scuff failure

Post by crazycuda »

I was hoping I could get some insite / opinions. I am playing detective on this one. Its a 403 olds with KB hyp pistons, pw was set at .0025.
The engine had roughly 280 miles on it before the piston noise happened. Customer explained they were on the highway for about an hr, it was "doing great" came off an exit ramp and they heard what they called a lifter noise. Drove it home it got real bad, then had it towed back to me. Supposedly it never went over 220 and oil psi stayed above 40. Anyhow when I fired it up it was making a piston slap noise but also a horrid squeaking. Pulled the motor, tore it down today and aside from one piston (#7) being the damaged everything looked perfect (bearings and cyl walls on the other 7 cylinders).
IMG_0723a.JPG
Looking at the burnt oil spot on the back of the piston I am thinking it overheated and the cyl went out of round.
IMG_0724a.JPG
The cyl wall marks/ damage has me a little intrigued, also there is no scuffing at the bottom 1/4" of the bore still fresh hone marks and the skirts do drop below the bottom of the cylinder
IMG_0722a.JPG
Ill be checking bore taper dia etc tomorrow Time to call it quits for the day.
Any thoughts or ideas?
Another gremlin, I did notice the lifters were no longer pumped up but that's another issue
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norm
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Re: Trying to determine skirt scuff failure

Post by norm »

Piston to wall clearance was two and a half thousandths?
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Re: Trying to determine skirt scuff failure

Post by user-23911 »

You normally see that sort of piston damage on a diesel that's got a leaking injector .
The piston gets too hot because it's pre igniting.
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Re: Trying to determine skirt scuff failure

Post by hoodeng »

Is the scuff damage on both front and rear of the piston ,or only on the thrust face? also the inside heat witness, thrust face or opposite face or both?

Also is the bore damage both sides?

Bore damage at the bottom of the stroke is sometimes very little as the piston is stationary at that point no matter what revs or load, and more pronounced at the top of the stroke as load and heat are at the peak so oil film breakdown is greater even though the piston is still stationary.

Cheers.
crazycuda
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Re: Trying to determine skirt scuff failure

Post by crazycuda »

norm wrote: Mon Jul 02, 2018 7:52 pm Piston to wall clearance was two and a half thousandths?
spec sheet called for .002 to.0025 for 4.100 and larger. I err on the loose side and went with .0025
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Re: Trying to determine skirt scuff failure

Post by modok »

I was just talking about this last week.
Clear example of "four corner scuffing"
Ran out of clearance for some odd reason.

If it's just one.....must have been something different about that one, or do they all look like they got a little hot?
Perhaps overadvanced timing? One cylinder somehow running lean?
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Re: Trying to determine skirt scuff failure

Post by user-23911 »

It was probably put together with too much timing advance too lean and then pushed too hard for too long.
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Re: Trying to determine skirt scuff failure

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hoodeng wrote: Mon Jul 02, 2018 9:53 pm Is the scuff damage on both front and rear of the piston ,or only on the thrust face? also the inside heat witness, thrust face or opposite face or both?

Also is the bore damage both sides?

Bore damage at the bottom of the stroke is sometimes very little as the piston is stationary at that point no matter what revs or load, and more pronounced at the top of the stroke as load and heat are at the peak so oil film breakdown is greater even though the piston is still stationary.

Cheers.
Only on the Thrust face, the other side of the skirt looks like new.
The heat mark inside the piston is on the same side as the scuffing.
The other side of the bore has no damage

One thing that is interesting, to me, for the amount of damage to the skirt there is none above the oil ring on the piston itself. The oil ring still moves freely in its groove. Yet the bore is marked to the top of ring travel.
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Re: Trying to determine skirt scuff failure

Post by modok »

Oh, that is strange that the other side looks fine.

I suppose the root of the problem could have been something else we can't see now, and it just took a long time after that to get that hot and ugly. if it scuffs bad enough, it might overheat as a result.
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Re: Trying to determine skirt scuff failure

Post by hoodeng »

It sounds like detonation ,, there is not a piston bore clearance issue as there is no damage on the off side [also KB Hyper-eutectic pistons can use minimal clearance without issue] the reason there is bore marking to the top of the ring pack line in the bore is because all the rings run over the damaged area and pick up trash that then marks the bore to the top of the ring line. The piston itself is diminishing in diameter at each land above the oil ring hence no damage to the piston in that area.[run it long enough though and you will not be able to pick the order of failure].

Surprisingly i have seen bores with this type of damage clean up with minimal stock removal, obviously there wound need to be no distortion.

Just out of interest ,were the comp rings prepped to KB's recommendations?

The heat patches inside the piston are caused by the rapid heat rise of detonation and skirt friction caused by oil film breakdown that the rest of the piston assy and oil splash can not dissipate fast enough.

A comment on Modok's four corner scuffing , that could still be detonation causing a rapid heat rise that can not be dissipated [or anything that will make an uncontrolled rapid heat rise] ,but the piston is now getting to a point that it is now running out of bore clearance.

Cheers.
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Re: Trying to determine skirt scuff failure

Post by Charliesauto »

crazycuda wrote: Mon Jul 02, 2018 10:38 pm
hoodeng wrote: Mon Jul 02, 2018 9:53 pm Is the scuff damage on both front and rear of the piston ,or only on the thrust face? also the inside heat witness, thrust face or opposite face or both?

Also is the bore damage both sides?

Bore damage at the bottom of the stroke is sometimes very little as the piston is stationary at that point no matter what revs or load, and more pronounced at the top of the stroke as load and heat are at the peak so oil film breakdown is greater even though the piston is still stationary.

Cheers.
Only on the Thrust face, the other side of the skirt looks like new.
The heat mark inside the piston is on the same side as the scuffing.
The other side of the bore has no damage

One thing that is interesting, to me, for the amount of damage to the skirt there is none above the oil ring on the piston itself. The oil ring still moves freely in its groove. Yet the bore is marked to the top of ring travel.

Scuffing only one side of the skirt is often caused by "fuel wash". Try do investigate if they may have had an issue with the carb, possibly needle and seat problem causing fuel to slosh out of the vents. This could have been exaggerated when they were pulling constant G's on the exit ramp.

That being said, looking at your photo of the block, the scuffing appears to be on the inboard side of the cylinder. Fuel wash scuffing usually occurs on the outboard side because as the fuel puddles on the piston, it migrates to the lowest point(gravity).

The burn marks on the inside of your skirt are likely just from the piston being heated from the scuffing. The heat did not cause the scuffing, the scuffing caused the heat.

If you had a clearance issue, both skirts would be damaged.

FWIW, I am also in Apex, NC
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Re: Trying to determine skirt scuff failure

Post by modok »

I agree with above. Once the cylinder and piston are "exchanging paint" the rings pick up some junk as well. Yes it may clean up with a little honing.
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Re: Trying to determine skirt scuff failure

Post by gmrocket »

I never run those things that tight in a 403. Minimum.0035 for a hot street motor. .004"+ if it's track only for a big 4.375" bore..I hate them things.

It might also be fuel wash..if it has the stock style intake #7 is where the fuel runs too if it's flooded on start.
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Re: Trying to determine skirt scuff failure

Post by bigmike »

"I did notice the lifters were no longer pumped up but that's another issue" Just the one cylinder that had the scuff failure? Are the cam lobes OK on that cylinder?
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Re: Trying to determine skirt scuff failure

Post by Charliesauto »

bigmike wrote: Tue Jul 03, 2018 12:03 am "I did notice the lifters were no longer pumped up but that's another issue" Just the one cylinder that had the scuff failure? Are the cam lobes OK on that cylinder?
Good point

If it's a flat tappet and the lobes are going away = lots of debris in the skirt.
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