Trying to determine skirt scuff failure

[Mark O'Neal]

Too lose, or too tight

Too rich, or too lean.

[rrp]

Was a plate used during honing and did it get a chance to.relax before final touch up? That wall moves a bunch.if it was set at .0025 without a plate, it was too tight. Another thing was if the motor was purged during the filling of coolant. I've had guys fill it up and stick the cap on and didn't wait for the thermostat to open.

[peejay]

The cam dropping debris idea is interesting, especially since it looks like the scuffing is on the outside where the crank would throw anything coming off of the cam. You'd be able to tell if any of the debris stuck in the piston was magnetic.

I wonder how the plug wires were routed, myself...

[Dutchman]

Had a similar problem with a 425 olds,2 sets of Pistons both looking the same way . Another shop brought me the engine after they tried and the second set failed.
Mild build with very reasonable cr. I was buying parts from the same supplier and used their name brand timing chain,as did the previous shop. When I degreed the timing events the cam was 10 degrees retarded. When they shot the hole for the pin in the upper gear it was way off. Placed a straight edge across the gear dividing it in half and you could see how the alignment pin was off to one side.
Indicated it on the mill and shot a new hole 180 from the old hole and put a new alignment mark as well.
Cam degreed within 1 of advertised.
Both my new timing set and the previous builders set checked the same.
Just because parts are new,doesn't mean their any good.

[hoodeng]

I would expect material coming off the cam to spread at least to the two cylinders directly in line with the lobe failure , i would also expect the trash to be evident on all surfaces of the piston and bore diameter ,the fact it is localized to only one piston face and corresponding bore surface suggests the problem started with the piston being the first victim.

Is there any evidence of fuel residue/ washed surfaces in the inlet tract, back of inlet valve?

All that said ,further inspection will reveal all.

Cheers.

[bigmike]

The cam dropping debris idea is interesting, especially since it looks like the scuffing is on the outside where the crank would throw anything coming off of the cam. You'd be able to tell if any of the debris stuck in the piston was magnetic.

I wonder how the plug wires were routed, myself...

You thinking about spark crossfire?

[Tom Walker]

IF, the piston clearance is good, IF no overheat, if no vacuum leak to THAT cylinder, then it may be a good idea to check spark crosstalk possibility. Is the bore round and straight?

[crazycuda]

"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?

I checked the lobes on the cam and they all looked ok (have not put a dia ind on them yet to be positive) , Did the old rudimentary lifter face check and all the lifters still appear to have a crown on them.

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.

[/quote]

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

Yes they were I learned that lesson when kb first came out , the hard way

[crazycuda]

Forgot to add, after checking the bore where the worst scrub is on the cyl, it has shrunk .002 and that's right above where the water jack ends and it goes into solid cast for the main web.

[Geoff2]

Piston only scuffed on one side, other side good? Binding wrist pin or tight in one spot?

[Truckedup]

In my experience with piston seizures on air cooled bike engine the wrist pin always seizes in the piston and or floating rod after severe detention or pre ignition.Pre ignition almost always blows a hole in the piston crown... What I don't know what comes first, piston seizure or wrist pin seizure...

[crazycuda]

Saw this and thought it might be interesting to share and get others personal opinions.
https://www.memoparts.com/img/cms/Docum ... Failue.pdf

Also wanted to say thank you for everyone's input.
I think I might have found one of the culprits, I was looking at the steel valley pan gasket and noticed that the bottom of it (where it meets the intake port) was not crushed flat like the remainder of the gasket (the spray copper-coat was transferred to both surfaces though) there was oil on the back of that intake valve (#7). I might be reaching here but I am wondering if the engine was running hot from the highway drive (been in the 90's+ here), on decel the engine sucked in a bunch of oil mist from the valley. Mix that with an orig 79 Q jet and running non ported (manifold vac advance per factory setup) became my recipe for headache

[gunt]

My dyno guy blew up a load of engines on me , and from what i'm use to that ran lean heated the film of oil burnt it out expanded the piston and that's the lot

[ptuomov]

A long document that might be relevant:
http://www.hartech.org/images/downloads ... terim).pdf