HELP! Looking for some advice on a cylinder plating issue

[MAF143]

Friend of a friend dropped off a MS391 for me to look at. It actually started but wouldn't tune at all. I pulled the clutch cover off and the PTO side bearing is no good. I tore it down and the plating in the cylinder is torn off in strips below the strato ports. It's like the skirt of the piston was digging into the plating there at TDC. I ran a flapper in there and got rid of what little aluminum xfer was in there. Then I stuck a new piston in to see if it would hang on the rough area in there. It was totally smooth in the bore and didn't hang at all and was not sloppy and fit well. I did ease the bottom edge of the strato ports with some shaped diamond files cause that edge was like a razor. I'm not sure if easing the edges on the piston would also help it last a little longer.

The original piston is trashed where the plating chips just tore it UP... Chicken or the egg. Did the plastic in the bearing go first and allow the piston to hammer the plating till it bit in from the slop and vibration OR did the plating fail first and the chips destroy the plastic bearing carrier killing the bearing??? Possible debris ingested via the strato port, but I would think that might cause more damage above the strato's??? Rookie here just spit-balling...

I didn't see any plating issues in other areas of the cylinder and above the strato ports everything looks good to go. The saw owner doesn't want to spend the money on a new cylinder (even an aftermarket at $110).

Bottom Line:
Can this be put back together with a new piston, bearings, and seals and be run for a couple years or will it just kill itself shortly after running it? I can try to talk him into spending about 50 bucks over his cap price to get an aftermarket cylinder cause I'd hate to give him a "fixed" saw that won't last. I'll discuss it with him either way cause I always want to be up front with everyone, but I'm just looking for a little advice as to if it would last at all or not cause I'm not experienced with this type of failure. If it was my saw I would probably put it back together and run it till it pukes again to see how long it would last.

 

[beaglebriar]

Cylinder is junk. Don’t bother fixing it if the owner refuses to pay to have it done correctly

 

[Ketchup]

X10. FUBAR.

 

[Woodwackr]

Friend of a friend dropped off a MS391 for me to look at. It actually started but wouldn't tune at all. I pulled the clutch cover off and the PTO side bearing is no good. I tore it down and the plating in the cylinder is torn off in strips below the strato ports. It's like the skirt of the piston was digging into the plating there at TDC. I ran a flapper in there and got rid of what little aluminum xfer was in there. Then I stuck a new piston in to see if it would hang on the rough area in there. It was totally smooth in the bore and didn't hang at all and was not sloppy and fit well. I did ease the bottom edge of the strato ports with some shaped diamond files cause that edge was like a razor. I'm not sure if easing the edges on the piston would also help it last a little longer.

The original piston is trashed where the plating chips just tore it UP... Chicken or the egg. Did the plastic in the bearing go first and allow the piston to hammer the plating till it bit in from the slop and vibration OR did the plating fail first and the chips destroy the plastic bearing carrier killing the bearing??? Possible debris ingested via the strato port, but I would think that might cause more damage above the strato's??? Rookie here just spit-balling...

I didn't see any plating issues in other areas of the cylinder and above the strato ports everything looks good to go. The saw owner doesn't want to spend the money on a new cylinder (even an aftermarket at $110).

Bottom Line:
Can this be put back together with a new piston, bearings, and seals and be run for a couple years or will it just kill itself shortly after running it? I can try to talk him into spending about 50 bucks over his cap price to get an aftermarket cylinder cause I'd hate to give him a "fixed" saw that won't last. I'll discuss it with him either way cause I always want to be up front with everyone, but I'm just looking for a little advice as to if it would last at all or not cause I'm not experienced with this type of failure. If it was my saw I would probably put it back together and run it till it pukes again to see how long it would last.

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He’s dead, Jim..

 

[drf256]

If it was your own saw, you could try to save. All the damage is below the exhaust roof. That being said, that’s garbage.

Most importantly, the owner doesn’t want to put money into it. Anything you convince him to do will be your fault when it fails again.

I’d do nothing to saw. Odd place to score that much. You really have to split case and do both bearings. I’d also inspect the crank bearing and the pin bearing. Not worth it, round file it.

 

[lehman live edge slab]

What’s the ring look like? I almost think the ring hooked a bad bevel on port and it went south from there but I could be wrong.

 

[Ketchup]

What’s the ring look like? I almost think the ring hooked a bad bevel on port and it went south from there but I could be wrong.

Looks like the bearing failed, a ball caught on the top of the PTO upper and deformed the piston. Rings couldn’t fully seat, opened too far in the stratos, deformed and stripped the plating on the down stroke.

 

[MAF143]

Thanks for the insights. I was thinking the jug would be no good, but without experience with missing plating before I just wasn't sure. It's a clamshell and I have it all apart. The crank feels tight so I'll try to get him to either replace the cylinder or forget about it.

I don't think the rings ever go down below the strato ports. The bottom of the piston skirt is mostly where it is torn up and has a jagged razor edge on it after the carnage. The top of the plating damage begins at the spot where the piston skirt is at TDC. I'm just not sure what event happened first... Not that it matters at this point. He said it would heat up and quit, not a sudden catastrophic failure. Then after it cooled it would start again and he would cut with it till it quit again. He said he did that several times... Wishful thinking can lead to bad things sometimes...

I've been unable to locate an aftermarket jug that matches this one. It's 49mm 391 but all the ones I can find have a totally different intake manifold pattern. The one I'm working on uses the same intake tract as my 362 and looks nothing like the 311 / 391 aftermarket jug pictures I've seen. I used to own a 391 that I replaced with my 362 and was surprised at the time at them having basically the same intake tract except for the air cleaner.

I talked to the owner and went over it with him that it could totally fail again very soon without a new jug and I advised to junk it or part out the good stuff from it. A new OEM jug is way more money than he wants to spend. He wants me to put it back together with the minimum parts cost possible and he'll run it till it blows... He has several other saws and this one has sat for 3 years till he brought it to me.

Thanks again for the advice.

 

[Tor R]

If he has chainsaws he can use, I would have said I don't bother putting this junk cylinder on.
All you're doing by putting this on is wasting your time. Once they start flaking as much as this one has, it's unlikely to stop.