Husqvarna 385xp piston damage with pics

[celliott]

Alright, so I got a pretty good deal on a 385xp. The compression was a bit low at 125-130psi, and the exhaust side of the piston looked a little scored, and pitted between the ring lands. It did run, though I didn't put it in wood to see how the power was. Other than that, it's about the cleanest saw I've ever seen used. It has a few layers of orange paint, you can tell it's been repainted a few times. 2001 model. I was wondering if it was so clean because it had been recently rebuilt, I had read the early 385's were known for bearing issues.

Well, finally got into tearing it down and checking out the piston. When I took the cylinder off, I found the impulse line was split on the bottom of the fitting, you couldn't see it otherwise. And then, the piston..

I've never seen a piston like this, on the internet or in saws I've worked on. It's pitted, and the top is worn down, the edges on the top of the piston are rounded over in places. Weird part is, there is no scoring I can feel on the cylinder. The cylinder may have been cleaned up in the past, I don't know. Didn't get any repair history when I bought it. The ports on the cylinder appear to have had the edges smoothed out. Also the top of the cylinder (combustion chamber correct word?) looks a bit "peppered" or pitted.
I did not find any metal or foreign objects in the crankcase.
The bearings have no play in them.
The rings appeared alright as well. OEM piston BTW.
Just a WAG, maybe the oem piston got reringed a few times, and it's just straight up worn out?
Or did the piston hit the top of the cylinder?
I really don't know what would have caused this  :P

What do you all think?  ???

I did clean the carbon off the top of the piston. It was carboned (not excessively) where the piston isn't pitted.

[celliott]

The saw DID have a cylinder gasket when I pulled the cylinder. Someone was probably going to ask  :D
I have a new meteor piston ordered, just wondering what to make of this damage and if it's cause for concern when I put the new piston in.

[ZeroJunk]

I think it is caused by detonation from too little squish. Maybe other things as well.

[sablatnic]

Take a VERY good look at the bearings. How is the air filter?

[Andyshine77]

Detonation from really bad fuel. The port edges are supposed to be chamfered, the rings would catch in them otherwise. If the piston was hitting the crown would look smashed and dented uniformly. Don't ask me how I know.   

[celliott]

Take a VERY good look at the bearings. How is the air filter?

No play in the bearings, but I'll see if I can get a mirror and check the inside of the bearings out.
The air filter was brand new when I got the saw  :

Detonation from really bad fuel. The port edges are supposed to be chamfered, the rings would catch in them otherwise. If the piston was hitting the crown would look smashed and dented uniformly. Don't ask me how I know.   

I know the port edges are supposed to be chamfered. Hard to see in the picture, but it looks like these edges were rounded over more than factory.

I had seen the madsens picture of detonation piston failure, and I didn't think mine was the same because it's so much more severe than the example they have  http://www.madsens1.com/saw_piston_fail.htm

How long would it take running bad fuel to make the piston get that bad? One tank? A couple fill ups?
I only run 93 octane in my saws, non-ethanol when I can find it, if I can't the fuel doesn't sit very long in the can before it gets dumped in my old truck.

[ZeroJunk]

When you get it back together make sure your squish is no less than 15 thousandths, or better 20 .

It's unlikely anybody shaved the jug, but who knows.

[Andyshine77]

Chris I don't know how long it would take to do that to a piston, it could be one tank or an accumulative effect. The rounding of the port edges could be from wear or honing. You can lightly run a sharp edge around but not in the ports, if it scratches the surface the plating has been worn through.

[sharkey]

Since you said it was painted a couple times that piston may have not come out of that cylinder.  The combustion chamber looks too clean to have been run very much.

When fuel ages all kinds of things can happen, one of which is the gas doesnt burn in a linear fashion and instead explodes.  The trapped heat can cause the top of the piston to liquify.  If the ignition has been advanced, via thinning the key or all together removed, set it back to stock until you become familiar with the way it runs.  Listen for ping.     

[celliott]

I can't verify for sure, but I would bet that the bottom end had been rebuilt at least once before. I visually inspected the bearings from inside of the crankcase and they looked fine to me. Still no play in the crank, or the big end bearing.
I almost wonder if the bottom end got done and the old piston put back in? But, I also have to wonder why anyone would put a piston in that kind of shape back in a saw. I kind of suspect this just due to how clean the saw is all over- the kind of clean you can only get when you totally tear a saw down.
The fuel that was in the saw when I got it is long gone.
I got the meteor piston last night, got it installed and everything back together but it was late so I didn't fire it up.

Sharkey, I'll pull the flywheel and check the key before I start it up. I don't believe it has a removable key, it's the husqvarna kind that is cast into the flywheel (I think).

Good thing I got a good deal on this saw or I might be a little upset at buying it  :D

[schmeg]

Deto. I've torn down several snowmobile engines that the pistons and squish areas look that way.

[Red Good]

Did you wire wheel the top of the piston off to clean it ? Looks like old damage from ingesting something . jmho Red 

[celliott]

Not wire wheel, steel wool and a bit of carb cleaner.

It's reassembled, it runs, compression after tuning was 150psi. Now I gotta run a few tanks through it and see how that goes.

[celliott]

Oh yeah and probably a muffler mod too  ;D