What do you think caused this damage? (PISTON PIC)

[biggus]

This is a pic of a piston from a STIHL HT75 pole saw I've been working on. I can see no sign as to what caused the damage, all the bearings are intact, circlip is there, no metal at the bottom of the crankcase etc.. The cylinder seemed fine (different thread) - tried the muriatic acid treatment, new piston - still low compression (actually it rose from 70 to 90 psi after the acid treatment and new piston) so, as some of you may have seen in my other thread, I just got in a new aftermarket piston/cylinder kit. But I'm still a little puzzled as to what caused this mess in the first place. I've seen pics of scored pistons due to overheating but this to me does not look like that - but I only have google knowledge in that regard - so could marks this deep be caused by overheating?

As always I appreciate hearing and learning from everyone who would like to chime in.

Thanks,

 

[Somesawguy]

It looks like something was in there. Could part of the muffler broken off?

 

[nmurph]

Overheating didn't cause that damage. Where's the upper ring pin? I don't see it. That was caused by something very hard getting caught bw the wall and piston.

 

[biggus]

Overheating didn't cause that damage. Where's the upper ring pin? I don't see it. That was caused by something very hard getting caught bw the wall and piston.

Upper ring pin is still in though not shown.

It looks like something was in there. Could part of the muffler broken off?

It's possible though there is nothing obvious. I will take a closer look at the muffler when time permits.

 

[Somesawguy]

This may be a dumb question, but is it possible that some dirt found it's way into the muffler?

 

[blsnelling]

Looks to me like it ingested something through the intake.

 

[Somesawguy]

Looks to me like it ingested something through the intake.

Good point. Something like the butterfly screw?

 

[biggus]

Okay, well I know no one can tell exactly what caused it but it helps to know what didn't cause it. I just wanted to make sure I completely ruled out the overheating part. To me it looked like the needle bearing lost some needles. When I taker her apart again I'll check the muffler, re-check the needle bearings, and run a magnet at the bottom of the case again just to make sure and to check for more clues.

Anything else I should check while she's apart?

 

[3000 FPS]

I am very surprised that the piston has grooves in it that deep and the cylinder was ok. Plus you say that it still has low compression after a new piston and ring. Not saying it is not true just suprised is all.

 

[Jacob J.]

Check the big-end rod bearing to see if that came apart. Also, an electrode could have broken off a spark plug
and caused that. I've seen that happen in the smaller engines before.

 

[biggus]

I am very surprised that the piston has grooves in it that deep and the cylinder was ok. Plus you say that it still has low compression after a new piston and ring. Not saying it is not true just suprised is all.

When I install the new kit I can take some pics. There were some darker colored smear marks which wouldn't even catch a finger nail. Everything was smooth which is why I originally just ordered a new piston and installed it w/o doing anything other than deglaze the old cylinder with a scotchbrite pad. The compression was at about 90 psi and after posting here someone mentioned metal transfer (I didn't even know what that was then) so I took it back apart and cleaned up the smear marks w/ acid. Even though the smears wouldn't catch a fingernail you can see where it was scuffing the new piston ever so slightly. It cleaned up nice but put it all back together and 90 psi. I figured the cylinder is just plain worn out and that getting a new cylinder and piston (since you can't order just the cylinder) will solve any problems.

 

[trappermike]

That is definitely not seizure damage but damage from some metal debris going through the motor.
Is that photo the intake side or exhaust side of the piston? There's a chance something got sucked into the intake port,but usually in these cases I've found that it's metal debris from the cages in the main bearings disintegrating. With the P&C off,look at the main bearings really closely with a flashlight,while rotating the crank around slowly,look for a twisted and chewed up bearing cage,or the balls piled together in one place with no cage.
Like someone else suggested I have seen some loose junk in the muffler fall into the cylinder,or check the entire intake,see if something(Carb screw,air filter screen etc.) came loose and went through... Rarely the little end on a piston circlip can break off and cause those gouges.
You don't want to assemble it with new parts until you find out where that stuff came from!

 

[biggus]

Check the big-end rod bearing to see if that came apart. Also, an electrode could have broken off a spark plug
and caused that. I've seen that happen in the smaller engines before.

The 'end rod bearing'? The one where the connecting rod attaches to the crankshaft? Funny but my IPL is not even showing one there. It's only calling out the two crank bearings and the needle cage where the wrist pin goes.

I didn't think of the spark plug electrode. That's a possibility - or maybe someone was swapping it out and got some junk on top of the piston. I'll ask more questions when I see the guy again.

 

[biggus]

That is definitely not seizure damage but damage from some metal debris going through the motor.
Is that photo the intake side or exhaust side of the piston? There's a chance something got sucked into the intake port,but usually in these cases I've found that it's metal debris from the cages in the main bearings disintegrating. With the P&C off,look at the main bearings really closely with a flashlight,while rotating the crank around slowly,look for a twisted and chewed up bearing cage,or the balls piled together in one place with no cage.
Like someone else suggested I have seen some loose junk in the muffler fall into the cylinder,or check the entire intake,see if something(Carb screw,air filter screen etc.) came loose and went through...
You don't want to assemble it with new parts until you find out where that stuff came from!

Sorry, I should've mentioned the pic is the exhaust side of the piston.

I will definitely take a closer look at all the bearings when I do this - I didn't spin the crank when I did it last time and my inspection mirror was too large to fit down there. I'll be more thorough next time.

 

[trappermike]

I agree with 3000fps,since you've only got 90 psi with a new piston,I'm wondering if there isn't a groove left in the cylinder matching the ones on the piston,seems likely.

 

[Jacob J.]

The 'end rod bearing'? The one where the connecting rod attaches to the crankshaft? Funny but my IPL is not even showing one there. It's only calling out the two crank bearings and the needle cage where the wrist pin goes.

I didn't think of the spark plug electrode. That's a possibility - or maybe someone was swapping it out and got some junk on top of the piston. I'll ask more questions when I see the guy again.

Yes, that's where the connecting rod is attached to the crankshaft. Your IPL doesn't show it but there's a caged bearing there. I'll bet it's blown.

 

[biggus]

Yes, that's where the connecting rod is attached to the crankshaft. Your IPL doesn't show it but there's a caged bearing there. I'll bet it's blown.

Hmmm, I may have overlooked that one - from the IPL it looks like you wouldn't be able to get a visual on it w/o taking the rod out. Is that end generally just pinned in or do you think the case will have to be split to get to it?

 

[biggus]

The IPL is not real detailed at that location....

Let me add this as well... when it runs (at the 90 psi) you can hear an audible clanging sound (it comes and goes) which I haven't pin pointed. I don't recall much, if any, play in the piston or rod or anything...

 

[Jacob J.]

Hmmm, I may have overlooked that one - from the IPL it looks like you wouldn't be able to get a visual on it w/o taking the rod out. Is that end generally just pinned in or do you think the case will have to be split to get to it?

The IPL is not real detailed at that location....

The connecting rod bearing isn't shown in the IPL because it's not meant to be replaced.

If it goes, you have to get a new crankshaft. You can eyeball it just by taking the cylinder off.
If the crank needs replaced, yes you have to split case halves. You're better off at that point
finding a good used replacement engine.

 

[blsnelling]

Wouldn't you typically expect to see needle bearing damage near the transfers?