What caused this piston scoring?

[ncvarmint]

I am getting a ms660 rebuilt and was wondering what caused this damage to the piston, as i don't know the history of the saw before i got it.
The cylinder has very little little scoring at about the 1-2 oclock area, but the rest of the cylinder looks fine. Just wondering what you all think?
trevor

Also is the gouges in the front and behind the crank rod normal, it almost looks like it has been rubbing the crank case?

 

[Jacob J.]

Looks like your saw ingested part of a ring? Something got between the crank and the case to cause that gouging. I doubt the crank did that all by itself.

 

[srcarr52]

Almost 360 scoring before the top ring... classic straight gas.

 

[Anthony_Va.]

It es indeed look to have been straight gassed. I'm lost on the crank rod smacking the case though. Did you find any broken peices of ring laying anywhere? Looks like a peice of ring is missing from your piston in the pics. How do the bearings feel? Big end rod bearing and crank bearings.

 

[ncvarmint]

It es indeed look to have been straight gassed. I'm lost on the crank rod smacking the case though. Did you find any broken peices of ring laying anywhere? Looks like a peice of ring is missing from your piston in the pics. How do the bearings feel? Big end rod bearing and crank bearings.

The missing ring is because i tried to take it off the piston and broke it tonite, the bearing have no up/down movement that i can feel. I am hoping it was straight gassed and that means hopefully no air leak. Hopefully a builder can tell us about the rod hitting the crank case
trevor

 

[blackoak]

The missing ring is because i tried to take it off the piston and broke it tonite, the bearing have no up/down movement that i can feel. I am hoping it was straight gassed and that means hopefully no air leak. Hopefully a builder can tell us about the rod hitting the crank case
trevor

It makes sense to go ahead and check it for an air leak after rebuilding this saw even though many are saying it looks to have been straight gassed. Pressure test it before you fire it up!!!

 

[LegDeLimber]

My guess would be that the previous owner let someone pull the recoil
while the jug was off and the rod dinged the cases.
you might look at the piston skirt to see if it could reach that nipple.
but I'd wager it was just spun/yanked over with the all of top end off.

[ caffeine for a bad migraine is sorta drivin my thinkin at the moment. ]

 

[Agrarian]

Could they be marks left from a tool used to split the crankcase?

Does the connecting rod show wear that lines up with the wear on the case?

 

[komatsuvarna]

Looks like a lot of trash in the bottom end...

 

[sachsmo]

You discounted Jacobs response.

perhaps not a wayward ring, but something else was in there fo sure.

That's not a straight gas score, that is a foreign object score.

You can even see a bit of it embedded in the skirt.

 

[timmcat]

My guess is that someone had put a a new crank in that saw at some previous time and the old crank failed due to overspeeding and the big end rod bearing let go, allowing the rod to hit the case. That amount of peening wouldn't happen from pulling it over without the top end on it unless you spun the crank with a drill for 10 minutes. In any case I'd tear that one down to bare bones and inspect everything before you spend any money on new parts.

 

[R/C Pilot]

Looks like someone pulled the rope after the top end was removed to me.

 

[sachsmo]

 

[sachsmo]

See all that "peppering" on the crown on that side?

 

[ncvarmint]

My guess is that someone had put a a new crank in that saw at some previous time and the old crank failed due to overspeeding and the big end rod bearing let go, allowing the rod to hit the case. That amount of peening wouldn't happen from pulling it over without the top end on it unless you spun the crank with a drill for 10 minutes. In any case I'd tear that one down to bare bones and inspect everything before you spend any money on new parts.

Too late for that, already bought everything to put it back together.
trevor

 

[ncvarmint]

My guess is that someone had put a a new crank in that saw at some previous time and the old crank failed due to overspeeding and the big end rod bearing let go, allowing the rod to hit the case. That amount of peening wouldn't happen from pulling it over without the top end on it unless you spun the crank with a drill for 10 minutes. In any case I'd tear that one down to bare bones and inspect everything before you spend any money on new parts.

All the bearing's are good i think, no up/down movement on the top and no up/down movement on the sides. I am going to clean the crank case out with some mixed gas and see if it cleans up. Wish the guy i got it from had left it alone so i could see what was inside the case in case he removed it
trevor

 

[ncvarmint]

Looks like a lot of trash in the bottom end...

You should have seen the inside of the carb, and the inside of the air filter. Like some poured it full of fine sawdust
trevor

 

[wilsonishere]

I ran into this before, check the main rod bearing GOOD I MEAN REAL GOOD make sure there is no cracks evident or pieces missing on the race, You can grab the rod an pull it in one direction then move the flywheel back and forth and get a little room on each side use a good light and then spin the crank over to make sure I do not what you to waste your money on a top end just to have the bottom let go. If it is fine clean the inside of the case out and do a good job. You would be surprised what a little saw dust can do to a new top end on start up. Hope it was just a case of some one let saw dust go through the engine.

 

[blsnelling]

You NEED to pull both crank seals and flush that crankcase THOROUGHLY! If you don't, you'll simply wash the debris between the bearing and the seals. Not a good idea. Then you don't have to worry about them being bad either.

 

[thomas72]

A ms660 is worth a good top end and crank.