O.K. Guys...What caused this???

[rms61moparman]

I picked up this 039 the other day cheap to play around with the porting.
When I pulled the cylinder, I found the piston looked like this.
What caused it???



View attachment 205878


Mike

 

[Bowtie]

I picked up this 039 the other day cheap to play around with the porting.
When I pulled the cylinder, I found the piston looked like this.
What caused it???



View attachment 205878

Mike

Just a guess, detonation from crap fuel.

 

[Stumpys Customs]

That is from pre ignition. Most likely from runnin low grade or stale fuel. See it all the time in dirt bike motors.

 

[Kenskip1]

Mike, I would take a stab and say possibly pre ignition or possibly detonation.Something is heating the outer surface.It appears to be symmetrical so I doubt that it is a carburetor issue.I will say a timing anomaly, Ken

 

[Bowtie]

That is from pre ignition. Most likely from runnin low grade or stale fuel. See it all the time in dirt bike motors.

Agreed. Is the rest of the piston in good shape?

 

[rms61moparman]

Could a bad seal cause it?
The P.T.O. side was pretty bad.


Mike

 

[blsnelling]

Yup, detonation. Bad gas would be my guess.

 

[rms61moparman]

Agreed. Is the rest of the piston in good shape?

It was scored.
I thought it would clean up, and it probably would, but I have a piston kit and new seals on the way.


Mike

 

[Bowtie]

It was scored.
I thought it would clean up, and it probably would, but I have a piston kit and new seals on the way.


Mike

You going to replace the seals since it is apart? They are cheap at a bearing shop. Too bad the piston is scored, or i would clean the top up with emery cloth and reuse it.

 

[rms61moparman]

You going to replace the seals since it is apart? They are cheap at a bearing shop. Too bad the piston is scored, or i would clean the top up with emery cloth and reuse it.

Heck those little score marks won't hurt it none.
I have reused many without any problem.
This one had a bit too much slop for me.
Yes, I know they are all a bit sloppy but this one was even too bad for me!!!LOL


Mike

 

[Stumpys Customs]

Could a bad seal cause it?
The P.T.O. side was pretty bad.


Mike

It was scored.
I thought it would clean up, and it probably would, but I have a piston kit and new seals on the way.


Mike

The seal prolly cuased the scoring. The pitting on top was cuased by detonation over a long period of time. New piston & seals, Some good fresh fuel and it shold be fine.

 

[tree monkey]

99% of the time thats caused by a piece of a bearing. check the head with a magnet. poor fuel will cause coking not this

 

[nmurph]

That was not from shrapnel. If it were, it would be all over the piston in random patterns. Was there a lot of carbon build-up in the combustion chamber? If so, it might have caused the squish to be too low and this cratering from the excess heat. Otherwise, poor gas as many others have mentioned.

 

[rms61moparman]

There was a little more carbon in the cylinder than I like to see but it wasn't nearly as bad as a lot I've seen.
It wasn't even close to the flaking off in chunks that many saws have.
I always run 87 octane pump gas and I've never seen anything like this in any of my saws.
It would be nice to be able to talk to the previous owner to find out what he ran to cause this.


Mike