MS-310 piston scoring....

[mrcaptainbob]

Here are

 

 the two sides of the piston. The cylinder is the same way. Carb side smooth, exhaust side heavily scored. Both sides of the rings, including the rings.

[mrcaptainbob]

What might have caused this?

[sawguy21]

Overheated. It was likely running too lean or insufficient oil. Could also be an intake air leak.

[mrcaptainbob]

Hmm... don't think it was insufficient oil in the mix. I'm pretty judicious about that. It's only ever tasted Stihl mix and always at 50:1 with maybe a dribble more oil in the mix. Always.
So that leaves the lean issue or air leak. I checked the carb boot and it's air tight. Will check the other tubes and hoses and verify. I presume lung pressure is sufficient for air leak test...?

[sablatnic]

Could be mixture, that was too old.

[sawguy21]

A Mighty-Vac or similar tool is needed to pressure test, the pressure needs to hold.

[mrcaptainbob]

The last couple times I used it there was evidence of a lot of 'splashing' from the muffler on the face of the wood being cut. Did not know it was blowing past the piston right out of the exhaust!