Burning up piston ms660

[kevind]

I'm new here so please be kind so I have a used ms600 ran great for months last year, the exhaust got loose, gasket broke off and killed first head and piston. Replaced with cheap head and piston because I needed it next day, also ordered a hyway replacement to swap out temp junk. Burned up temp and fused ring to piston near exhaust. Put in hyway cut 2 days and fused rings just on the side of exhaust. I'm running 40:1 and unfortunately did not tune carb so I'm hoping I'm just running too lean. I'm not maxing out the saw when cutting so what should be my first step to resolving this issue. I have rebuilt a 029 with has run great for years and also own a 251c which I despise but use quite often. This 660 is the only one troubling me. Also, not a pro just a weekend cutter helping friend clear trees downed because of massive fire 3 years ago.
Thanks

 

[ballisticdoughnut]

First step is to vac/pressure test the saw. If it has an air leak it will continue to cook top ends.

 

[Bob Hedgecutter]

The exhaust coming loose and loosing the muffler gasket did not cause your first burn up- likely heat did- if it is burning the piston just to the left (viewed from the front) of the exhaust port.
The fact it also did the same thing to the cheapo top end and then again to the Hyway one- should ring a few alarm bells for you and even if using cheap top ends it is going to get expensive fast at hours of run time versus new top ends.

You likely are running lean- like untunable lean, so turning the mix screws is not likely to help much- if at all.
Too much rouge air is likely entering the system somewhere, like through a rubber hose or manifold tear- or main oil seals.
Couple that with dirty cylinder fins, dirty recoil cover and air entry restrictions, dull chain on a saw being worked hard and heat will kill the piston.

Quite likely the original cylinder can be cleaned up and reused with a new piston and rings- maybe the next two as well.
Photos of the damage will help explain the situation.