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.

 

[ZeroJunk]

It is practical to replace the seals when you rebuild the top end.
A tear in the impulse or manifold should jump out at you when you put a new cylinder on.
So, if you did change the seals the spring could have popped out. They won't last long that way. I have done it.

Buy or borrow a tachometer. I like 12,500 no load max myself. Never seemed to gain much running leaner.

And, if you can tune it to 12,500 no load and it will idle properly then you don't have an air leak to amount to anything.

Then you can look at bad gas, dull chain, air inlets clogged, etc.

 

[cookies]

After that many failed cylinders and pistons I would tear it down to the crank and replace bearings, cranks seals(get a extra set they are easy to muff up), hoses, case gasket rebuild the carb and slap a new oem cylinder/piston kit on it...get and use the little cone seal installer

 

[lone wolf]

OP pics of the carnage?

 

[kevind]

This is the 026 i was given, thought it was 029 it looks the same. I'll post the 660 next.

 

[kevind]

OP pics of the carnage?

This is the 660, other head was honed so no pic of it. Some of the scoring near the rings is from me seeing if i could easily recover rings but they were welded in pretty good

 

[cookies]

OP pics of the carnage?

Don't do it, these guys get off on carnage photos!!
When pistons come out looking like that the saw needs a teardown, all that missing aluminum, cylinder plating and whatever else went through the crank/rod/wrist pin bearings and the crank seals got hot hardening the rubber.
Spend extra time learning to adjust the carb/fuel ratio a little fat/rich once fully heated up