MS391 scored piston while milling

[Yukon Stihl]

Yes...when the saws were not built as tight, and didn't run as fast or as hot

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plus no EPA

 

[ZinTrees]

update, got the new piston in, saw fired up and ran for a little bit, then died, and didnt want to restart, once we got it to restart, my dad tried to do some tuning (messed with the screws while pulling limiters), tried to get it to run, then it wouldnt start again
any ideas?
hoping the piston isnt jacked up again (Has less that 2 minutes of run time)

 

[J D]

How well did you get the cylinder cleaned up? Pull muffler & check it would be my first thought... Next would be pressure/vac test, especially if you didn't do one when you put it back together

 

[ZinTrees]

the base of the cylinder seemed to have some light pitting, other than that it was almost new looking
the piston had very light scoring when we pulled the muffler, gonna see about a pressure test

what all does a pressure test need? do you just put the cylinder under pressure (with the piston at tdc) or do you individually check everything from the intake to the muffler?

 

[J D]

the base of the cylinder seemed to have some light pitting, other than that it was almost new looking
the piston had very light scoring when we pulled the muffler, gonna see about a pressure test

what all does a pressure test need? do you just put the cylinder under pressure (with the piston at tdc) or do you individually check everything from the intake to the muffler?

This explains it fairly well despite the photos being AWOL. https://www.arboristsite.com/community/threads/poormans-guide-to-vacuum-testing.42768/
Test is important to ensure cylinder is sealed & won't be sucking in air somewhere that will cause it to run lean & toast the piston/cylinder... especially if that's why the saw is being repaired in the first place

 

[Canyon Angler]

I'm far from an expert, but I think you want it pig-rich after a rebuild (at least for the first few minutes) just to be on the safe side of leaning it out and tearing it up again.

Whenever I'm in doubt about the tune, I richen it up. Better too rich than too lean.

 

[ZinTrees]

update, saw wouldnt idle, ran ok wide open, pulled it apart again, and honed the cylinder (Yes, your not supposed to)
now it wont hardly run, probably just gonna get a new cylinder and piston, or a new saw

 

[sean donato]

Yes...when the saws were not built as tight, and didn't run as fast or as hot

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Yes, I'm aware. I felt it pertinent at the time it was decent tid bit of information to share. not that I advocate for using straight weight oil, as we have much better options available.

 

[sean donato]

update, saw wouldnt idle, ran ok wide open, pulled it apart again, and honed the cylinder (Yes, your not supposed to)
now it wont hardly run, probably just gonna get a new cylinder and piston, or a new saw

Most likely had a air leak, that was the first cause of your issues. Better get set up for pressure and vacuum testing. Make sure you didn't blow a crank seal or whatnot.

 

[ZinTrees]

Most likely had a air leak, that was the first cause of your issues. Better get set up for pressure and vacuum testing. Make sure you didn't blow a crank seal or whatnot.

I will see about getting set up for that, thanks Sean

 

[Mad Professor]

one more time.... milling with. a 60cc saw is asking for trouble. Clamshell only worse trouble........got both?

 

[ZinTrees]

one more time.... milling with. a 60cc saw is asking for trouble. Clamshell only worse trouble........got both?

well, its what we have
we are buying a 661 when money allows

 

[SEAM]

Or find an affordable old boat anchor like a Stihl 050/051/075/076. They may feel a bit heavy and slow but get the job done...

 

[ZinTrees]

Or find an affordable old boat anchor like a Stihl 050/051/075/076. They may feel a bit heavy and slow but get the job done...

I been looking around, thanks man

 

[Wow]

OK, Here is my opinion. Milling and cutting wood is similar, but a bit different. Milling is generally at full throttle for minutes at a time. This generates a lot of heat. Especially with modern saws, already set lean. Normal wood cutting is usually full throttle but not for extended lengths of times. As mentioned with milling the heat generated literally saturates the entire cylinder. The heat is your enemy. How do we get rid of the heat? Richen the mixture and add more lubrication. As mentioned with my 041, the saw was used with 30 weight oil for years at what ratio I have no idea but the dealer mentioned 16/20 -1.Yes this is a lot of oil buy todays standards. The 041 cylinder fins were caked with saw dust, but the saw showed no signs of any over heating issues. What I am trying to say is richen it up and run say 32/1. I would make a bet that your saw will thank you for this. In my personal opinion 50/1 is the very bare minimum. The EPA doesn't care about your equipment. In all my saws I run 32/1 Amsoil. Never had a overheating or oil related issue

You and I must be members of the SAME CHURCH..I swear you are preaching MY sermon...THIS 50:1 message reminds me of the story of the tower of Babel where all of us spoke the same language until an Angel looked through a hole in the sky and saw humans building a TOWER to Heaven..God asked, "How far have they Gotten?" Pretty far the Angel said. God asked, "is the labor force Mexicans?: The angle replied not that I can see,, just some white guys and a few blacks. God said, NO problem.. They will never make it. They'll be fighting and the project will end.. Bring me another beer and lets finish this Game of Chess. Nothing to worry about. Of course you probably heard the CBS and NBC news version..Ha. Ha Don't believe it..Ha. Ha..BUT OIL saves saw engines.. For Saws it's the real Savior..Praze de LARD.and give 10% to the oil companies..ha...ha

 

[Dennisthemenace]

I don't think a 391 is a good saw for milling. I doubt that even Stihl would recommend it. Plus I think the poster said 50/1?
I wouldn't try serious milling with anything less than a 046/461.
I even worry about my 394 with 32/1 when I've done a few slabs lol.
I do have a lightly used 039 with a small muff mod and it is a great mid range saw but I wouldn't put on a mill