MS391 scored piston while milling

[ZinTrees]

scored the piston on my dads 391 the other day while milling, saw went way down on power then just died

I havent pulled the cylinder off yet, but im just wondering, how much damage do yall think there will be? and could it be honed out and just put a new piston in?
exhaust side of the piston is super bad, will snag a fingernail pretty easy, im gonna go pull the plug and look in the cylinder and see what I find

 

[GeorgiaVol]

Can't hone a chainsaw cylinder. Might could be cleaned up with acid or sandpaper though if it isn't too bad. If so, new piston and rings, find out why it scored, and rebuild.

 

[Kenskip1]

I hate to ask this, but what mix ratio are you using? It seems as if I have read about more saws being ruined lately than say, in the past 15 years. The ole timers milling with saws had them running for years. My 041 was used for milling for many years. The owner traded it in for a newer saw. I got hold of the 041,removed the cylinder along with the muffler then removed all the carbon from the exhaust port. He had apparently used straight 30 weight oil for years. The piston and cylinder were perfect. The muffler was filled with carbon. Saw is heavy, loud, and vibrates but runs well.

 

[ZinTrees]

50:1 fuel mix

reason the saw blew? im assuming it got too hot, and piston ring broke

the scoring on the cylinder isnt too bad, but you can feel it sliding a small flathead screwdriver around (yes, I was careful not to scratch it any more)

I like the sandpaper idea, might have to try that

 

[ZinTrees]

Can't hone a chainsaw cylinder

why is that?
are the tolerances just that tight? or is the steel just that hard?

 

[GeorgiaVol]

Walls are coated. Nikasil.

 

[ZinTrees]

ahh ok, thanks

 

[Kenskip1]

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

 

[GeorgiaVol]

I would hate to open this oil can of worms up, but I agree. For general cutting I use 40:1.
I am thinking about milling though and I think I will try your suggestion.

 

[Kenskip1]

I would hate to open this oil can of worms up, but I agree. For general cutting I use 40:1.
I am thinking about milling though and I think I will try your suggestion.

I fowled spark plug is much less costly than other parts.

 

[GeorgiaVol]

Way easier to change too.

 

[ZinTrees]

I was actually telling my dad a few days ago, to run heavier fuel mix when milling, he said that makes less power

even if it does make less power, I feel its worth it in the long run
I will relay your info to him

 

[buzz sawyer]

I was actually telling my dad a few days ago, to run heavier fuel mix when milling, he said that makes less power

even if it does make less power, I feel its worth it in the long run
I will relay your info to him

if you use more oil, be sure to re-tune the carb. More oil = less gas = leaner mixture.

 

[ZinTrees]

ok, so richen it up some

how much is too much? ive seen people saying to lean it out till it runs rough, then back a little, I guess an easy way would be tune till it sounds like a stock saw should?

 

[tomalophicon]

ok, so richen it up some

how much is too much? ive seen people saying to lean it out till it runs rough, then back a little, I guess an easy way would be tune till it sounds like a stock saw should?

You've got it backwards; richen the high till it runs rough and smooths out during loaded cutting.

 

[ZinTrees]

that little mistake may have been my issue all along
thanks dude

 

[tomalophicon]

that little mistake may have been my issue all along
thanks dude

No problem. Good luck.

 

[RED-85-Z51]

Yes you want to richen it until it falls off, 4 strokes, then a touch leaner, it should smooth out in the cut. If you go leaner you make great power but it burns up

Sent from my SM-G996U using Tapatalk

 

[ZinTrees]

Yes you want to richen it until it falls off, 4 strokes, then a touch leaner, it should smooth out in the cut. If you go leaner you make great power but it burns up

Sent from my SM-G996U using Tapatalk

makes sense

 

[J D]

Madsen's has a good guide... For milling ideally use a tacho & adjust H to a few hundred rpm below max