MS250 Piston Runs Good but look at Piston!

How do you think this saw will run?

Exhaust side top pretty scored, but rings are still ok:




Bottom not quite so bad:


Should I replace the piston ASAP? Can it cause more damage - overheat - damage cylinder worse etc? What symptoms would I expect - I cut with it for about an hour doing some limbing recently - it ran great believe or not.

I did order up a new meteorite piston - Should I NOT delay in putting it in? Or should I just keep running it as is?

I just rebuilt a MS 250 that looked better than that and it was straight gased



I would check the cylinder really close also

I would rebuild it IMO

Rings are still ok?:msp_w00t:
You should pull the cylinder off and clean it up with Muriatic acid then put it together with new piston and RINGS and cyl base gasket.

MS250 Piston Runs Good. My first question is....COMPARED TO WHAT.

I would be looking into why this is happening. If you dont stop running this saw at this point it may be irretievably damaged if not already.

rob066 said:

I would be looking into why this is happening. If you dont stop running this saw at this point it may be irretievably damaged if not already.

Click to expand...

Pressure test and vacume test would be in order.

Stop running it immediately if not sooner.
Check the fuel line (most likely culprit for the damage if the fuel mix is ok).
No base gasket in this saw. Get some Threebond 1194, Yamabond 4, or Hondabond 5 to seal the clammy halves together.

nmurph said:

Stop running it immediately if not sooner.
Check the fuel line (most likely culprit for the damage if the fuel mix is ok).
No base gasket in this saw. Get some Threebond 1194, Yamabond 4, or Hondabond 5 to seal the clammy halves together.

Click to expand...

I seen a strange case of a blowup. The fuel filter was defective and put debris inside the carb. It all went to the inlet screen choking off the fuel. This made the saw lean. Not this is neccessarily the case but something to look at.

Seals and Bearings will run you about $70 - there are two different seals make sure you get the right ones

Piston and Rings about $45

Carb kit $10

All hose intake boot and filters $35

Cylinder depends on were you get it

milkman said:

MS250 Piston Runs Good. My first question is....COMPARED TO WHAT.

Click to expand...

Compared to my memory, and it ripped through limbs of red and white oak with ease, I expect if I ran it longer than an hour I might start seeing more issues.

rob066 said:

I would be looking into why this is happening. If you dont stop running this saw at this point it may be irretievably damaged if not already.

Click to expand...

Agree 100%, think I found out why:

Fuel system clogged up - probably due to a gummed up carb - from old ethanol gas in it too long?? that is my guess, after a thorough cleaning of carb, fuel filter and hoses and fuel vent, it returned to 'good' working order - despite the piston. Mine from new, about 9 years old, never dumb enough to run straight gas, but old gas, and left gas in it for long periods 8 months or more.. maybe at some point my mixture was lean, used cheap oil mix.

I have learned to treat my mix with care - and add a bit more than the 50:1 ratio - I always use the little bottles and have a 1 gallon tank, so it is pretty hard to screw it too bad. I will also try real hard to keep fresh gas in it and dispose old mixes instead of using them now.

I did run it hard quite a bit in some big oaks, through it's life - could this be due to hard use or overheating?? I imagine I cut up around 20 cords with it.

I will stop running it, new piston will be in later this week, hopefully cylinder is not too messed up.

Hermann said:

Agree 100%, think I found out why:

Fuel system clogged up - probably due to a gummed up carb - from old ethanol gas in it too long?? that is my guess, after a thorough cleaning of carb, fuel filter and hoses and fuel vent, it returned to 'good' working order - despite the piston. Mine from new, about 9 years old, never dumb enough to run straight gas, but old gas, and left gas in it for long periods 8 months or more.. maybe at some point my mixture was lean, used cheap oil mix.

I have learned to treat my mix with care - and add a bit more than the 50:1 ratio - I always use the little bottles and have a 1 gallon tank, so it is pretty hard to screw it too bad. I will also try real hard to keep fresh gas in it and dispose old mixes instead of using them now.

I did run it hard quite a bit in some big oaks, through it's life - could this be due to hard use or overheating?? I imagine I cut up around 20 cords with it.

I will stop running it, new piston will be in later this week, hopefully cylinder is not too messed up.

Click to expand...

20 cords is nothing for a Wild Thing, never mind a 250. That is not an "issue." (Lordie, how that word is abused!)
Chainsaws are designed for running @WOT for many hours. Using otherwise is abuse.

It should run almost forever on 50:1 in good health, IF you use a good synthetic oil. Much more important is air/fuel ratio, most especially the high-speed. If it's right, saw will 2-stroke happily @WOT in a cut, but if you lift the bar slightly it'll 4-stroke. Extremely simple check- takes a few seconds.

Brad Snelling has a video on YouTube demoing that check. If your saw ever fails that check, deal with it NOW. Or risk melting a piston really quickly. (Buddy looked funny at me when I did that with his 250, but grinned when I told him the mixture was spot-on.)

I forgot to mention how important it is to keep your chain SHARP. Not kinda sharp, but razor sharp. At all times. Hand-filing with guide like Granberg's is really easy. Dull chain makes engine work much harder and longer- bad idea. Dull chains can be spotted by dust being produced, rather than chips.

Also, regularly clean "cooling system." Compressed air works great.

Are you serious? That thing is toast! You might be able to clean up the cylinder and install a new piston.

The bright shiny spots in the first pic are little globules of aluminum melted off the piston and blown into the exhaust. You can also see where all the flow in your exhaust is by that clear spot. Amazing that it didn't stop running.

Problems like this don't go away by themselves - they get worse. Fix this saw now. The longer you run it in this condition will be proportional to the cost in parts.

I will tear it down soon, will post some pics of cylinder and whole piston removed. New Piston should be here in a couple days.

Update, Cylinder out:



Light scuffing, felt with fingernail around exhaust port, next muratic acid. I found that the Flywheel Magnet was rubbing the Magneto - how bad does that f' things up - I assume not enough to roach the exhaust side of piston?

How closely should I examine the crankcase bearings? After the binding mentioned above the flywheel seems to spin freely and not too much play, think I should pull the flywheel and clutch and take it out to examine the bearings more closely??

bump for update.

So do you think I can put in the new piston in and be good to go? I think my fuel system clogged and ran lean?? Caused this and problem solved or I should investigate further?

Need to learn to make a Poll:

(1) Stop torturing myself and get a MS 26x
(2) Fix it up, be happy
(3) I will #### it up again regardless..

Hermann said:

bump for update.

So do you think I can put in the new piston in and be good to go? I think my fuel system clogged and ran lean?? Caused this and problem solved or I should investigate further?

Need to learn to make a Poll:

(1) Stop torturing myself and get a MS 26x
(2) Fix it up, be happy
(3) I will #### it up again regardless..

Click to expand...

The flywheel rubbing the magneto tells me it has a worn bearing and seal which caused an air leak which caused a lean seize!

Which brings this to mind.



For want of a nail the shoe was lost.
For want of a shoe the horse was lost.
For want of a horse the rider was lost.
For want of a rider the message was lost.
For want of a message the battle was lost.
For want of a battle the kingdom was lost.
And all for the want of a horseshoe nail.

well I think I found the nail,

one of the crankshaft bearings is loose on the shaft - how bad a problem is this? Could this be the cause?




Geez, went to replace a piston and now I have this?:msp_scared::



Anyway, quite a learning experience already, Where do you guys source your bearings and seals?

Hermann said:

well I think I found the nail,

one of the crankshaft bearings is loose on the shaft - how bad a problem is this? Could this be the cause?

Geez, went to replace a piston and now I have this?:msp_scared::

Anyway, quite a learning experience already, Where do you guys source your bearings and seals?

Click to expand...

You need to slow down and weigh the repair costs at this point. Is the bearing still tight on the crank or did it spin and wear a groove in the crank or case half? I always use stock bearings and seals so I don't have a problem with China crap. What is the saw worth running used? What will it cost to repair is the question is it worth it? Now you need a piston rings both bearings and seals should be done . Sounds like a couple hundred right there.If the cylinder is wrecked i would put the whole thing on hold. Go figure out your cost and decide if you want to do it.