Stihl MS250 rebuild.

[dodge man]

I got all the parts cleaned up and am waiting for new parts. Pic is the parts after cleaning. I ordered the jug and piston made by Hiway and a couple of gaskets. About $100 total. Looks like I will have reuse my crank and rod. Any tips for getting the piston off the rod and the new one on?

 

[ericm979]

Here is a pic, the exhaust side is up.View attachment 668832

There's no detonation damage and the crown does not appear overheated. You said the intake side looks good. So it's most likely the seizure was from lack of lubrication. The seizure marks match that diagnosis but it's hard to tell from them alone. Lack of lubrication could be from separated premix but modern oils do not drop out of suspension as readily as old ones did. It might be from poor quality oil but even cheap two stroke oil should be sufficient for a chain saw, which has a low power/cc. Most likely cause of lack of lubrication was lack of oil in the premix, like running straight gas by accident or making a mistake with the ratios and not getting enough oil.

I always shake up my can of saw premix before pouring into a saw, and use a Ratio-Rite to measure the oil out so I don't have to do math in my head. You can rinse the oil out of the Ratio-Rite with some of the gas to make sure you get all of it. I recommend using a good synthetic oil. I like Redline Allsport, because it works and it makes the exhaust smell good or at least less bad. I've torn down engines I ran it in and they are pretty clean with no signs of lack of lubrication. For my saws 50:1 using good oil, is fine. 40:1 would be ok too but you'll need to clean the spark arrestor a bit more often.

 

[ericm979]

I got all the parts cleaned up and am waiting for new parts. Pic is the parts after cleaning. I ordered the jug and piston made by Hiway and a couple of gaskets. About $100 total. Looks like I will have reuse my crank and rod. Any tips for getting the piston off the rod and the new one on?

Use a pick to get under a circlip (first rotate it so one end is at the relief if it has one, some Stihl pistons dont). Once you get it started you can pry it out. Do not reuse the circlip! There's probably new ones with the piston. When you have one circlip out you can push the pin out that side of the piston. If it's tight in the piston, you can warm the piston or use a puller. I've made them out of all thread and PVC pipe and a washer sized to fit inside the circlip but engage the pin. You can warm the piston with rags with hot water or a heat gun. Warming the piston is safer. On a lot of two strokes the pin's supposed to be a light interference fit in the piston.

Once the piston's off, inspect the pin and the small end bearing. I'd replace the bearing while you're there, they don't cost much.

When you get the rings, check the gap by gently pushing them into the bore and using a piston to get them square. The gap spec should be in the shop manual. I have gotten mis-marked rings from a major manufacturer. I dress the corners of the rings with a fine stone so there's not a sharp edge to pull the locating pin out of the piston. With a piston this small you might be able to compress the rings with your fingernails especially if there's a chamfer on the bottom of the cylinder. But ring compressors will make that easier. There are cheap sets of plastic ones for chain saws that work pretty well.

Getting the pin in is the reverse, warm the piston or use your puller. The circlip can be hard to get in. I use a set of needle nose pliers to get it started and then gently pry it in with a tiny screwdriver or push with a small socket that just fits in the pin hole. Try not to bend the clip any more than you need to get it in. The circlip ends should be either at the top or the bottom of the hole, not at the sides. So the huge g forces on the clip don't compress it and allow it to come out.

 

[ruffdog]

Use the correct sealer to seal the clam shell as that is a notorious place for them to suck air. Any air leaks will cause a erratic idle.

 

[dodge man]

I use either Amsoil or Stihl two cycle oil. It’s possible I or my son put straight gas in but not likely. I do not normally burn a gallon of fuel just through my saw but also use it in a leaf blower and weed eater and they are running fine. I did loan it out once but I don’t even think they burned a tank of fuel through it. I guess it only takes one screw up for it to happen though. The saw still ran and made good power, just did not idle.

 

[ArlyA]

If you forget oil in the fuel, wouldn't the bearings warm up rather quickly??

 

[sd455dan]

Good points by all, only other thing I could think of is using sealer on the gaskets or the new end seals/ clam shell

Not a bad time for new fuel lines to.

about sealers -only a few are rated for 2 stroke cases because of the fuel oil mix. The ones I have heard good things about are 3 bond 1127, 1215 Yamabond and Honda bond HT and what Stihl recommends Dirko HT

i have used the Dirko HT and it has held up well

I would also second doing a vacuum/pressure test after reassembly. not to much pressure 5-7lb should be plenty.

Keep us posted on how it goes, definitely Nice to have a back up saw. :thumbsup:

 

[dodge man]

Thanks for all the answers. How do you pressure test? Plates to cover up the ports?

 

[ericm979]

If you forget oil in the fuel, wouldn't the bearings warm up rather quickly??

They are all roller or ball bearings, which need only a thin film of oil. The piston is more like a plain bearing. Those need a constant flow of oil because the oil IS the bearing. And the piston crown is hot, which makes things worse. It's really common for a low oil seizure to not affect the bearings at all, just the piston and cylinder. If a bearing is affected it'll probably be the wrist pin bearing. If the rollers look overheated (they turn blue) then it needs to be replaced. I just replace then anyhow since even good ones are cheap.

 

[dodge man]

One reason I do not think I used straight gas is I keep it in a 5 gallon can, which is hard to fill a chainsaw with, where my two cycle mix is in a two gallon can. I could have mixed it too lean or not have shaken the mix. At this point no way to really know. Just guessing but the saw would have 100 to 200 hours on it. Either way, I woukd say at some point it got short changed on oil.