Reuse or replace piston?

[hometheaterman]

After deciding I wanted to rebuild this MS250, I started pulling it apart to see exactly what I needed in addition to the crank. I really don't see any scoring that I can feel with my nail on the piston or the cylinder. I couldn't get a good picture of the cylinder, and at the top I can see a few marks where you can tell the rings have just been sliding back and forth, but it isn't any scoring that I can feel with my nail or anything and it's smooth to the touch. However, there is a bit of discoloration and a fair amount of carbon buildup on the top of the piston. Since this is my first rebuild and you all are a lot more knowledgeable than I am, am I good to go reusing this? Or would you recommend replacing it while I have it apart?

 

[Vintage Engine Repairs]

With the machine marks being still visible on both the rings and piston, I think most people here would clean off the carbon, chuck it back in and be good to go. Being a bit anal, I’d check the clearances for my own curiosity and peace of mind.

The only true way to know if a piston is within spec is to measure skirt to cylinder wall clearance as well as piston ring end gap. Although there are machine marks still visible, there is wear at the very base of the skirt and that is a fair bit of carbon passing the rings.

For skirt clearances - the simplest method is to drop the piston in the cylinder until it bottoms out at top dead centre, bring it back a few millimeters and slide a feeler gauge between the skirt and cylinder. it’s the least accurate method, but for basic engines like a firewood saw, it will be fine.

Regarding the rings, place them inside the bore a cm or so before top dead centre measure the gap with feeler gauges again.

I’ll go and check my notes as to the widest clearance recommended for both and get back to you, unless someone beats me to it.

 

[cookies]

I'd replace the rings at a minimum and inspect the wrist pin and bearing and likely replace the bearing anyway. Piston looks good to reuse but I would soak it until the carbon wipes off.

 

[FamilyTradition]

Agreed. Remove carbon from the piston and cylinder, replace rings, inspect the rest and replace as necessary. Good luck with your build!

 

[ericm979]

I'd replace the rings, wrist pin bearing and circlips at a minimum.

 

[Mad Professor]

Need to clean all that carbon off P/C. Then see post #2

 

[lone wolf]

OP what brand of 2 stroke oil are you running? The piston looks OK the machine marks are still there. New rings as mentioned and clean everything. Are you planning new seal,gaskets and a pressure test? What prompted you to rebuild it how did it run?

 

[hometheaterman]

Thanks guys. It looks like my next step is going to be to soak this thing in something so I can clean the carbon off. Then I'm going to check the ring gap and decide if I should do rings from there. The wrist pin bearing feels tight. Is there a way to check it other than just for play by hand?

 

[hometheaterman]

OP what brand of 2 stroke oil are you running? The piston looks OK the machine marks are still there. New rings as mentioned and clean everything. Are you planning new seal,gaskets and a pressure test? What prompted you to rebuild it how did it run?

I've been running Stihl oil since it was new, but it was the orange bottle stuff that I bought several years ago. I've read that it can cause lots of carbon and to stick to their silver bottled Ultra oil. I plan to switch over to it. I think I may have run one can of bottled premixed gas during that time, but it was pretty much only Stihl oil.

I'm rebuilding it because it snapped the crank off at the flywheel. After fixing a torn vent hose causing it to run poorly, it had been running excellent prior to the crank breaking. I'm planning on picking up a another crank, then replacing it and the crank seals. I'm just undecided on if I should do rings, wrist pin bearings, etc. I am going to pressure test it once done.

 

[Husky77]

I've been running Stihl oil since it was new, but it was the orange bottle stuff that I bought several years ago. I've read that it can cause lots of carbon and to stick to their silver bottled Ultra oil. I plan to switch over to it. I think I may have run one can of bottled premixed gas during that time, but it was pretty much only Stihl oil.

I'm rebuilding it because it snapped the crank off at the flywheel. After fixing a torn vent hose causing it to run poorly, it had been running excellent prior to the crank breaking. I'm planning on picking up a another crank, then replacing it and the crank seals. I'm just undecided on if I should do rings, wrist pin bearings, etc. I am going to pressure test it once done.

Oh now you have said it, you mention that word "oil" haha. Piston is looking good except the obvious so give it a good clean and it will be fine and new rings. Wrist pin as you call it is fine, you would feel if it wasnt. Good luck with your quest for the "right" oil, all will work but some are definitely better than others and partly down to how you run the saw too. Mostly keep your chain sharp, and I mean sharp, not half sharpnornit will do. It's always worth stopping and giving the chain a few light strokes with a file than pushing on just to finish and going past the point so your chain needs grinding. Good luck

 

[lone wolf]

I've been running Stihl oil since it was new, but it was the orange bottle stuff that I bought several years ago. I've read that it can cause lots of carbon and to stick to their silver bottled Ultra oil. I plan to switch over to it. I think I may have run one can of bottled premixed gas during that time, but it was pretty much only Stihl oil.

I'm rebuilding it because it snapped the crank off at the flywheel. After fixing a torn vent hose causing it to run poorly, it had been running excellent prior to the crank breaking. I'm planning on picking up a another crank, then replacing it and the crank seals. I'm just undecided on if I should do rings, wrist pin bearings, etc. I am going to pressure test it once done.

Well price a new piston maybe . How are the crank bearings? Why do you think the crank broke?

 

[hometheaterman]

Well price a new piston maybe . How are the crank bearings? Why do you think the crank broke?

The bearings seem good to me. I don't feel any play and they feel smooth, although I'm certainly no expert.

I really don't know what happened with the crank. If I was to take a guess, and this is 100% a guess. The saw was recently having trouble starting when hot and also would act weird at idle. It acted like it had an air leak. Spraying some brake cleaner in the side cover made it bog. I thought this might be an air leak at the crank seals. I ended up removing the flywheel so that I could spray some soapy water on the seals while. I had pressure on the motor.

To remove the flywheel, I did the old trick of loosely threading the nut back on the crank, holding the saw by the flywheel, and hitting the nut on the end of the crank with a hammer. It popped the flywheel loose, but I'm also wondering if it somehow fatigued the metal. I torqued the nut to Stihl specs with my torque wrench when I put it back together. It broke not too long after that. I'm not sure if the metal was already fatigued and it just happened or if the two were related. It's a complete guess that the two may be related.

The crank seals ended up being fine, and the leak was in the vent hose. After replacing it, the saw ran amazingly until the crank broke.

 

[Husky77]

That's a strange one, of all the parts to break that is probably the strongest part. I guess some thing you never get to the bottom of.

 

[hometheaterman]

That's a strange one, of all the parts to break that is probably the strongest part. I guess some thing you never get to the bottom of.

I agree. I never would have expected it. It's such a strange failure. That's why I have no idea if the flywheel removal above had anything to do with it, or if it was just a freak failure.

 

[lone wolf]

I agree. I never would have expected it. It's such a strange failure. That's why I have no idea if the flywheel removal above had anything to do with it, or if it was just a freak failure.

If it was the sprocket side I was say you tightend the chain too tight all the time and beat on it. Flywheel side may from hitting it since it happened right after.

 

[Tulameendude]

The top of the piston looks pitted...you have it apart...so just change everything.

 

[lone wolf]

The top of the piston looks pitted...you have it apart...so just change everything.

Now he needs to figure out what caused the pitting.

 

[Tulameendude]

Now he needs to figure out what caused the pitting.

Well I'm not a small engine mechanic, but I do credit myself with a fair mechanical aptitude...so my guess would be...and I use the word guess here...is maybe the gap between the fly wheel and the ignition coil may be out of adjustment causing the spark to be a little retarded...possible pre-ignition...or maybe the wrong/rich oil mix...causing carbon build up on the piston, thus hot spots that may be causing the pitting...maybe some food for thought!

 

[Captain Bruce]

I've been running Stihl oil since it was new, but it was the orange bottle stuff that I bought several years ago. I've read that it can cause lots of carbon and to stick to their silver bottled Ultra oil. I plan to switch over to it. I think I may have run one can of bottled premixed gas during that time, but it was pretty much only Stihl oil.

I'm rebuilding it because it snapped the crank off at the flywheel. After fixing a torn vent hose causing it to run poorly, it had been running excellent prior to the crank breaking. I'm planning on picking up a another crank, then replacing it and the crank seals. I'm just undecided on if I should do rings, wrist pin bearings, etc. I am going to pressure test it once done.

New Engine. There are issues with the balance of your old jug.....

 

[huskihl]

I can’t tell if the piston is pitted or not. It looks like there is a heavy carbon buildup and the piston below might be fine