Rebuilt my Stihl MS 250

[curticat]

I should blame Stihl, but don't imagine I would have much luck.  It died in action, and wouldn't start again.  Turned out the cylinder was scored deeply from a foreign object (tiny piece of what looked like hard steel wire) found in the bottom of the pan.

Anyway, I bought a kit from HL Supply and put it in.  That's a finicky beast to disassemble and re-assemble.  When finally complete it started right up but wouldn't stay running.  Re-started several times but then wouldn't fire at all.

I tried numerous things until I ran across this forum, and saw something that caught my eye regarding the flywheel key.
I didn't remember noticing one, and recall that the flywheel was pretty tight going on to the shaft.  I don't think I would have totally missed lining things up, but it  is possible.  In any case, when I pulled it today it looks like I probably destroyed the built in key during assembly.  Unless, it sheered during those couple of start-run-fails.

I also noticed that it rubs the magneto (?) at one point on the cycle, but clears it everywhere else.  It may be because I have mal-shapened the flywheel when I nutted it down?  But it does make me wonder if the shaft (Cylinder assembly) is perfectly aligned in the housing.  I hope I didn't do something a little off when I threaded into the new cylinder kit.

What is the spacing supposed to be between the flywheel and magneto pickup?  I saw a video where it looked like the guy only slid a paper thickness between the parts.

Has anyone else run into a similar alignment problem?  I saw one person stating that the fit of parts on that model is poor.

I will order a new flywheel and try again, unless somebody has an extra around that they would be willing to part with for a few bucks?  The part number is 1123 400 1203A  Photo attached to show the state of the key. 



 

Thanks for any help or comments.  It's my first re-build, so....

Curtis

[samandothers]

Welcome to the Forum!

I have a 250 that runs pretty good though it can be finicky when hot.

[dougand3]

Make sure the engine pan recesses for the cyl bolts are big enough to accept the bolts heads. I rebuilt a 250 and the new Huztl pan recesses were too small a diameter. When pan was bolted to cyl, the engine sat too high in the plastic chassis. I noticed cuz worm gear spring/hook was not centered.

Flywheel - the key is just a locator. The taper holds flywheel in position. Clean crankshaft and flywheel really well with carb cleaner prior to install. You can file the old cast key out and use AL or Cu wire as key. 

http://forestryforum.com/gallery/albums/userpics/36829/Stihl_MS250_clamshells.jpg

[curticat]

Great thoughts, Dougand.  Thank you.

My key is/was part of the the flywheel, so not sure how filing away the remains would allow a copper wire to serve the purpose, but I will give it a try as a locator, and torque down to allow the taper to do it's job.  

Do you know the torque spec by chance?

I will check the pan recesses as well, though might need  to pull it out (again) to do so.

Spacing on between wheel and pickup?

[Al_Smith]

Before you do a quick fix bear in mind you can snatch a brand new flywheel from flea bay for next to nothing .Like 15 bucks free shipping .There's a bunch of them,evidently perhaps you aren't the only one who had problems .Stihl made a blue million of them,firewood cutters and they did okay on that .Tree service,not so good .You should be able to find the torque specs for the nut on the internet .Righty tighty,lefty loosey.Tight is tight too tight is broke . ;D 

[Al_Smith]

While I'm thinking about it .You can't lap a flywheel to the shaft with a built in key .What you could do is heat the flywheel up to expand it with a heat gun and get kind of a shrink fit . Old trick from an old coot .

[DelawhereJoe]

To add to the last comment if its cold outside you can put your saw out there or in the freezer to shrink the shaft, heat the flywheel then quickly put them both together. Just make sure everything is completely lined up the way it should be.