Looks like a plastic bearing cage failed. Good thing about them, the piston and cylinder tend to survive.
Stihl MS 390 carnage - disassembly.
- Thread starter MacAttack
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I hope you're right!
An improperly tightened flywheel will shear the key. Your old one will still be good, just index it and tighten it properly. The keyway is only there to ensure proper ignition timing.
I've seen saws eat the main bearings because of pre-ignition. There were telltale signs on top of the pistons. I've done a 290 rebuilt with a 390 kit, even then I considered it underpowered for its weight. Kind of a goofy muffler mounting system on it, doesn't allow for an easy muffler mod.
That being said, parts are readily available and although there are some difficulties in assembly, it's not the worst saw to practice on.
I've seen saws eat the main bearings because of pre-ignition. There were telltale signs on top of the pistons. I've done a 290 rebuilt with a 390 kit, even then I considered it underpowered for its weight. Kind of a goofy muffler mounting system on it, doesn't allow for an easy muffler mod.
That being said, parts are readily available and although there are some difficulties in assembly, it's not the worst saw to practice on.
Thanks for the input!
I don't need a high performance saw...if it ends up a runner I'll be happy.
I'm going to try and remove the bits of trashed key from the flywheel with a dremel.
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Should drive out with a pin punch- then if you are not going to re key- lap the flywheel to the crankshaft with some lapping/grinding paste- there is a lot of galling on the taper bore of your flywheel, clean it up a bit- clean off all the lapping compound, index it to original position and tighten the nut down well.
Last night I tried everything in my tool box to get the remaining key out of the flywheel...it's not budging. The flywheel taper is kind of messed up from spinning on the crank, if I want to do this right I'm probably going to just look for a good used flywheel.
the first 2 of those i looked at the bearing keepers were missing thier keepers- I third one a guy phoned me up and he asked me why it lookd like little chunks of coal in his muffler- I said that was what was left of the keepers, imo garbage bearings- be nice to see if a 6203-c3 steel caged braring would fit it- very common bearig for a lot of saws- a small nail set migt get that sheared key out
I THINK the "key" is actually cast into the flywheel?
Making progress on the disassembly:
Making progress on the disassembly:
The key is a part of the flywheel casting on these saws.
It's worth a shot to try lapping the flywheel to match the crankshaft taper. Use some valve lapping compound from the auto parts store.
If it doesn't work, no big deal. The flywheel will shift, the engine will start hard or not run right and you will have to buy a replacement flywheel.
If lapping works this is a great opportunity to experiment with adding a little timing advance for more power.
It's worth a shot to try lapping the flywheel to match the crankshaft taper. Use some valve lapping compound from the auto parts store.
If it doesn't work, no big deal. The flywheel will shift, the engine will start hard or not run right and you will have to buy a replacement flywheel.
If lapping works this is a great opportunity to experiment with adding a little timing advance for more power.
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Yes, sorry, thought you were aware the key was "part" of the flywheel- I was meaning to get the remains of the key out of the crank stub.
That part fell right out....i was foolishly grinding away at the flywheel thinking I was grinding a separate "key" until I realized it was part of the flywheel, haha.
Thanks for the help!
Don't remember a saw, but I have put blowers back together with no key when one of the steel starter spacers came out , hung on the magnet, and spun the flywheel. Just eyeballed it. Ran fine.
Chainsaw Pete
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You might try capping one end of the hole, pouring penetrating oil of your choice into the now cone shaped cavity, and letting it soak for a few days. Drain the juice, uncap the hole and heat the entire (aluminum?) flywheel with a hair dryer until it is hot to touch - but not blistering hot. Then try again.
Some of the larger hardware stores sell rubber and cork stoppers that should do the trick to close off one side of the flywheel's hole.
Or start your search....
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That wont help in this instance- the key on 039/390's is part of the flywheel casting- not a separate steel key, so no amount of soaking in anything is going to remove the nub of the original key on the flywheel bore- indeed if a clean shear it could stay there without removing, as an indicator of correct timing- but only if you do not intend to lap the bore of the flywheel to the crankshaft.
I'm not quite confident that I'd get my ignition timing right without the key...maybe if I had a degree wheel and a top dead center gauge, haha.
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Just mark on the flywheel where the old key was and line that mark up with the slot on the crank.
Or have you made a huge divot grinding off the stub remains?
I luckily didn't damage the flywheel, so I could try marking it and lining up with the keyway in the crank.
There's used OEM Stihl flywheels on eBay for around $25, so that's also a good option, pending what I find when I open up the engine.
