Crofter
Addicted to ArboristSite
Is any brand or model of saw prone to this. Is some particular operating practice more likely to cause failure. Are the plastic flywheels an attempt at lessening crank breakage.
What is the Cause of Crankshafts Breaking?
- Thread starter Crofter
- Start date
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Crank breaking is usually caused by worn crank bearings. The crank starts to move around on its bearings, and after a period of time viloa broken crank.
I think the plastic flywheel thing was a effort by stihl to get certain saws to rev out faster so they where better for limbing.
I think the plastic flywheel thing was a effort by stihl to get certain saws to rev out faster so they where better for limbing.
leweee
ArboristSite Operative
crank breakage
Over tensioned chain, jammed chain, shock load & crank fatigue are all causes of crank failure. You brake something Frank?
Over tensioned chain, jammed chain, shock load & crank fatigue are all causes of crank failure. You brake something Frank?
ehp
Addicted to ArboristSite
we have had alot of 372's break the crank in the last couple years now this is not saying they have a problem because they donot, first the 372 is about 90% of the saws used here for logging, and after listening to them tell there storys it was this happening alot of the time, the person would be limbing with the saw wide open and knock the chain brake on, bye bye crank
I was first thinking they were stuck and pulling hard on the saw to get out because of where it was broke
I was first thinking they were stuck and pulling hard on the saw to get out because of where it was broke
njforestfire
ArboristSite Operative
My 372xp (R.I.P.) broke the conn. rod instead of the crank. This ever happen to anyone else for this model??? Manufactured in 1998. I was the second owner and it was never apart. Mix was always run at @45:1.
ehp
Addicted to ArboristSite
nope never seen a 372 rod break even on the racesaws, and i use that rod on other small race motor that turn a real high rpm
njforestfire
ArboristSite Operative
Maybe I should have played the lottery that day.ehp
Addicted to ArboristSite
Maybe, i hope your luck is better than mine, most saws donot break the rod, the 3120 with a stock rod would be the worst once you take the rpm limiter out of it even on gas they break very easily and on alcohol they break real easy , that is why we put the 084 rod on the 3120 crank, she is good to go now
Ekka
Addicted to ArboristSite
I had a Stihl MS460 only 6 months old break the crank and bend the conrod. Stihl repaired it free!
Ended up that part of the piston ring broke and got jammed between the top of the piston and the exhaust port. They said "it's just one of those things that sometimes happens" and totally rebuilt it.
I always run at 40:1 fuel, run slightly rich, and don't overtighten the chain, but regardless sometimes the crankshaft bearings especially on the drive sprocket side get a bit of wear and you need to change them. When I clean the saw or take the bar off I check to see if there's any movement.
Ended up that part of the piston ring broke and got jammed between the top of the piston and the exhaust port. They said "it's just one of those things that sometimes happens" and totally rebuilt it.
I always run at 40:1 fuel, run slightly rich, and don't overtighten the chain, but regardless sometimes the crankshaft bearings especially on the drive sprocket side get a bit of wear and you need to change them. When I clean the saw or take the bar off I check to see if there's any movement.
Lawn Masters
Addicted to ArboristSite
Good habit Ekka. I like to check my saws periodically for wear like that but havent seen any evidence of it yet.
thompson1600
ArboristSite Guru
What method do you use to check for end play or bearing wear?
I use a gauge, but have never been taught a real sure way to make sure I'm doing it right.
Tom
I use a gauge, but have never been taught a real sure way to make sure I'm doing it right.
Tom
Lawn Masters
Addicted to ArboristSite
I just feel for any movment that isnt normal and that determines whether or not I need new bearings. (havent needed new ones yet)
Fish
Tree Freak
Cranks always break on me, when I put my maglev system on a saw without tempering
with a slow marinating of garlic cloves and thistle seed oil, and at least a dab of
liquid h2o. Otherwise, it is not an issue, or more over, a "non" issue, which seems to
dominate during the winter months.
with a slow marinating of garlic cloves and thistle seed oil, and at least a dab of
liquid h2o. Otherwise, it is not an issue, or more over, a "non" issue, which seems to
dominate during the winter months.
spacemule
The Peanut Gallery
Fish, don't you mean liquid H<sub>2</sub>O?Fish said:
That's your problem--you should be using solid H<sub>2</sub>O.Ekka
Addicted to ArboristSite
I know it might be old hat to revive this thread but I had a thought of what the problem could also be and it wasn't posted on here.
We tension the chain on a hot saw and then put the saws to bed a night which is nice and cold (freezing around your parts) and then the chain contracts.
Somewhere I read that you should loosen off the bar nuts and slacken the chain off .... this could put a lot of stress on the crankshaft and over time form a weak point full of stress fractures, also the bearings would be copping it too.
So good practice IMO
We tension the chain on a hot saw and then put the saws to bed a night which is nice and cold (freezing around your parts) and then the chain contracts.
Somewhere I read that you should loosen off the bar nuts and slacken the chain off .... this could put a lot of stress on the crankshaft and over time form a weak point full of stress fractures, also the bearings would be copping it too.
So good practice IMO
bgadway
ArboristSite Lurker
I had the crank break on my 362xp.. It broke outside the case on the clutch side of the saw have no idea how I was cutting small stuff maybe 3 to 6 inc stuff.......anyone else ? ? ?
