I have a Jonsered 451EV that for some reason all of a sudden it shears flywheel keys after every 5-10 minutes of running. I never had any problems with this saw before. Does anybody have a clue what could be going on?
I keep shearing flywheel keys
- Thread starter SGT_KQ
- Start date
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Two things, either the flywheel nut is not being tightened enough or the taper in the flywheel its self has been damaged. The taper is what holds the flywheel to the crank, if damaged it will move and shear the key. The key is just an alignment device to align the flywheel for timing, when the nut is tightened properly the flywheel is jammed tightly to the crank and will stay put.
A damaged taper can be corrected by using valve grinding compound between the crank and flywheel to reseat it. 3-4 applications of compond and rocking the flywheel back and forth will restore the taper. The compound must then be thoroughly removed, the bore and crank stub must be squeaky clean and dry before resssembly.
A damaged taper can be corrected by using valve grinding compound between the crank and flywheel to reseat it. 3-4 applications of compond and rocking the flywheel back and forth will restore the taper. The compound must then be thoroughly removed, the bore and crank stub must be squeaky clean and dry before resssembly.
Excellent detailed instructions. Only thing I can add is that Clover valve lapping compound is cheap and easy to find and comes in two grits to a container. I reckon that'd do the trick nicely.
It will, I have done more than 10 setups and all have turned out well, saws are still running and no more sheared keys.
promac850
formerly promac610
I get it on there as tight as I can with a normal 3/8 drive ratchet and the flywheel held stationary with a strap wrench, oil filter wrench, or the like. (I don't recommend the screwdriver trick on flywheels, especially newer ones... they can't take the stress) Have not encountered problems so far.
I agree. My 076 kept tossing its flywheel and I did exactly what Jerry details here and no more issues.
Chris B.
I can`t say for the 410 specifically as I don`t have specs for it but most saws can be torqued to 20-25 ft lbs. Good and tight with a 3/8" drive ratchet only would put it on tight enough.
Is the nut tight when you find the key broken? Clean the joint between the crank and the flywheel well before you start. Just like the clutch will tighten itself onto the crank because of the engine rotation, the flywheel side will loosen if the flywheel is not seated when torqued because of the rotation. Slip a socket over the nut and give a light rap to the flywheel with a brass hammer and then check your torque specs to see if anything has changed. The flywheel key needs to be the correct size and in line with the crank, rounded side down inserted into the crank. You can use a little blue loctite on the threads, but make sure the flywheel is seated all the way down against the shoulder of the crank.
Terry Syd
Addicted to ArboristSite
One thing to check, especially after lapping a flywheel, is where the end of the crank taper hits the end of the taper in the flywheel. If there is a ridge at that point, it will prevent you from getting a good contact between the two surfaces.
There's two remedies, one, take a tad off the end of the crank taper so it doesn't hit the ridge and the other is to carefully go into the flywheel and grind out the ridge in the flywheel. I usually grind out the ridge in the flywheel.
There's two remedies, one, take a tad off the end of the crank taper so it doesn't hit the ridge and the other is to carefully go into the flywheel and grind out the ridge in the flywheel. I usually grind out the ridge in the flywheel.
Little Al
Addicted to ArboristSite
100% answer, might be worth checking the taper with engineers blue first it will give you some idea where the flywheel is/isn`t seating, then to the lapping stage
You could check Al , but the compound will only remove the high spots anyway.
To my recollection i cant remember finding the nut loose when it shears, but after it shears even with the nut being tight the flywheel will somewhat spin on the shaft. Its not like you can spin it by hanmd though and of course its fine until the compression stroke. Maybe im doing this wrong but i was sticking a piece of flat stock into the cylinder and then tightening that nut.
All good advice. I had this happen in my 268. Cleaned the shaft and flywheel tapers and no more problems. Remember, the key is only for alignment. The taper is what locks the parts together. My SP125 has no key on the clutch side.
Terry Syd
Addicted to ArboristSite
"Maybe im doing this wrong but i was sticking a piece of flat stock into the cylinder and then tightening that nut."
If that means that you are sticking a piece of flat stock into the exhaust port to keep the piston from moving, yeah, you're doing it wrong. You can deform the cylinder liner/piston that way. We've seen some stuffed up cylinders from people using flat screwdrivers in the exhaust port to keep the engine from rotating.
Try using a section of rope fed into the cylinder through the sparkplug hole. Make sure the piston has closed the exhaust port and then feed the rope in and around the cylinder. A larger diameter rope creates a greater rod angularity to the crankpin, which helps to keep from bending lightly constructed rods.
If that means that you are sticking a piece of flat stock into the exhaust port to keep the piston from moving, yeah, you're doing it wrong. You can deform the cylinder liner/piston that way. We've seen some stuffed up cylinders from people using flat screwdrivers in the exhaust port to keep the engine from rotating.
Try using a section of rope fed into the cylinder through the sparkplug hole. Make sure the piston has closed the exhaust port and then feed the rope in and around the cylinder. A larger diameter rope creates a greater rod angularity to the crankpin, which helps to keep from bending lightly constructed rods.
Boleclimber
ArboristSite Guru
You may want to try a dab of tooth paste applied to the crank and then put the fly wheel on. I also use blue loctite on the threads.
I have used this trick on a few saws with good results.
I have used this trick on a few saws with good results.
Boleclimber
ArboristSite Guru
Using toothpaste is a trick taken from motocross mechanics.
Tootpaste dries to a glue like substance and the grit adds surface area and holding ability. The bond continues to allow easy fly wheel removal.
I had a 262XP that had a sheared keyway and would not hold time. I dabbed toothpaste, set the timing, and tightened the nut with blue loctite. Been running 4 years, no keyway, timing has stayed constant.
I have used it on a few 394XP's to.
Using toothpaste as a search word I found another thread discussing this technique.
http://www.arboristsite.com/chainsaw/98002.htm
Tootpaste dries to a glue like substance and the grit adds surface area and holding ability. The bond continues to allow easy fly wheel removal.
I had a 262XP that had a sheared keyway and would not hold time. I dabbed toothpaste, set the timing, and tightened the nut with blue loctite. Been running 4 years, no keyway, timing has stayed constant.
I have used it on a few 394XP's to.
Using toothpaste as a search word I found another thread discussing this technique.
http://www.arboristsite.com/chainsaw/98002.htm
Last edited:
BrianBryan
New Member
13 years later and this post is still relevant. I damaged by flywheel/key and thought I was screwed but I just tightened the flywheel and it works now, minus the key. Thanks!
BrianBryan
New Member
Oil filter wrench! good tip, thanks
