Problem with Stihl BR 400 blower

[trooper1954]

Hi all,
I have a 30 year old blower that has served me well and given me no problems until now. The problem is it keeps breaking woodruff keys....I have replaced three myself, and each time the key breaks after a very short while, and the blower is a bear to pull start. I finally took it to a shop...they replaced the key and as soon as I got it home it broke> When I contacted them they told me they wouldn't work on it again!
Can someone tell me if this is more than likely a crank shaft issue, and if so, do I have to replace the crank shaft or should I start with changing the bearings first? Any help to determine the cause of this would be appreciated...with the plug removed it turns over fine on the pull cordwhen there's no compression. I don't want to throw it in the trash as it's in good shape and has never given me problems before. Where to start??
Thanks.

 

[Motown]

I would consider replacing the flywheel. It goes on as an interference fit to the crank, maybe after spinning a couple times isn't wedging in place.

 

[gregs]

I agree with that^^. The key is locating the flywheel/magnet to the crankshaft for correct ignition timing. Its the taper that locks it in place. If its spun multiple times the inner flywheel taper is probably worn. The other option is to apply some loctite sleeve retainer or even red threadlocker and tighten it tight. Also make sure the nut contacts and doesn't bottom out on the threads.

 

[The Cobbler]

how snug does the flywheel sit on the shaft with no key? I'm thinking you have a loose or poor fit on mating surfaces or not getting proper torque

 

[M6erfan]

Make sure you have the correct washers in the correct order under the crank nut. If the flywheel is a little loose on the crank you can use some lapping compound and rotate the flywheel back and forth on the crankshaft. Check for an even dull finish and repeat until you have a lapped joint. I've do this on motorcycle cranks. The flywheel key is just a locator, it has very little shear strength.

 

[Wakefield]

Also the nut that secures the flywheel needs to be torqued all the way to the specification,not looser. At least on lawnmower engines that nut (or the starter clutch that acts like a flywheel nut on older B & S engines) if not tightened could cause the flywheel to shift bending or breaking the key

 

[Lasu]

The flywheel center bore and the crankshaft must be flawless + cleaned with a degreaser, the key correctly, oem nut.
Tightening torque 25Nm. ( BR400)

The whole machine should be inspected and tested properly.

 

[joel_400]

Nothing else works maybe remove the key, clean everything and put some valve lapping compound in between the crank and the flywheel. Spin the flywheel and lap it in then see it you're getting a good pattern. If so then clean it real good and try again. Just make sure it's super clean as any compound or dirt could cause issues. Maybe inspect the taper of the flywheel for cracks while it's off again as well. Just a thought. Good luck!
Joel