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Briggs and Stratton sheering flywheel keys


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  #1  
Old 11-26-2007, 08:40:02 PM
Mac Leod Mac Leod is offline
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Default Briggs and Stratton sheering flywheel keys

I have a 3 hp briggs engine, slightly modified. I have not yet had it running, I can get the engine to fire and run for a few revolutions but then the engine pops back through the carby and the flywheel key sheers. I know the key is just there for timing the engine not as a retainer for the flywheel. I tried lapping the flywheel bore and the crankshaft together. it helped some but still, as soon as the engine starts, the key breaks. HELP

Thanks

Mac leod
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  #2  
Old 11-26-2007, 08:44:25 PM
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Default Re: b/s sheering flywheel keys

Slightly modified, how?? If the compression has been increased considerably, it might be that the extra "poop" when it fires causes more snap on the crankshaft than the key can stand. If you haven't, already, try belting it up to something so the driven machine acts as a dampener.
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Old 11-26-2007, 08:47:33 PM
gene w gene w is offline
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Default Re: b/s sheering flywheel keys

if this eng. was set up w/ an aluminum flywheel & a blade you will need an aux flywheel or iron flywheel also ign timeing & comp. factor in
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Old 11-26-2007, 08:53:43 PM
J.B. Castagnos J.B. Castagnos is offline
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Default Re: b/s sheering flywheel keys

Are you torqueing the flywheel to specs?
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Old 11-26-2007, 09:18:17 PM
Mac Leod Mac Leod is offline
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Default Re: b/s sheering flywheel keys

do not knowtorque specs. have tried both an aluminum flywheel <light mower type> and stock heavy iron flywheel. yes, compression has been increased.

still no luck

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Mac Leod
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  #6  
Old 11-26-2007, 11:03:15 PM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: b/s sheering flywheel keys

Mac,
first look at the crankshaft. Does the key fit snugly in the keyway? Does the tapir on the shaft fit exactly into the flywheel? Are you using the pressure washer that is OEM under the starter assembly, or a flat washer) The factory washer actually helps grip the flywheel. Are you tightening the starter assembly tight enough? When you place the flywheel on the shaft, is there any radial movement (can you turn the wheel before the crank begins to move) ? You may have to add an auxillary flywheel to the output shaft of the engine, in order to slow the accelleration of the crank, when the engine fires. If you have the flywheel retainer good and tight, and the crank still shears keys, you will have to reduce the accelleration rate of the crank.
Andrew
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Old 11-27-2007, 09:42:06 AM
Patrick McNallen Patrick McNallen is offline
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Default Re: Briggs and Stratton sheering flywheel keys

I seem to remember that some B/S keys for mower applications are or were pot metal. They were designed to shear if the blade hit a rock so that the inertia of the flywheel wouldn't twist and bend the crankshaft. If you have a hopped up engine, you might have to use a steel key.
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Old 11-27-2007, 09:48:13 AM
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grub54891 grub54891 is offline
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Default Re: Briggs and Stratton sheering flywheel keys

Had one years back that would do that.It would start and run all day, but shut it off and no restart.broke the key every time.Turns out the crank was bent/twisted.Got it for free so really no loss but it took awhile to figure out.
Tom
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Old 11-27-2007, 01:49:17 PM
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Default Re: Briggs and Stratton sheering flywheel keys

The most common fault for this symptom would be lack of torque on the flywheel retaining nut. The torque should be 45 to 50 Lb Ft. Not Lb In. The only thing that holds the flywheel from slipping and changing the timing (sheering the key) is the friction tension between the flywheel and crankshaft taper, so it needs to be relatively snug. The flywheel key is only for positioning the flywheel, not for driving it. Does the engine kick back or jerk the rope from your hand when starting? If so, everything else being setup properly, I have run into one case where the ignition system module was faulty (requiring replacement) for the cranking / starting timing resulting in flywheel key sheering. (Assuming you are not using ignition points). The other thing that will cause kickback and snap the rope from your hand is not enough flywheel weight. Just as a point of illustration, many Tecumseh push mowers without the blade installed will not start. The blade is needed for the weight. Hope something here helps!!!
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Old 11-27-2007, 04:05:05 PM
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ListerDiesel ListerDiesel is offline
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Default Re: Briggs and Stratton sheering flywheel keys

Quote:
Originally Posted by grub54891 View Post
Had one years back that would do that.It would start and run all day, but shut it off and no restart.broke the key every time.Turns out the crank was bent/twisted.Got it for free so really no loss but it took awhile to figure out.
Tom
I seem to recall (can't find my master manual at the moment) that there are a selection of keys for the engines, depending on the type of engine.

Might be worth checking up on which key and key material you should have.

Peter
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Old 11-27-2007, 10:07:47 PM
SecondMonkey SecondMonkey is offline
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Default Re: Briggs and Stratton sheering flywheel keys

I've had the same problem before, sheared like 5 keys in a row. I forgot the damn washer that goes on top of the flywheel - amazing how much of a difference that little thing can make.
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  #12  
Old 11-27-2007, 10:20:03 PM
Bill Sherlock Bill Sherlock is online now
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Default Re: Briggs and Stratton sheering flywheel keys

Any type of lubricant between the flywheel and crankshaft will also cause crimping or shearing of an aluminum key, especially with a cast iron flywheel. Make sure the mating surfaces are wiped clean.

Bill
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Old 11-28-2007, 11:29:42 PM
Mac Leod Mac Leod is offline
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Default Re: Briggs and Stratton sheering flywheel keys

cleaned the shaft, tightened the fly wheel up <went out and got the proper tool> put a washer in....all is well, the engine fired right up. Thanks for the advice

Mac Leod
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Old 11-28-2007, 11:57:18 PM
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Randy Hart Randy Hart is offline
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Default Re: Briggs and Stratton sheering flywheel keys

Mac Leod, I noticed you said " put a washer in " did you use the domed Briggs washer or just a washer ? it needs to be a briggs washer and > cup down and it should work.
Randy Hart Ohio
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Old 11-29-2007, 12:49:59 AM
Mac Leod Mac Leod is offline
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Default Re: Briggs and Stratton sheering flywheel keys

I know I need the domed washer...it will be a few days before I can get to the mower shop to pick one up. for now it will have to wait.

Thanks for all the help...this is going to be a good little mini bike engine: ported and polished. polished crank and rod, adapted float carby. shaved head...lot of work---I was getting ready send it off to the corner of the shop as I kept sheering keys...not now

Mac Leod
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  #16  
Old 11-29-2007, 10:10:45 AM
K D Redd K D Redd is offline
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Default Re: Briggs and Stratton sheering flywheel keys

B&S does not service the domed washer any longer. I tried to buy some a couple of years age. Have they started building them again?The washer they supply when I tried buying some was flat. But you need some type of washer as the washer will let you tighten the flywheel nut/starter clutch tighter. You need to run the cast iron flywheel if you are not running a mower blade.

Kent
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  #17  
Old 11-29-2007, 10:25:29 AM
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Elden DuRand Elden DuRand is offline
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Default Re: Briggs and Stratton sheering flywheel keys

The domed washer for the flywheels is called a "Bellville" washer. I haven't checked but, if you have the dimensions, you may be able to get them at: MCMASTER.COM

Good luck!

Take care - Elden
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