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I hope someone here has experienced this problem....and managed to find the cause.
I have experienced it on two briggs engines. The first was an older "L" head 12hp. The present one is an OHV engine 31P977-0575-E2.
The flywheel keys shear upon start up attempt. There are no obstructions, the engine turns freely by hand, the pto is not on, the valves are adjusted properly, the keyway is not damaged or misshapen (worn). The engine(s) were running fine until the key sheared. On the "l" head engine I finally replaced the aluminum key with a steel key which worked for about a year. It then sheared again and subsequent replacements with steel keys resulted in immediate shearing when attempting to start. Just got a backfire and it sheared. The OHV key was also finally replaced with a steel key which only lasted for four start cycles before shearing. I've talked to friends at the John Deere dealer where I last worked and they have not had this problem. Am I the only one? I hope to find someone here who has come across this phenomenon and found a cause if not a cure. I thank you for any comment or help you can offer. :dunno:
 

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How tight are you getting the flywheel nut?? We has a similar issue we finally solved by using air impact instead of battery powered impact.
 

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Ditto having had same problem...Did you tap the key down into the grove before putting on nut or bolt....? I have also learned from a mower shop...add a concave washer under the bolt head or nut...sheering is definitely due to loose flywheel..even if the bolt is tight..
 

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Note to self on that one suspicion! Thanks! :ThumbUp:
 
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