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Discussion Starter #1
Went to boost the 314 on the weekend and after a couple seconds of cranking there was a sharp sounding bang. After which, the starter motor would only spin and not crank the engine. Had a quick look and could see it had sheared the armature.

Pulled the starter today and swapped with one from my parts tractor...314 up and running. Before starting, I did pull the plug and turn it over by hand, no odd noises and no missing teeth on the flywheel.

But what the heck happened and is it going to happen again? Was working fine the last couple years since the rebuild. A google search did turn up a couple dead threads on an identical failure, but no resolution or cause determined.

I've seen bendix's, springs, bushings and brushes fail in automotive starters, but have never seen an armature shear like this.





Anyone know or care to give a best guess?

Al
 

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My guess is you were just unlucky to have an armature with a manufacturing defect that went undetected. I can't imagine how much force would be required to snap a good armature shaft, but it would be a hell of a lot more than what a starter motor could generate.
 

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Kind of difficult to determine the actual cause of break from pic, but I'm thinking stress failure. Also thinking that in some time of the starters life, the pinion end got dropped/hit/bent. When starter engages flywheel is gets straightened out and the relaxes after 180º, then straight, then bends back. Another possibility is a little runout in the flywheel and/or the pinion on the starter. The bending will introduce stresses into the shaft until, "Bang" and it's now a 2 piece shaft with no stresses! Just guesses on my part though! Bob
 

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Discussion Starter #4
Maybe...best explanation I've heard thus far. Doesn't seem like something guys are running into on a regular basis...which is good.

Al
 

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You might need to shim the starter. I have a 112 with a K301 that was hard starting. I got a new replacement starter thinking the old one was going, and after putting the new one on I still had the same issue. Found that when the starter gear meshes with the flywheel gear it is too tight. I'm going to shim it out with a couple of thin washers.
 

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Discussion Starter #7
Don't think it was a backfire, those are pretty obvious and it wasn't "that" loud...but again...somewhat plausible as an explanation too.

Shimming is something I had not considered. Use to be common with automotive starters...I'll check into it.

Al
 

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My guess: Just fatigue after nearly 40 years of use allowed a “defect” (imperfection) to grow and then snap.
 

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Likely just fatigue. You can see the shaft was cracked for some time by the darker spots in the break. Or, if it's not that old, (as it appears) (sure is clean for all of those years) just blame it on lousy quality. As for any quality control or inspection by the manufacturer for parts like this? I doubt if they bother with any of that.
 
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