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08-16-02, 07:51 AM   #1  
lawn mower

I hit something hard with the blade of the lawn mower. The engine stopped. Apparently the pin inside is broken. It is an old Briggs and Stratton. How do I repair this, is it complicated?

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08-16-02, 01:26 PM   #2  
You may have sheared the shear pin.
This is a pin (really a small rectangular piece of metal) that connects the magneto to the engine shaft.

You will have to pull the magneto, take what's left of the pin to a hardware store (for sizing) and buy a new pin. You may have to cut the pin down (lengthwise). Then simply reassemble with the new pin, the magneto will only go on one way with the pin installed, so no timing or anything like that will be affected.

That magneto is a large spinning mass, when it's stopped suddenly, it takes the pin out, rather than say, snapping or twisting the shaft it's attached to.

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08-16-02, 07:25 PM   #3  
That is correct...except...The key is made of an alloy that is designed to break before the flywheel does, or the crankshaft does. DO NOT use any key other than what is intended for it. The key is made to shear before damage occurs to the engine when it comes to a sudden halt. If you use a steel key, or any key not soft enough, the flywheel could explode if the situation ever occurs again. The flywheel is made of aluminum, and turns at high RPMs. If a key that is too hard is used, the flywheel could break rather than the key. You don't want fragments of metal turning at 3500 RPM to go flying around you. Buy the correct replacement at your lawn mower repair shop....don't try to make one.

To change the key, remove the top engine shroud, pull the flywheel with a puller, replace the key, reassemble. Let us know how it goes!

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