20 hp shearing keys


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Old 10-10-20, 09:49 PM
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20 hp shearing keys

Riding mower shears keys. I replaced the connecting rod, camshaft seals and gaskets. New oil pan on a 20 hp. Briggs. Put it all back together and adjusted the valves. Torqued the flywheel bolt to 100 ft. Lbs. Started it up and it sheared the key. Tore it back apart and checked to make sure that the timing marks were right. And they were. Cleaned valves, checked all internal parts. All good. Put it back together and sheared the key again. Readjusted valves, double checked torque on flywheel nut. It started and ran. But when I tried to start it again, it sheared the key. I'm running out of ideas. Can anyone help me please? Crankshaft is free on the bottom. No pulleys or belts attached.
 
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Old 10-11-20, 04:06 AM
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Is there a kickback or backfire when it shears?

Shearing the key that quickly, with no back fire would point to a poor fit of the flywheel to the crank. It may be a worn bore on the flywheel. If you could post a picture of the flywheel sitting on the crank without the nut and a couple of both ends of the bore, it may show the problem.

With a good fit assemble with no grease.
 
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Old 10-11-20, 08:21 AM
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20 hp shearing keys

It did backfire. The first time, it was so violent, it cracked the flywheel keyway. Had to change flywheels. It sounds like timing, but the timing marks are on the money, and the flywheel magnet is just leaving the ignition module arm at TDC. Can I lap the flywheel and crank together to improve the fit?
 
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Old 10-11-20, 08:59 AM
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It did kick back? Then that is likely what is doing the key shearing. It's timing almost for sure. When you put the flywheel on and align the keyway with the key push the flywheel on by hand and turn it back and forth on the crank to make sure the key is seating. Then torque the nut.

Then if you are still getting the kickback, don't run it like that the problem has to be worked out.
 
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Old 10-11-20, 03:38 PM
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I had a 16 hp kohler rebuilt for me last year that developed a serious backfire when the engine was started. At first it backfired cold, but as time went on, it backfired less and less as it got more hours on it. It woke the dead when I had to start it the other day while it was very hot, (about 10 hours running on the rebuild) With spot on timing and valve adjustments, I figured it must be the new valve guides expanding and not allowing the valve that needs to be closed to hang up just enough to cause the problem. Probably not YOUR problem, but if you put new valve guides in it, it could be. Good luck!
 
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Old 10-11-20, 04:30 PM
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I am going to pull it back apart and double check that the lobes on the new cam are in exactly the same location as the ones on the old cam. It has to be something simple that i am overlooking. I didnt replace the valve guides, but with the cam and connecting rod brand new, i would think it has to be the cam. Ebay ordered, they may have sent the wrong one. Thanks for the possible solutions.
 
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Old 10-11-20, 05:23 PM
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When you lost the first key, without regards to how strong the backfire, the key should have simply sheared. A cracked flywheel can happen with the key out of place and the flywheel torqued against it.

But since it is ongoing there must be something more.
 
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Old 10-11-20, 10:54 PM
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You can check the timing of the cam by watching the valves in relation to the piston. You can rotate the engine to TDC not on the compression stroke and as you move one direction, one valve will open and as you go the other, the other will begin to open. It's called rocking the valves. As the piston rocks from TDC one way or the other, the valves also do the same in direct relation to the piston. If not, the valve timing is off. I suspect something isn't seating properly with the flywheel and the key shears while it's running and causes the backfire, then the engine dies. Does the flywheel wobble or move at all when mounted on the crankshaft?
 
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Old 10-12-20, 06:59 AM
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The key seems to fit Snuggly in the slot. The weird thing is is I have tried 2 flywheels and 2 crankshafts and get the same results. Valves are seating properly and adjusted correctly, but still the same result
 
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Old 10-12-20, 11:04 AM
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Compared to the original crankshaft and flywheel, the second crank/flywheel are identical?

Also the key shearing only started with the overhaul you did and started with the crank/flywheel set which were running OK prior to the overhaul?

Is that close to what you have?
 
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Old 10-12-20, 12:35 PM
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When I bought the engine it had a broken connecting rod, a bent camshaft, a cracked case, and a broken oil sump pan. I thought I could pull it together and make something good out of it, but apparently that's not going to happen. The key was sheared on the flywheel when I took the motor apart originally. Don't know if that was the cause for all the damage inside or not.
 
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Old 10-12-20, 01:00 PM
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PJ Max gave me credit for sticking with a tough project, but this exceeds my determination by far!
 
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Old 10-12-20, 01:23 PM
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To clarify: you have had one crankshaft and a cam and two flywheels? I understand the two flywheels.

Is there a chance you have a mismatch on the flywheels to the crankshaft. That would affect timing and could give you a kick back.
 
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Old 10-12-20, 06:06 PM
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At this point, anything is possible. It might be just a loose nut behind the wrench. Think I'll start looking for another motor. This one has shot my confidence to he!!. Thanks for your time guys.this is the second crank. The flywheels were used, but supposed to be the same part #. First flywheel was the original that cracked.

n
 

Last edited by Toby Myers; 10-12-20 at 06:14 PM. Reason: Add on
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Old 10-12-20, 10:32 PM
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With the bent camshaft, I would suspect damage to the boss that holds the camshaft in place. They often crack when it gets struck enough to bend it.
 
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Old 10-13-20, 08:43 AM
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Yes it was damaged. Another reason for changing out the case. I will work on the flywheel to crank theory and see where that leads.
 
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Old 10-13-20, 11:18 AM
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Ok, I wasn't sure if you checked that on the cover as well or not.
 
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Old 10-25-20, 10:09 AM
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Well, I made one more attempt at it. This time I switched the flywheel and lapped the crank to the flywheel before installing them. It seems to gave done the trick. Thank you everybody for your input. I probably would have given up on it if I didn't have somebody to bounce this off of. Think I'll try another one.
 
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Old 10-25-20, 12:48 PM
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Glad you got it! Thanks for the update!
 
 

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