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JerH
05-18-2004, 11:34 AM
How does one fix the shear pin on a lawnmower? I ran over a hunk of metal with mine the other day, and my neighbor said I'd probably need to replace the shear pin. I went to my handy owner's manual, and couldn't find a word about doing this. Can anyone help me?

Ezstrete
05-18-2004, 11:44 AM
How does one fix the shear pin on a lawnmower? I ran over a hunk of metal with mine the other day, and my neighbor said I'd probably need to replace the shear pin. I went to my handy owner's manual, and couldn't find a word about doing this. Can anyone help me?

If the erngine is running smoothly, and the blade is spinning, leave it alone.

JerH
05-18-2004, 12:22 PM
It isn't running - I should have mentioned that in my OP. It will start up but immediately putter out.

Bill H.
05-18-2004, 12:39 PM
It ain't got no gaaas in it.

Here's my understanding: you pull the cord, and it pulls freely (i.e. the engine isn't frozen). It kinda sputters, but doesn't start (i.e. it begins to start, and likely some smoke comes out of the exhaust, but it doesn't fully start). True?

If so, it's likely that running over something is coincidental and unrelated. First thing I'd try is to put a new spark plug in it. Make sure it's gapped right. And, do make sure it's got gas. If those don't do it, I would play with the carburator adjustments next, but I'm not sure if you're comfortable with that.

BrotherCadfael
05-18-2004, 01:00 PM
I got my lawn mower out Saturday morning, and spent an hour trying to get the it running. I stood there and contemplated yet another summer spent fighting that goddamn mower, which is seriously underpowered, jams at the drop of a hat, and a drive system that works when it feels like it. I think it was designed for suburbanites with a 10x20 lawn who mow whenever there is a half-inch of growth.

I stood there and thought about it for a while, and threw it in the back of the car, called my son to come along, and went out and bought a new one!

The new machine cut right through the tall patch over the septic field without even revving down. Not a hint of clogging on the discharge chute, the drive works perfectely, and, while this may make me Officially Over-The-Hill (TM), it's got electric start!

Wahooo!!!!

Finagle
05-18-2004, 01:01 PM
Lawn mowers have shear pins? News to me. But a quick Google search informs me that at least some mowers have em.

If you have a shear pin, it's essentially just a bolt that's designed to fracture in a sacrificial way, rather than destroying your blade or engine. So the engine would run, but the blade wouldn't turn. Replacing it would be a matter of buying the right part at your local dealers, removing the old one, and putting in the new.

But...if your manual doesn't say anything about a shear pin and your mower isn't running immediately after you hit something, then I suppose it's possible you don't have a shear pin and you bent the crankshaft or something equally ominous. Any signs of oil leakage? Anything blocking the blades from turning? (Note: unplug spark plug before checking this.)

NutMagnet
05-18-2004, 01:40 PM
Shear pins are/were common on any motor-driven device that might jam. Rather than having the blade break and come flying off, the shear pin shears and the motor spins to a stop while the blade just stops. Later models did away with the shear pin replacing it with a sacrificial coupling where the blade meets the shaft.

The motor stops because the blade acts as a flywheel. Without a blade, the motor sputters and dies as the OP observed.

The trouble with most of these gizmos is that the crankshaft bends anyway- one of the things the shear pin was designed to prevent. Very often the crankshaft can be straightened though, but old-timers will only do it once (metal fatigue/hairline cracking).

NutMagnet
05-18-2004, 02:11 PM
You know reading carefully is a good idea before running off at the keyboard. I mistook Brother Cad for jerH about buying a new lawnmower.

Replacing the shear pin is easy.

Mowers with shear pins have a single bolt connecting the blade to the crankshaft. I believe they are all "correctly" threaded - i.e., righty-tighty/lefty-loosey. Spray the nut and the joint between the blade and shaft with penetrating oil. Remove the bolt. Tap the connection while rocking the blade to pull it off. This may be a little difficult as bits of the shear pin my have jammed in the joint. Another pair of hands is useful here. Once the blade is off, check it for "bentness" and balance. A straight edge will tell you if it's bent. It can be hammered straight. You can buy a little cone-shaped balancer that the blade sits on. It has a pivot which allows the blade to float and "find" level. If it measures evenly from each tip to a level surface, it's balanced. Sharpening it (more on one end than the other) balances it. Sharpen the light end until it's sharp, then the heavy end until it's balanced.

The most common shear pin is halfmoon shaped. The curved side sits in a slot in the crankshaft. The blade housing has a slot which fits the flat side of the shear pin when you slide the blade back on the shaft. Other shear pins are just rectangles of soft steel, with no curved side. The principle for installing them is the same.

Replace the blade and bolt. Start the mower. If it vibrates excessively:
the blade is bent; or
the blade is unbalanced; or
the crankshaft is bent.

If it runs normally, mow yer brains out.

cornflakes
05-18-2004, 02:33 PM
If your lawnmower is anything like my Briggs&Stratton, the cotter key that locates the flywheel on the crankshaft has sheared. Pull off the top cover, remove the flywheel nut and you will be able to see if the flywheel and crankshaft slots are lined up.

Then again, my experience is that a sheared flywheel key results in advanced timing. The mower fires backwards when starting and tries to pull the rope out of your hands.

justwannano
05-18-2004, 06:34 PM
In my experience,I've kept lotsa mowers running over the years and even worked in a small engine repair shop,you've probably sheared the flywheel key.
There are several symptoms but put if it was running and you hit something hard and the mower won't start you've sheared the flywheel key.

justwannano
05-18-2004, 06:36 PM
A cotter key is something entirely different.

cornflakes
05-18-2004, 07:01 PM
A cotter key is something entirely different.Wow. After doing a Google search, I get the impression that my relatives and I were the only people to call a cut piece of keystock a cotter key. Maybe this is derived from the definition "a wedge-shaped or tapered piece used to fasten together parts of a structure." No, I wasn't referring to a cotter pin.

Johnny L.A.
05-18-2004, 08:29 PM
As stated, the key is designed to fail to prevent damage to the engine.

My friend gave me his new lawnmower with the house. He'd used it three times. I loaned it to a friend, and he used it once. I used it, and hit a stump. The key failed as it was supposed to, but not in time to save the crankshaft. The crankshaft on the B&S engine is bent. It will cost $120 to replace it. The lawnmower repairman told me I should buy a new mower (which I already had).

The flywheel key engages the flywheel, and the flywheel (I'm told) controls the timing. That's why the engine will sputter a little and then die -- it's out of time.

I might replace the key, but the engine might shake too much to use. I don't think I'll replace the crankshaft, as it's an expensive part and I already have a replacement mower.

justwannano
05-18-2004, 11:37 PM
Quote Johnny L.A.
I might replace the key, but the engine might shake too much to use. I don't think I'll replace the crankshaft, as it's an expensive part and I already have a replacement mower.

It will also ruin the oil seal and sooner or later you'll lose the oil.

Johnny L.A.
05-19-2004, 08:58 AM
Aw, I was hoping the next post would be something like "Oh, you can get a new crankshaft for ten bucks from ______!"

ZipperJJ
05-19-2004, 11:55 AM
The new machine cut right through the tall patch over the septic field without even revving down. Not a hint of clogging on the discharge chute, the drive works perfectely, and, while this may make me Officially Over-The-Hill (TM), it's got electric start!

Wahooo!!!!

Slight hijack but I have the same old mower that you just replaced.

What did you get? I must have one. This one I have is a POS and it's like 2 years old and it takes me 2 hours to cut an acre lawn!!!!!

justwannano
05-19-2004, 01:58 PM
Quote Johnny L.A.
Aw, I was hoping the next post would be something like "Oh, you can get a new crankshaft for ten bucks from ______!"

Naw but I do have a really shiny bent one in the basement you can have.

Several years ago Mom was cutting the grass and hit the water shutoff marker. Bent the crank. I decided to replace it. Not a really big deal just kinda expensive. IIRC $85.00 for the crank. The whole mower only sold for $125.00 brand new.
Damned if she didn't do it again
:D

BrotherCadfael
05-19-2004, 02:01 PM
Slight hijack but I have the same old mower that you just replaced.

What did you get? I must have one. This one I have is a POS and it's like 2 years old and it takes me 2 hours to cut an acre lawn!!!!!It's a
TORO Recycler 20031 Electric Start (http://toro.com/home/mowers/recycler/20031.html). It has a good Consumer Reports rating, and Toro is currently running a special which gives you a free upgrade from 2 year to 3 year warranty. Price was $420, and they gave me $75 for my POS. Not bad.

ZipperJJ
05-19-2004, 02:10 PM
Hmm I have a Toro recycler, self-paced mower and that is the POS I'm talking about :D

I think we need to go to a tractor. We need more POWER!

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