20 hp johnson broke shear pin

jwilkey84

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Jun 26, 2005
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524
Well I finally got the 1972 johnson 20hp running good. I was taking it on a test drive and after about 3 minutes the lower unit made a sound like I had hit a small tree branch. I kept driving and it made the sound again, I kept driving and on the thrid time it made this noise the shear pin broke. In all actuallity I don't think I was hitting anything in the water, but it was making that same kind of noise. It would clunk, like maybe it was skipping a gear in the lower unit. When I first got this motor last fall, the guy told me that the lower unit was shot and I thought he was full of it. The shear pin was broke then, so I figures that he thought the lower unit was shot because he couldn't go anywhere. What does this problem sound like to all you pros. Could the lower unit gears be bad, or the clutch dog? Why would that break the shear pin. I have never actually been into the gears of a lower unit, but I am ready to learn.!! It cant be that hard can it? Does it take any special tools.
 

samo_ott

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Jun 18, 2006
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Re: 20 hp johnson broke shear pin

Yep, rounded clutch dog and/or rounded gears.
 

F_R

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Re: 20 hp johnson broke shear pin

Classic description of a worn clutch dog and forward gear.
 

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jwilkey84

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Re: 20 hp johnson broke shear pin

will I need any special tools then, like a spanner wrench or anyting?
 

jwilkey84

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Re: 20 hp johnson broke shear pin

The motor sounded like it was under a lot of load also. I had it on a 12' lund. The nose of the boat would stick up really high (scary) and the motor would sounde like it was under a lot of load, then maybe 5-7 seconds later the motor would go ahead and shoot the boat up on plane. It was like it was hesitating to get the boat on plane. And this has nothing to do with the weight of the boat or anything like that, I can loat this boat in the back of the truck by hand. The only other thing I can think of is the fact that the motor is a long shaft and the boat is designed for a short shaft motor. I was just using this boat to try out the motor. Any other ideas that these details might stir up?
 

jwilkey84

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Re: 20 hp johnson broke shear pin

are there any more ideas concerning this problem? If not, I will tear into the lower unit. Will I need any special tools, that are really expensive?
 

jwilkey84

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Re: 20 hp johnson broke shear pin

Well, I tore it all apart and found a set of really good looking gears to me. They aren't worn and as far as I can tell nothing in the whole assembly is worn. There were a FEW tiny flakes of metal in the housing, like smaller than ground black pepper. I took the assembly totally apart and cleaned everything up with some brake parts cleaner and I think everything looks good. The only thing I would question is a part that moves back and forth as you shift gears. It is the only piece on the lower horizontal shaft that actually sits on the splines and moves on a set of 2 ball bearings that are spring loaded. Is this the "clutch dog"? If not what is the clutch dog? Anyway this piece looks as thought it has a little wear on the ears that actually engage the forward and reverse gears. Does this sound like the culprit or could I have just hit something in the water and broke the shear pin? I will try to post some pics of this piece, but sometimes I have trouble attaching pics.
 

jwilkey84

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Re: 20 hp johnson broke shear pin

unless someone can tell me how to reduce the size of my pics, I cant post them. Each of my pics is like 3.9 MB and this website won't allow anything near that size in the jpg form.
 

Xcusme

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Re: 20 hp johnson broke shear pin

The easiest way is to open a free account at photobucket.com. Upload your pics there, then come here and post links to those pics. There's a "How To" here on iboats.
 

F_R

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Re: 20 hp johnson broke shear pin

Well, I tore it all apart and found a set of really good looking gears to me. They aren't worn and as far as I can tell nothing in the whole assembly is worn. There were a FEW tiny flakes of metal in the housing, like smaller than ground black pepper. I took the assembly totally apart and cleaned everything up with some brake parts cleaner and I think everything looks good. The only thing I would question is a part that moves back and forth as you shift gears. It is the only piece on the lower horizontal shaft that actually sits on the splines and moves on a set of 2 ball bearings that are spring loaded. Is this the "clutch dog"? If not what is the clutch dog? Anyway this piece looks as thought it has a little wear on the ears that actually engage the forward and reverse gears. Does this sound like the culprit or could I have just hit something in the water and broke the shear pin? I will try to post some pics of this piece, but sometimes I have trouble attaching pics.

Yes, that sliding part is the clutch dog. It takes surprisingly little wear on the ears to make it jump out of gear. Wear on those ears are accompanied by wear in the gear where the dog bangs up against it as it rotates. You are probably looking right at the problem and don't recognize it.

The dog is forced into disengagement by torque, the gear rotates 1/2 turn and the dog slams back into engagement, giving that hit-a-stump sensation. All the slamming also beats the shear pin to death. BTW, you are supposed to be using a stainless steel pin..are you?
 

jwilkey84

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Jun 26, 2005
Messages
524
Re: 20 hp johnson broke shear pin

yes, I am using the omc shear pins that are specific for this setup. I have read the arguments on here about using an old bolt and stuff like that, but I would rather break a shear pin than something more $$$.
 
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