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Shear pin Failure

S

Sam110

Regular Contributor
I was on the river today doing some carb 'tuning' with my '65 Johnson 18 hp FD-19 and it was running pretty good. I had just leaned down the HS needle and could tell I picked up a few more rpm so I made another adjustment but it didn't have any effect either way on the rpms. I throttled back to a slow trolling speed for a few minutes and then throttled up quickly. When I did the motor coughed and nearly simultaneously the motor revved up and I was going no where. Luckily I was only about a 1/4 mile from the dock and I paddled it back. (up stream of course)

Once at home I pulled the prop and found the shear pin had sheared. I am happy it was only the shear pin but now I have to question . . Is it uncommon for a shear pin to shear for such a mild motor sputter (cough / sneeze) or would it be fair to guess that the shear pin was already weakened over the years prior?

I have been told a shear pin is an item I should have as an 'extra' for such a time but I don't think I could have changed it with the minimal tools I had with me, it took some prying and pounding to get the prop off.
 
But the steel pin from the factory was a " drive pin " with the prop having a rubber shock absorbing hub.--------Not supposed to shear.------But yes if it is a brass pin in there they can shear just froim repeated shifting and when you least expect it.
 
Could have been from the extra umph from adjusting the high speed needle as well. At least you know the prop hub is in good shape. Is it possible you bumped a rock or log in the river? I always take a spare prop, hardware (pin cone cotter key) spark plugs and wrenches.
 
Reminds me of the time I sold a new 2hp and the buyer asked about need for shear pins. I explained that it has a stainless steel "drive pin" and it is not supposed to shear. OK, so the guy takes the motor up north somewhere in the wilderness, puts it on a canoe, and goes way up the lake and shoots a moose. Loads the moose into the canoe and heads back. Wouldn't you know it, he hits a rock and breads the unbreakable drive pin. No spare, because I told him he didn't need one. He was not a happy camper when he came to see me when he got back home.

BTW, those drive pins can wear out from many shifts and/or rock dredging. Especially if the hole and slot are worn.
 
I was out in the middle of the puget sound during a strong tide when I lost my first prop. That will make your hole pucker. That was a long tow with high winds and 5ft swells.
 
So . . . I should replace the pin with a steel 'drive pin' ? The one that sheared was definitely brass. I guess the one's sold here on Marine Engine are steel it doesn't say. Think I'll order several that ought to keep me from ever having that problem again ! lol I see the same pin is used in my 9.5 Evinrude too so I'd better order several. (think I'll back off that HS needle a wee bit too !)
 
Brass ones can barely withstand the normal torque. Correct one is 307949. It is stainless steel. Get a couple of 306394 stainless steel cotter pins also. Lots of props have been lost due to steel cotter pins rusting out.
 
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