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  1. #1

    Default Shear Pin information

    I have a 5.5 hp Johnson CD-21 motor from 1964 and managed to have my first ever shear pin failure. I could use some infomation from some veterans who have experienced more than a few. I think it failed from fatigue during a shift rather than hitting a rock since I was having trouble with the idle and was trying to shift at a higher than normal rpm. Also I don't think I was in shallow enough water to hit something. I think the shear pin had been previously replaced with one that didn't match the original specs. The failed pin was 5/32 dia and I can easily fit a 3/16 drill bit through the drive shaft and hub. Do you agree that a 3/16 pin should be used to replace it? Having a smaller dia would create more clearance between the parts which would allow a harder hit on the pin as it accellerates through that clearance with both the prop shaft and the propeller hub and the smaller dia would also likely have a smaller shear force unless there was a material difference. The other puzzling thing was that the three pieces of the failed pin had a total length greater than the o.d. of the propeller hub at the drive hole. The rubber cone would still hold it in place though it would have to be forced over the ends of the pin, but I don't think the pin length should ever exceed the o.d. of the propeller drive hub, should it? Lastly, does anyone have a CD-21 and a spare shear pin or an old parts manual so they could tell me the dimensions of the correct shear pin or perhaps an OEM part number. Thanks for any help.

  2. #2
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    Default Re: Shear Pin information

    Sounds like you know what you are doing. Part number is 302333 and it is 3/16" x 7/8" , stainless steel. A clutch inside the lower unit releases whenever you tangle with a rock, and resets itself automatically.

  3. #3
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    Default Re: Shear Pin information

    The prop hub has a rubber bushing pressed in just replace the pin and go fishing. It is not a shear pin someone just jerry rigged it the bushing should spin if the prop hit an object underwater.
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  4. #4
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    Default Re: Shear Pin information

    Are we sure that the 64 models had a rubber hub in the prop ?

  5. #5
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    Default Re: Shear Pin information

    My bad it does have the slip clutch in the lower driveshaft in the gearbox.
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  6. #6

    Default Re: Shear Pin information

    Thank you for your replies, but I don't think all the information provided is accurate. I finally found a parts diagram on this MarineEngine.com site which let me see the parts involved. The 1964 CD-21 does not have a rubber bushing in the prop nor does it apppear to have a clutch in the lower unit. A part that is called a "shifter" (P/N 0329795) sits on the spline of the prop shaft and engages the forward (0377152)or reverse bevel (305216) with sort of a two knob drive design which I don't see any way it can slip unless it throws the shifter back to neutral somehow. The later dog clutches that are used have springs that hold things in place and allow the clutch to slip, but this design does not have any of that. Am I missing something that would allow this design to slip if the prop hits something? The part number provided by fdgator is the part number that is shown in the diagram, but if it is stainless steel then I would think it was a "drive" pin and wouldn't have shear properties that would protect the drive train. I do agree on the size of this pin, though, just question whether stainless was ever used in a pin that was designed to shear. I am leaning toward trying to find pin of this size in brass or something that would shear and then going back with that. Anyone have any additional thoughts? Thanks

  7. #7
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    Default Re: Shear Pin information

    Wrong.---------The slip clutch is found in the pinion shaft assembly just below the waterpump !!----And I agree with using a brass shearpin too.

  8. #8
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    Default Re: Shear Pin information

    Sorry, but there is indeed a slip clutch in the lower unit. It is item #10 on the diagram, part number 376073. However OMC called it a pinion shaft and shock absorber assembly. It is a two piece assembly with a heavy spring inside. When you hit a submerged object, the spring winds up and loses its grip on the upper section, which then can rotate till the obstruction is clear, then the spring relaxes and grips both sections again.

    Yes, it is a stainless steel drive ​pin, not intended to break under normal circumstances.

    EDIT: racerone and I were typing at the same time.

  9. #9

    Default Re: Shear Pin information

    Gentlemen - thank you for all your help. I was looking for some sort of clutch down on the propeller shaft near the clutch dog or shifter or whatever and didn't think to look on the vertical drive shaft. Although I'm not sure I would have identified it there if I had looked. Since this doesn't need a shear pin I will just locate a solid drive pin in the size mentioned by fdrgator and put it back together. Once, again, thanks to you all for the info.

  10. #10

    Default Re: Shear Pin information

    I recently bought a 3 H.P. 1960 Johnson. I noted the shear pin was steel. I haven't touched an outboard motor in over 50 years but there was no way I was installing a steel shear pin in my lower unit. From my past I remember shear pins being brass. The O.E.M. shear pin was 0.002" smaller than the brass pin (cut from a brass rod) I replaced it with. The O.E.M. pin was slightly "S" shaped and one of the prop blades has a slight ding on the leading edge indicating that the pin probably attempted to shear at at some point. I WON'T run s steel shear pin or as Johnson calls it a "drive pin".

  11. #11
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    Default Re: Shear Pin information

    It's your motor, do as you want. You want to be changing pins out on the lake, that's fine. But is is supposed to have a stainless steel drive pin. The rubber ring in the prop takes care of shock loads.

    Now let's but this thread back to rest in the archives.

  12. #12

    Default Re: Shear Pin information

    Quote Originally Posted by fdrgator View Post
    It's your motor, do as you want. You want to be changing pins out on the lake, that's fine. But is is supposed to have a stainless steel drive pin. The rubber ring in the prop takes care of shock loads.

    Now let's but this thread back to rest in the archives.
    Thanks. I did read another post where the writer also swears by stainless shear pins. I've never had any exposure to engines larger than 20 C.I. I doubt my little 3 H.P. 1960 Johnson is going to stress my brass shear pin - but a rock will. Thanks for the advice and I think you're right on a 50 H.P. engine.

  13. #13
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    Default Re: Shear Pin information

    Geeze, I grew up in a part of the country that had a lot of fishing motors and rock bars. Seems brass shear pins were the most popular. Cheaper than gears and ect. JMO.

  14. #14

    Default Re: Shear Pin information

    Quote Originally Posted by Boobie View Post
    Geeze, I grew up in a part of the country that had a lot of fishing motors and rock bars. Seems brass shear pins were the most popular. Cheaper than gears and ect. JMO.
    Having only run small engines in my youth I never saw a shear pin made of steel until I purchased my little 3 H.P. in January of this year. Possibly larger engines with forward/reverse would chew up a brass shear pin. Dunno.

  15. #15
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    Default Re: Shear Pin information

    Does anybody really think a skinny little 1/8" diameter stainless steel drive pin will be able to break a 7/16" diameter stainless steel drive shaft or bust off hardened gear teeth? C'mon, even in the worst case scenario, the drive pin will break before any of that happens.

    Mercury motors made their fortune by advertising their motors did not have any shear pins at all. Say what you want, they didn't break gears or shafts by hitting underwater junk.
    Last edited by fdrgator; 02-25-2018 at 08:49 AM.

  16. #16
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    Default Re: Shear Pin information

    The last time I changed a shear pin, I beached the boat to do it. But first, I had to disturb this little fellow's nap.
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  17. #17

    Default Re: Shear Pin information

    Quote Originally Posted by fdrgator View Post
    The last time I changed a shear pin, I beached the boat to do it. But first, I had to disturb this little fellow's nap.
    I'm glad we don't have those little fellows in Michigan. The worst we could encounter would be a snapping turtle.

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