Shear Pin Johnson 6R72D

I have had to replace the shear pin on my 6R72D (great little motor, by the way) several times in the last couple of months. I think/know the first time was from running over deadfalls in a low level river - my own fault. But the others have been out in the middle of a lake where I KNOW that I didn't run into any thing. Why does it keep breaking. Could it have anything to do with how tight I have been tightening the Prop Nut - How tight should it be? Also, how much space should there be between the prop and the gearcase housing. It almost looks like there is a part missing...I have about a half inch between the two pieces - is this right or do I have a bigger issue to deal with.

Maybe I should go to a Stainless Steel pin...?

Also, what is the correct size for the pin? (Part #307949)

Thans in advance for any help.

I'll put up a pic of how it looks later today...

Re: Shear Pin Johnson 6R72D

Do NOT use a steel pin. Look at the hole in the prop shaft. I'm betting it is elongated so the rocking action of the prop due to acceleration and deceleration is shearing the pin. The slot in the prop may also be worn allowing the same rocking motion. Perhaps the pins being used are too small thus allowing the hole to be elongated.

Re: Shear Pin Johnson 6R72D

Silvertip said:

Do NOT use a steel pin. Look at the hole in the prop shaft. I'm betting it is elongated so the rocking action of the prop due to acceleration and deceleration is shearing the pin. The slot in the prop may also be worn allowing the same rocking motion. Perhaps the pins being used are too small thus allowing the hole to be elongated.

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From what I saw when I googled 6R72D Johnson, your motor is most likely from the early seventies??? If so, it should have a stainless steel drive pin. My '73 20 hp has a stainless pin, and when I ordered a spare from the dealer, a stainless one arrived. Some of the antique motors had brass shear pins. As the name suggests, the pin would shear when the prop hits something. Guys would lean over the back of their boat to replace the pin and fall overboard. The industry went away from this design and went to a stainless drive pin with a hub in the prop that is allown to spin. Make sure that the pins you are using are correct and not generic brass pins from the hardware store

Re: Shear Pin Johnson 6R72D

Good point. I haven't owned a pin-drive outboard except for my own 40's Johnsons. Those are bronze and I make them from welding rod. Never-the-less, check the hole in the shaft and the prop. Doesn't matter if the prop has a rubber hub as that rocking motion is still present.

Re: Shear Pin Johnson 6R72D

307949 is indeed a stainless steel DRIVE pin. It is not intended to shear.

Having said that, your motor might have a warn clutch dog, making it jump out of gear and instantly slam back into gear. The impact of when that happens will shear a brass pin.

So, bottom line is first install the correct pin. Then if you notice a jarring sensaton like hitting a rock when running, but you know you are in deep water, time for a clutch dog and probably forward gear.

Re: Shear Pin Johnson 6R72D

Thanks Guys,

Anyone know what size Prop I should use? I am going to replace it.

Re: Shear Pin Johnson 6R72D

I have a 72 6 HP in my garage that still has the original 3 blade aluminum prop. I will get the number off of it for you when I get home later today.

Re: Shear Pin Johnson 6R72D

Okay, just looked at the prop that I have.

It is a 8 x 7, three blade, with OMC part # 387334. Hope this helps.

Re: Shear Pin Johnson 6R72D

I also agree that it's probably an elongated slot in the prop. Or , maybe it' the wrong prop? There should not be a 1/2" gap in there. Do you have a picture?

Re: Shear Pin Johnson 6R72D

The gap actually went away when I finally installed a new shear pin...(or should I say Drive Pin...)