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Old 15 February 2011, 16:13   #1
Country: UK - England
Town: Bristol
Boat name: Dont have one yet
Make: Dont have one yet
Length: 3m +
Engine: Dont have one yet
Join Date: Apr 2009
Posts: 26
Old tohatsu prop drive

I have a 1984 Tohatsu 12hp and last time out the brass shear pin that was fitted failed. I didn't have a spare so the row back to shore was good exercise.
I also have a very low hours 1995 Suzuki 5hp for estuary / river pottering.
The newer Suzuki has a nylon drive bush as well as the shear pin. The older Tohatsu simply has a shear pin through the prop shaft which engages in a slot in the back of the prop.
This arrangement seems somewhat ALL or NOTHING with no designed cushion during acceleration unlike the nylon drive bush arrangement.
The chandlers advised that all shear pins are now stainless steel, replacing the older brass ones so I have made some 5mm pins from SS.
Can anyone advise if this will continue to happen, even with a stronger shear pin, and whether there are any improvements I can make to the drive as overall the Tohatsu is of quality build, no corrosion and 100psi compression in each cylinder.
Sorry for the long winded thread.
Any suggestions would be much appreciated.

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Old 15 February 2011, 16:24   #2
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Country: Ireland
Make: Redbay Boats
Length: 9m +
Engine: 370hp
Join Date: Feb 2008
Posts: 12,154
The shear pin is designed to "fail" if you clip something - possibly you fouled something for a moment? The idea with these is that you always carry a spare. A lot of these engines have a rubber mount for a couple of pins somewhere on the frame or under the cowl.

I'd guess that £2.50 brass pin just wears over time. The stainless pin will probably wear whatever pricey part it is touching instead

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Old 16 February 2011, 10:24   #3
Country: USA
Town: Oakland CA
Length: 3m +
Join Date: Feb 2005
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Originally Posted by Sibbob View Post
The chandlers advised that all shear pins are now stainless steel, replacing the older brass ones so I have made some 5mm pins from SS.
That surprises me. Stainless is much harder than brass, and the shearing force for a like-sized SS pin vs brass must be *much* higher (an assumption on my part; never had a metallurgical course.)

A quick look on the web, while it didn't yield a ton of hits, all had brass shear pins as the primary product, with "Stainless available" as an aside.

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