Shear Pin Failures

SR500F@aol.com
Fri, 27 Nov 1998 15:24:50 EST


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West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
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Folks, forgive me as I lapse into "engineer mode"......

I have a Mercury 2.2 which is essentially the same as the Nissan and Tohatsu
2.5. These are close siblings to the 3.5 horsepower models. A quick look at
the Nissan web site shows identical bore and stroke but a different gear
reduction ratio and a higher operating rpm range (differences in port height
and carburetion, perhaps?) for the 3.5. What this indicates to me is that the
higher output of the 3.5 is obtained in much the same way as driving a car in
third gear gives more "power" than in fourth gear. The increase in gear
reduction will cause greater torque to be passed through the shear pin under
all conditions when compared with the 2.2/2.5. Some of the 3.5 hp engines are
equipped with the "forward - neutral" clutch which may be an influencing
factor in the shear pin failures, since those who are complaining of the
failure here on the list all seem to have the option.

First of all, my Mercury came equipped with a spare shear pin and cotter pin
in a little rubber doo-dad mounted to the inside of the spark plug access
cover - If the other motors are similarly equipped then there should be no
excuse for not having a spare pin when one is required.

But, back to the failures, it is probably not the pins that are at fault here,
but the hole in the drive shaft into which the pin fits that initiates a
failure of the pin. If the hole becomes enlarged over time and pin becomes a
loose fit in the hole the drive torque, shocks, etc. will no longer be passed
across a surface to surface contact but rather over an edge to edge contact. A
half circle at one end of the hole, and a second half circle at the other end,
a much higher concentration of load than over a larger surface of the pin.
This high load concentration may cause some local deformation of the pins,
this deformation could behave as a "stress-riser" leading to a fatigue failure
in that spot and will also be a favorite spot for corrosion to start also
leading to an eventual failure. Even if the hole is the correct size, a sharp
edge or burr (a manufacturing defect?) at the ends of the hole could have a
similar effect on the pin.

So, to those of you having problems, I would suggest examining the condition
of the hole into which the shear pin fits - if the hole is elongated (out of
round) a new propeller shaft may be required, if there is only a sharp edge,
that could be cleaned up with a small file or sandpaper.

Reverting now to "long weekend" mode.......

Happy Thanksgiving,

Dave Kautz
P-15 #1632