Shear Pin Size

Pull the pin out of your Evinrude and mike it.
Brass sounds like a winner to me.

 

You should use brass. It is soft to work right. Stainless is too strong, also even the best stainless will rust in salt water. I brought a buddies shear pin into where I worked. We had a shop there and we found a few short lengths of brass rod thrown into the almost worthless metal corner that was perfect. A minute on the ban saw and a minute with some emery cloth and we had about 20. Enough for the Canada fishing trip and many years as well.
I wonder if you can get a more modern prop that will fit your motor. One with rubber bushings.

 

I'm pretty sure that a 1978 15hp Evinrude doesn't have a shear pin. I have a 1976 15hp Evinrude and it has a slip hub prop, no shear pin. Could not find a shear pin on the 1978 parts diagram. Could it be a 1968?

 

In some applications, a shear pin was used along with the rubber hub. The purpose was to reduce wear on shaft/hub splines.
We are a ninety-three year old boat dealership and have an incredible number of parts and pieces which are now pretty much useless. We have drawers full of shear pins which are mostly brass. My grandfather bought shear pins by the diameter to match the more popular engine models and usually longer than needed. They could be clamped in a vice and hackisawed to proper length. If you know the diameter and the length you need, I might be able to fix you up at no charge. If you have a rubber hub, you wouldn't really have to use a shear pin but it should help protect against spline wear. I am headed for a second knee replacement in a few days and may not be prompt to reply.

 

I can remember shearing pins twice one night driving home in the dark. Only had 1 spare. Tried a nail but too soft. Did a McGuyver and took the receiver pin from my brother's Rem 870. Fit and Worked perfectly. Got home and replaced it with a shear pin and for fun, tried it in the 870. Actually fit fine. Go figure.

 

Here's the exploded diagram for the gear case of the 1978 15 hp Evinrude, there's no shear pin (#44 is a cotter pin). Have you actually seen a shear pin in it? If so, you either have a different year motor or someone has cobbled a different year lower unit onto it.

 

Interesting, 'plankton. Now that you mention it, my old 1978 Merc 20hp had a rubber cush drive.....you could well be right that outboards went to cush drives before 1978.

 

My '75 Johnson 25 horse uses a shear pin.

 

Prior to 1974, Evinrude hadn't made a 15hp for several years, it went from 9.5hp to 18hp. In 1974 it introduced the low profile case 9.9 and 15hp, both of them had slip hubs, they were basically the same motor. By changing carbs and removing the restrictor in the exhaust you could turn a 9.9 into a 15hp. They dropped the 18hp line in 1974. The 25hp continued to have a shear pin until 1978, when they went to the slip hub prop. There are still a few outboards with shear pins, but they are mostly very small 2-8hp engines.