On 24/01/2020 09:29, FMurtz wrote:
Brass for shear pin 15 hp outboard?

Whereabouts? does it see seawater? That would be a good reason for using
a high quality brass (or other cuprous alloy). In "dry" locations mild
steel is the usual thing: cheap, convenient, breaks relatively cleanly.
Stainless steel is less suitable because it is relatively ductile and
work hardens, so may deform but not fail cleanly.

I would say that subject to suitable corrosion resistance (and galvanic
compatibility with the other parts) the other materials properties are
not particularly important.