Nexus
I started on a Nexus, was really good machine with the exception of your problem. I tried using pieces of finishing nails, etc., but what seemed to fix it for me was to make sure to not flood (hydraulic) when starting (this will break the shear pin), and, before using a new one, disassemble the shaft, gear, pin combination, and using a tapered grinding wheel in my dremel, chamfer the sharp edges on each side of the hole that's through the start shaft. These are really sharp, and with the constant back and forth motion on the pin while running the bird, cut the shear pin. This fixed my problem, and yes, the nail fix worked great, and saved buying the whole assy. when it did shear. Just try nails til ya find one thats close, and if it's a little loose, knurl the nail any way ya can. I used cutting pliers to 'knurl' it so I got a tight fit. Hope this helps, Rick. Oh, and if the nail is a little big, sand it down.