The only real question is what alloy they are made of. Failing that, I can just test a known good mil spec pin. I have a 1000 foot-pound torque wrench and could set up a dummy junction and just test the torque to breakage, get a number, and try pins made from all the common alloys until I find one that shears at about the same value of torque.
Another fast way to test it would be to take a working truck, engage the PTO with the engine off and in low gear. Put the winch in neutral. That means the PTO shaft is locked on the PTO side and free on the winch side. Put a large pipe wrench on the PTO driveline with a very long cheater stick, and load the cheater stick with some known, heavy load, like a sack of concrete or whatever. Move the load out along the cheater stick until there is enough torque to break the shear pin and make note of how far (feet) out the load(pounds) was when it broke, and there you have the foot-pounds of torque, ignoring the relatively small contribution of the wrench and stick, which you could account for, but probably not worth it.
Then try several alloys of aluminum pins until you find one that shears off at about the same torque. There is a risk of damaging the PTO, of course. However, if you plan on using the winch for anything, you are going to stress it anyway. The designers surely specified the pin appropriately for the system. I'd be cautious about the test pin. Make sure it is a genuine US Military issue pin and not something that someone just made and sold to someone as the real McCoy. Irv
