Rigging Pin Size

[Storm]

For 5/16 inch dia 1x19 wire, I am able to buy fittings that are for 1/2 inch dia pins or 5/8 inch dia pins (for example Hi-Mod mechanical fittings). I am not able to find documentation on the strength of pins in shear. Will a 1/2 inch dia 316 stainless pin have the same strength as the 5/16 inch dia wire, or would it be the weak link.
Regards

[Brion Toss]

Hello,
If you run pin strength (pi x r squared x strength of metal), you will see that a 1/2" pin is stronger than 5/16" 1x19. The problem is that it is not stronger enough. Pins -- and chainplates and tangs -- need to be at least a third stronger than the wire, in order to deal with vulnerability to fatigue and corrosion, as well as the weakening effects of threading rod for studs, and shaping barrels, for instance. Taking all this into account, it is not impossible to end up with a turnbuckle that is weaker than the wire. That is why 5/8" is the standard pin size for 5/16" 1x19.
The reason that you can nevertheless get 1/2" terminals for that diameter is that those terminals are meant for 7x19 and similar constructions, which are much weaker than 1x19. 1/2" is the appropriate pin size for those constructions. If people don't realize this, or if perhaps they are just being cheap, they will put a too-small pin size on 1x19.
Fatigue failure of undersized turnbuckles is not rare. On a related note, some standards are now specifying 3/4" pins for 3/8" 1x19, due to failure rates with 5/8" pins.
Fair leads,
Brion Toss