We may have reached peak dumb,
@AlbatrossCafe, nobody answered our questions.
Called SPC. Castle nut is m14x1.5. I ordered off Fleabay for $5. I answered my own question. I'm smart!
Trying to learn how suspension works.
Here's a dude installing an adjustable rear shackle. He's forcing/compressing the rear leaf pack. Is that effectively what "preload" is? If he flattens it too much, the leafs won't have any more compression and won't absorb road imperfections any more?
Preload is easier to explain with coilovers. In the below pic, my spring perch (which are the two adjustable collars holding the bottom of the spring) is set at a certain height. If I wanted to "increase the preload", I would twist those collars so that they moved higher. This would compress the spring. As a spring is compressed, more weight is required to have it compress further. Pre-compressing the spring is what is known as preload.
There a few reasons to doing this. Mostly, it is about getting lift height. People will crank up their springs so that the truck doesn't make them sag as much at rest and ultimately it makes them taller.
There are a couple of downsides to this. #1 you touched on kind of... more preload means stiffer spring. A stiffer spring with no weight change in the truck will mean harsher ride. #2 is that if I crank my spring up to make the truck taller, the shock is going to be more extended while at my now-normal ride height. With a more extended shock, I'm going to have less droop travel. So going in a pothole might be more harsh as well because the shock might top out before it reaches the bottom of the pothole.
