The leather nuts of the old days were usually thicker than those of today. So, they had a bit more 'bite'. So, it's usually wise to use a nut that is slightly larger than the original nut as long as doing so won't impinge on the parts that the nuts are holding in place. Also, larger nuts are much easier to drill out to the correct diameter for the rods, and they're easier to install -because it's easier to hold onto them.

With regards to correctly 'sizing' the hole in the leather nut, I've been unable to find terms that adequately explain how tight they should feel when you thread them onto the rod for the first time. The old timers made the center hole just big enough so you could put paraffin on the rod and screw the nuts on by hand, but what might seem fairly tight to one person could feel relatively loose to another. It really depends on the strength of their fingers.

Leather nuts must never be so tight that they squeak or 'jump' when you turn them. Also, they shouldn't be so loose that they're 'easy' to adjust. (See how difficult that is to explain?) I suppose I could use a term like pretty snug, but that's so subjective that it's almost meaningless. For sure, they shouldn't be so tight that you can't twist them with your thumb and index finger. If they're that tight, you run the risk of breaking the part to which the rod is connected.

Regarding the installation of leather nuts on things like the control rods in air motor governors and stack cut-out boxes, it's imperative that the rod be held securely in place when the nuts are screwed onto the rod. Failure to take this precaution could lead to damage of the part to which the rod is connected inside of the box. I just use a pair of pliers to hold the rod in place with one hand while turning the nut onto the rod with the other hand.

Regarding the nuts that are used to secure a rod to a flange finger, the nuts should never hold the pneumatic fingers so tightly that the pneumatic falls from the fully collapsed position to the fully open position any slower than it does when the bottom nut is not on the rod. You can test this by putting both nuts on one rod and putting only the top nut on an adjacent pneumatic. Then collapse both of them, and then let both of them open at the same time. If the one with both nuts takes longer to open than the one with only one nut, the lower nut on the one with both nuts is too tight.

Another way to look at all of this is to realize that leather nuts are NOT lock nuts. They are, in effect, adjustable collars and they should be adjusted such that the part being moved moves in concert with the part doing the moving. In other words, there shouldn't be any delay, free space, or 'lag time' between the parts. As soon as one part starts moving, the other part should start moving. They should move in coincidence with each other.