hub size

can you use or has anyone tried using a mtn bike hub on a 20" wheel. I was thinking of using a Chris king hub and didn’t know if it would work?

Unicycle hubs are fundamentally different from bicycle hubs (unless we’re talking about giraffe or other non-standard unicycles).

A unicycle hub (as I’m sure you are aware) has a axle that is fixed to the hub shell. Sometimes the axle and the hub flanges are all one piece. In other cases the axle is inserted into a hub shell and held in place with splines or keys. The frame then attaches to bearings on the axle such that the frame can rotate around the entire hub/axle.

A bicycle hub has a axle that is designed to rotate freely inside of the hub shell. Generally the axle on a bicycle is bolted to the bicycle frame and is fixed in position. The hub rides on bearings around the axle which is secured to the frame.

In any case, you could not use an off-the-shelf bicycle hub on a unicycle.