If I had room I would prefer a self contained ball sprag bearing that had it's own wear surfaces instead of a needle sprag bearing that has to ride on a hardened shaft. I just hardened my shaft in my trans and will have to put some miles on it and maybe a race or two before I disassemble it and check for wear. I previously used drill rod, which is pretty hard, but showed premature wear from the needles digging into it. It never failed to grab, but it was only a matter of time. I'm no engineer, so I don't know hardness specs. The bearing manufacturer may be able to supply the requirements for their bearings, but if you use one with a built in wear surface, you can run just about any readily available shaft like go kart or mini bike keyed jack shafts. Hope this helps.