Hi i also used these material for my bugatti piston pins.Piston wrist pin
to make piston shafts I use inductively hardened shafts that are available from diameter 3mm (less than 1/8 inch) in inches from 1/4) the advantage is that the core is still soft, the outside is hardened and ground to h6 tol , is round, straight and cylindrical. to protect the cilinder wall i used teflon pads
the axes are used in linear motion technology ( https://www.thn.nl/files/stream/documents/THN-TS06-NL.pdf)
You can turn them to length with hard alloy inserts,no need to grind.