I am rebuilding a 100# Erie steam hammer. I need to hone the cylinders that the throttle valve and the motion valve run in. That means I will need to build up the valve bodys and machine them to fit the larger cylinders. What should I build the bodys up with? Babbitt? Aluminium bronze? What should the clearence in the valve cylinders be. I will be running it on air.
I am thinking of using 4140 turned, ground and polished to make a new rod. Erie originally used 4340 for the rod. What should I use for piston rings?

Grant Sarver knows a great deal about this - I think he has done it a couple of times. Not sure if he's a member here but does post occasionally across the street on Sparky's site. Memory serves that there was a recent discussion about this very subject.

As I recall, the tolerances should be very tight because air doesn't build the "ooomph" that steam makes. Steam runs with relatively loose tolerances because of the heat - air needs good sealing surfaces.