A cast iron piston / piston ring in a cast iron bore is best, but a cast iron piston in a steel bore is not too bad. I use both in my hot air motors, one of which has a cylinder made of a bit of steel water pipe, that almost didn't work, as I bore ut the pipe to size the weld gave way, I clamped it to close the gap, then ran a weld along the seam on the outside, it worked, the piston is old cast iron window weight, and after many hours of use since 1993 there is no measurable wear on either item.
Ian S C