Some clarification. I think a monel piston will continue to function longer than plated piston, although it might be with sloppy action and lots of oiling. On the other hand, as soon as the plating wears through to the brass a plated piston is useless because brass on brass sticks like crazy. This has been my experience with several vintage Olds and with a Getzen student horn that had a lot of wear. Until the plating wears through the plated piston will slide better because, as Ivan said, the plating is harder than a monel psiton.