NPS 3 Schedule 80 tubes have an interior diameter of 73.66mm, and a common piston size is 73mm. The 0.66mm falls within the normal gap around the piston for the rings, which varies from 0.58-0.79mm in most engines. So am re-designing it around this new size.
Do I follow you correctly, that you're suggesting a liner out of 3" schedule 80 pipe? Material wise, that stuff is junk steel. I'm not sure it'd be suitable for running rings on. And my notes list a nominal id of 2.900" (which is your 73.66mm) but most of the pipe I've seen is neither round enough (usually it's lumpy or eggy, definitely not round) or finished well enough on the inside to run pistons on (you might have a weld seem, definitely won't have a machined finish on the inside), so you'd need to bore it in order to use it, and it might take a significant cut to get it to clean up (might need .030" on the diameter) Regardless, I don't think a 73mm piston would work in that application.
And, .58mm to .79mm seems like a big clearance to me (that's .022" to .031" in numbers I relate to). For reference, Wiseco recommends .004" (.10mm) piston to wall clearance on a 4" (101.6mm) bore small block chevy piston. I'd expect for the nominally 3" parts you're talking about you'd want to be down closer to .003" (.076mm).
Hope that helps keep things on track...
Well, do remember this is the first time I've looked at a piston based motor. (previous work has been on electrics or turbines) As for being junk steel, depends on the pipe source in my experience. As for round enough, you could use 3" schedule 160, which would be an internal diameter of 2.624, and bore it out. And, you can also get solid iron cores to do the same thing if need be as well, such as here:http://www.grainger.com/Grainger/Rod-Stock-2KUP9
But, that brings to me to, what hardness would be suitable for such work?