Hi OZ, the stuff I flattened was from a piece of what we call "black iron" pipe (it's steel actually), schedule 40 (the usual weight) and I believe it was 1-1/14" size. It comes in black, and galvanized.
In my experience it's a difficult steel to try to turn -- leaves a rough finish -- at least with my abilities and machines. I tend to get out the old file to smooth it up on the lathe. The cast iron of the plumbing fittings machines, and finishes out a lot better. And annealing it as I did seems to get rid of chilled spots.
Wall thickness measured a bit over 1/8".
I realized at the end of the day, that I'd made my pieces too small by about a quarter inch.
So I'll repeat again today!
--it really only takes ten minutes to saw an flatten, so not a big deal -
ps.I don't know what scaffolding tube is, but I do also have electrical conduit scrap in a variety of sizes, and fence post tube in several sizes. They're thinner. Tube instead of pipe. I have regular and some heavy duty pipe up to 4" diameter.
I've got some other ideas for engines from pipe. Something about that stuff, just makes me want to build things!