I have a 4" tube which I want to scribe lines on. The two lines will be
parallel to the tube axis and should be exactly 180 degrees apart, i.e.
on the opposite sides of the tube.

OK, here's an odd idea: Take two files, and clamp them both to a bar,
maybe 6" apart, so they are both perpendicular to the bar. Rest the
bar on the tube, and turn it at an angle from perpendicular until both
files contact the tube, then drag along.

I have no idea how well it would actually work, though. :/

--Glenn Lyford