Originally Posted by Scubawpb
2" pipe is 2" on the ID inside dimension. 2" tube would be 2" on the Od outside dimension. Most t-tops, towers, etc are made with schedule 40 pipe which is polished, then anodized or lightly brushed then anodized. The grade is usually 6463. Mill finish(bare) aluminum is available 6463 or cheaper 6063. 6061 is harder and might crack if you bend it. To order pipe you ask for schedule 40, 10, 80, etc as explained by DavidM.
Not as simple as that... 2" pipe isn't 2" on either but has to be consistent OD for threading tools to work.
For complete list of ID & wall thk for various schedule pipes see