Very often these clamps don't tighten down on very small pipes (1" and smaller) due to the bolts used. You could simply replace the bolt with a longer one however.
If you're using much smaller pipe however, perhaps you don't need such a large clamp. What type fixture are you using?
Thanks, Dennis! I actually haven't installed a fixture yet, so I am free to use a different fixture and a different clamp. This is just the clamp I ran across that looked like it would work well with ARRI 650 Fresnels (and smaller like 350 and 150). A two inch pipe seems like more than I should need. I'm thinking a one inch pipe would do. Is there another clamp you know of that would work well for that?
You shouldn't be buying clamps separate from the fixture.
Buy the fixture and the clamp that goes with it. Arri makes an excellent clamp -- use it. Why waste time shopping for separate clamps?
If you do feel a need to "shop around" for clamps then go to THE LIGHT SOURCE (coolclamps.com). They have every clamp you can imagine.