Pipe is usually measured by its inner diameter, where tubing is sold by its outer diameter . . . or something like that, right?
In any case, if it's still 14.75" to the bottom of the lower pipe at 20" to the top of the upper pipe,
I don't care about the diameter
We're sure about those dimensions, at least? I don't have field schematics at home, and as far as I can tell, they're not available online.