SpudFiles

I am making a piston for a 2" valve(3" tee) using this as a guide:

http://www.instructables.com/id/Colossa ... on-valved/?

3in pipe is not 3.00in. You would think this would be the case, but no. 3in pipe is really 2.946in pipe. Therefore, PVC rod that is 3.00in in diameter will not fit, especially after the 3in pipe is solvent welded (which causes the pipe to shrink). So if you have a lathe, this part is easy. If you don't, well, better get sanding.

Here's my question. I ordered a 3"x3" aluminum rod already, and this guy said that I need to machine the 3" rod to 2.946". Right? Well, I measured the inner diameter of a 3" PVC pipe and it's 3.050"! Did I just waste $20 on this aluminum that's too small???

Average 3" sch 40 pipe is 3.042. With a gap that minimal your performance will be fine. You can use that as your equalization hole. You did not waste 20$ no worries.

Bit unrelated but...
Where did you get the aluminium so cheap?

Cheap? I call that expensive!
But anyways, you could also add an o-ring to make up for the diameter loss.