Pipe thread sizes refer to the inside of the pipe, but then to standardize the threads, the inside varies a bit depending on the schedule (wall thickness, pressure rating) of the pipe. The following is approximate: 

    NPT   nipple OD
  -----   ---------
    1/8     .40
    1/4     .54
    3/8     .68
    1/2     .84
    3/4    1.05
  1        1.31
  1-1/4    1.66

The 3/8 NPT fittings are pretty standard, and mate up reasonably well with 5/8 ID hose. But, I find you're better off not sealing on the threads on the end of the pipe nipple. If you need a 2 inch stub, buy a 4 inch nipple and cut it in half - then you'll have two stubs with smooth ends for the hose. Or, if money is no object, just buy the barbed hose adapter fittings.

Alberg30: PipesAndHoses (last edited 2009-07-27 19:22:08 by localhost)