Bison Pumps 1 1/4” NPT Schedule 120 drop pipe has a wall thickness of 0.25 inches (compared to Schedule 40 at 0.133 inches). It has a coupling bell on one end that has a diameter of 2.125 inches while the pipe itself has a diameter of 1.66 inches. We must use the larger bell diameter when determining whether a system will fit inside a well based on the amount of available space. Both ends of the drop pipe are threaded and the male end comes with Teflon tape applied. It can be purchased in either 8 or 4 foot lengths.