What oil sight glass size do I need? Oil sight glass size refers to physical dimensions, but the standard for size is not always as straight-forward as measuring the pipe. We can all chuckle about the sizing question and we are asked all the time how to measure.
Measuring the pipe can be misleading.
A ¼” NPT has an inside diameter of ¼” and an outside diameter (OD) of 1/2” (.50)
A 3/8” NPT has an inside diameter of 3/8” and an outside diameter of 5/8” (.625)
A ¾” NPT has an inside diameter of ¾” and an outside diameter of 1” (1.00)
A ½” NPT has an inside diameter of ½” and an outside diameter of ¾” (.75)