NPT threads are ''self sealing'' since they are slightly tapered; the diameter slowly becomes bigger towards to top. - 20.8mm is the approx. diameter of this plug in the middle of the plug; including the thread itself. Note: For easier identification of the correct plug; the diameter (approximately measured in the middle of the plug; including the thread itself) is given in millimeters. Note: Pipe plug sizes are totally different when compared to common US bolt sizes.