NPT threads are ‘self sealing’ since they are slightly tapered, the diameter slowly becomes bigger towards to top. – 13.5mm 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.
