Used on transmission oil pan front side. OEM replacement reference 45893-99NPT threads are ''self sealing'' since they are slightly tapered; the diameter slowly becomes bigger towards to top. Note: 16.8mm 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: 3/8 size pipe plug size is much larger than regular 3/8" sized bolts.