Pipe threads are tapered threads to provide an airtight and liquid tight joint. A 3/8" machine thread tap and a 3/8" pipe thread tap are compared in figure 6-29 show their differences. The 3/8" machine thread tap will cut machine threads in a hole so that a 3/8" cap screw, having the same thread can be screwed into the hole. The 3/8" pipe thread tap will cut pipe threads in a hole so that a 3/8" threaded pipe can be screwed into the hole. Because pipe diameters are measured and given as inside diameters, and the Wall thickness of the pipe must be taken into consideration, the 3/8" pipe thread tap in figure 6-29 is noticeably larger than the 3/8" machine thread tap. It should also be noted that the pipe thread tap is tapered, but the machine thread tap is not.
The N.P.T., which formerly stood for National Pipe Thread, is still used as a carryover and now refers to the new name for the same thread, American Standard Taper Pipe Thread. This standard taper is 3/4" per foot.
To select the proper tap drill for the pipe tap that you will use to tap a hole to take a given size of threaded pipe, refer to table 6-3. For example, if you want to drill and tap a hole to take a 3/8" pipe thread, find the 3/8" pipe tap in the Pipe Tap Size column. Opposite this, in the Tap Drill Size column, is 19/32", the proper tap drill to use for a 3/8" pipe tap.
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