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BASIC INTERNAL THREAD MEASUREMENT

Because small internal threads are most often made by tap- ping, the pitch diameter and fit are determined by the tap used. However, internal threads cut with a single-point tool need to be checked. A precision thread plug gage (Figure I-342) is generally sufficient for most purposes. These gages are available in various sizes, which are stamped on the handle. An external screw thread is on each end of the handle.
The longer threaded end is called the go gage, and the shorter end is called the no-go gage. The no-go end is made to a slightly larger dimension than the pitch diameter for the class of fit that the gage tests. To test an internal thread, both the go and no-go gages should be tried in the hole. If the part is within the range or tolerance of the gage, the go end should turn in flush to the bottom of the internal thread, but the no-go end should just start into the hole and become snug with no more than three turns. The gage should never be forced into the hole. If no gage is available, a shop gage can be made by cutting the required external thread to precise dimensions.
If only one threaded part of a kind is to be made and no interchangeability is required, the mating part may be used as a gage.

BASIC INTERNAL THREAD 342

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