What is the purpose of a thread plug gauge and how is it used?

• A. To measure external thread diameter.
• B. To seal threads during operation.
• C. To cut threads on metal pieces.
• D. To verify internal thread size and pitch; insert to check tolerance; ensure go/no-go checks.

Answer: (D)

The idea being tested is using a thread plug gauge to confirm that an internal thread conforms to its specified size and pitch by performing go/no-go checks. A thread plug gauge has two ends: a go end that represents the lower limit of acceptable size and a no-go end that represents the upper limit. When you insert the go end into the threaded hole, it should screw in smoothly to the required depth, indicating the thread is not undersized and the pitch matches. Then you try the no-go end; it should not screw in to the same depth (it should stop before reaching that depth), which shows the thread is not oversize. If the go end fits and the no-go end does not, the internal thread is within tolerance. If either end fails, the thread is out of tolerance and needs correction.

This is why the other options don’t fit: a plug gauge isn’t used to measure external diameter, seal threads, or cut threads. It’s specifically for verifying internal threads against their specified limits using go/no-go checks.