Thread Plug Gage
Does a thread plug gage tell me the exact pitch diameter?
No. It gives a pass/fail functional verification of the internal thread against allowed limits, not a direct numeric measurement.
Why is the NO GO side limited to two or three turns?
The NO GO end verifies the thread is not oversized beyond the permissible functional limit, especially pitch diameter. Entering more than two or three turns indicates the threaded hole is too large for reliable mating-part assembly.
Can a thread plug gage be used on external threads?
No. Thread plug gages are designed exclusively for internal threads. External threads are checked with thread ring gages.
A thread plug gage is a fixed-limit go/no-go inspection tool for verifying the functional size of internal threads, such as tapped holes. The GO end must enter smoothly to full engagement; the NO GO end must not enter past the allowable limit, confirming the thread is within tolerance for mating-part assembly.
On a CNC cell or in final inspection, a thread plug gage acts as a fast attribute check rather than a dimensional measurement tool. Operators use it on production parts before assembly or downstream operations to confirm a tapped hole will accept the proper fastener without binding, cross-threading, or excessive looseness. The GO end is inserted by hand with very light force and must run through the full threaded length freely, verifying that the pitch diameter, lead, and flank form are functionally acceptable. The NO GO end is then applied and should not enter more than about two to three turns, depending on the applicable standard. In high-volume tapped holes, this catches tap wear, incorrect offsets, wrong thread class, poor burr control, or damaged first threads before scrap multiplies. It does not measure threads directly; it compares the hole against known functional limits.
Forcing the GO end: excessive force makes a dirty hole or damaged gage shave material during insertion, turning a good thread into a false reject or a bad thread into a false accept. That accelerates gage wear and corrupts results.
Ignoring chips, coolant, or burrs: debris packed in the first threads or a raised burr from chamfering prevents the GO gage from seating fully, mimicking an undersized thread even when tap geometry is correct. This drives unnecessary rework.
Using the wrong gage class or a worn gage: a plug gage that does not match the specified thread series, class, or calibration drifts from the true functional limit, causing false acceptance or rejection and eroding process control.