STI Gauges

CoilThread® STI (Screw Thread Insert) Thread Plug Gauges are designed to check the accuracy of the internal thread of an STI tapped hole. Gauging the tapped hole prior to installation of the insert is critical to ensure the accuracy of the finished thread. If the tapped hole gauges correctly, the finished thread will also be accurate once the insert is properly installed and seated.

It is not necessary to gauge the installed insert. Gauging of the tapped thread is specified because a standard gauge may not freely enter an installed insert. The insert will seat itself once a bolt or screw is installed and tightened (Ref. NASM33537). In fact, it is not possible to gauge a locking type insert because of the locking coil.

Features & Benefits

  • Available in 3B or 2B (Unified); 4H or 5H (Metric) Class of Fit.
  • Available for a full range of Unified and Metric thread sizes.
  • Ensures accuracy of the finished threads.
  • Lower cost than similar gauges from competitors.

We offer two types of STI Thread Plug Gauges (for unified sizes):

Working Gauges

Recommended for production use since their greater tolerance allows for more wear.

Reference Gauges

Pitch diameters at or very close to the basic size and are to be used as master gauges.



CoilThread® Gauges include a "GO" and "HI" (NO GO) member. The Working Gage "GO" member has a 0.005 wear allowance on the pitch diameter. The "GO" member checks the basic pitch diameter, major diameter minimum and lead. The "HI" member checks the maximum pitch diameter. The minor diameter must be checked separately with a graduated pin set.

Part Numbers

Example of a CoilThread® Gauge part number:

CG C - 02 X - 3B

 1   2    3  4     5

Tool Type
Thread Designation
Size Designation
Style / Type
Class of Fit
CG - CoilThread® Gage (STI) C- Unified Coarse
F - Unified Fine
M - Metric
--- X - Working
W - References
Example 2 of how to put together a tool's part number. 
Tangless® or Tanged Tool Type of Tool   Metric Size Free Running or Locking
Blank= Tanged CT


Insertion Tool M 2.5-12 F= Free Running
2= Tangless®           L= Locking

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