Because connection failure in the oil field industry can be extremely expensive to repair; the connections made on oil field drill pipes working thousands of feet below ground or ocean are critical. To assure against screw thread failure while working in these rigorous conditions the screw threads are made to extreme and exacting American Petroleum Institute (API) standards. Controlling these standards is the American Petroleum Institute (API) which licenses the use of a Monogram. The Monogram is applied by licensed screw thread gage makers as an oil field industry seal of approval to the screw thread gages they manufacturer.
American Petroleum Institute (API) oil field screw thread gages are available as working screw thread gages or master screw thread gages.
API Master Screw Thread Gages
American Petroleum Institute (API) oil field Master screw thread gages can be owned by anyone. Depending on your use for the master gage the certification will be important. If you intend to work as a calibration laboratory and certify other peoples working gages it will be important to have your master API screw thread gages directly certified by the National Institute of Standards Technology. If your intended use of the API gages is to verify your own in-house working gages, then certification against a certified set of master gages is acceptable. The closer to NIST the less error is accumulated, but the closer to NIST more cost and lead-time is required.