The American Petroleum Institute (API), headquartered in Washington, DC, USA is a
world leader in the development of standards for all aspects of the oil and natural gas
industry. When using API sanctioned gages for the inspection of products or calibration
of other gages, it is critical that the user have a high quality gage, manufactured using the
latest manufacturing and inspection technology available. Gages should be manufactured
so that they meet or exceed API specifications.
Each API working gage shall be accompanied by a certification listing the values with
dimensions measured for each critical element, as well as a measured standoff to a
certified master gage. Each API master gage shall be accompanied by a certification
from an API qualified agency such as the National Institute of Standards and Technology
(NIST) located in Gaithersburg, Maryland, USA, the National Physical Laboratory (NPL)
located in the United Kingdom, or the National Institute of Metrology (NIM) located in
Beijing, The People’s Republic of China, as described in the API specifications. For API
Specification 5B, the API approved certifying agency shall perform a full inspection on
all critical elements as well as a mating standoff, and provide a report detailing the results
of the inspection. For API Specification 7, the API approved certifying agency shall
perform a full inspection on all critical elements as well as a standoff against the
appropriate regional or grand master gages and a mating standoff. A certification shall be
provided detailing the results of the inspection.
The gage tolerancing defined by API is designed to allow the gage manufacturer
tolerance for manufacturing the gage while still providing the end user with tolerance for
wear of the gage. Due to the desire for maximum wear life of gages, it is often requested
that gages be manufactured on the plus side of the tolerance. Manufacturing to the plus
side of the tolerance does not always provide desirable results. It is possible that
manufacturing to the plus side of the tolerance will lead to one laboratory performing
inspection and standoff near the maximum limit while another laboratory, using different
master gages will find that the gages exceed the allowable tolerances and they will
question the validity of the gages. This situation can arise very easily for API
Specification 5B gages because the taper ratio is 16:1 and even small differences in
diameter can result in large differences in standoff. For this reason, it is PMC Lone
Star’s practice to manufacture all gages as close to nominal size as possible. When there
is a difference between the API accredited manufacturer’s reported results and another
API accredited laboratory’s reported results, it does not mean that the gages are out of
tolerance and should be rejected. It simply means that the two inspecting laboratories
have master gages which differ in size from one another. In cases such as this, the
inspection results provided by the API accredited manufacturing and inspection facility
should be accepted and used with the gages. It is also recommended that the results
provided by the local laboratory be recorded for future comparison when checking for
wear. Manufacturers of product threads shall conform to the gaging requirements as stated in API Specification 5B, Fifteenth Edition, April 2008 Paragraph 6.1.2.
