Per API Spec 7 - Obsolescence Rotary Shouldered Connections
In 1964, at the API Committee on Drilling and Service Equipment, a new series of rotary shouldered connections was adopted in order to improve the service performance of drill collars and tool joints. 11 of the 13 NC sizes were approved as standard.
For the new style of connections, the size designation is a two digit number indicating the pitch diameter of the pin member at the gage point and the style is referred to as the number style or numbered connection – NC. These NC connections utilize a V thread form having a .065 inch flat crest and a .038 inch rounded root. This form is specified as the V-0.038R form. This V .038R form mates with the V-0.065 form that is employed on all API internal flush ( IF ) connections and on the API 4 inch full hole ( FH) connection.
All API IF and FH connections are now considered obsolescent and appear in the appendixes I,J, and K of API specification 7.
Of the new NC connections there are 5 sizes that have the same pitch diameter, taper, pitch, and length of thread as the corresponding size in the IF style and one corresponds to the 4 FH connection.
These 6 connections in the number style can be gauged with the same gauging practice and same gauges required for the equivalent connections.
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