in the course of the development of the petroleum industry,API series specification of oil pipe special thread plays an indispensable role. The special thread for oil pipe is mainly divided into two categories: the oil drilling tool joint thread used for the connection of downhole tools and drill string components, and the oil casing joint thread used for the connection of oil casing.

（1） internal flat thread

This type of drilling tool joint thread connection outer thickening or inner and outer thickening drill pipe, forming drill pipe joint inner diameter, pipe body thickening inner diameter and pipe body inner diameter equal or similar diameter. All sizes of threads are usedV-0.065 flat-top flat-bottom triangular tooth type, this tooth type is a flat tooth bottom, the tooth top is 0.065 inches (1.651mm) wide. In addition to 51/2IF, other specifications of the thread because the structure size and the corresponding digital thread is exactly the same, so it has interchangeability. This type of thread is easy to cause stress concentration due to its tooth shape structure. API has eliminated it, including 41/2IF and 4IF, which are 410, 411, 4A10 and 4A11, which were once widely used in our oil field, and replaced by NC50 and NC46 digital threads.

（2）eye-penetrating thread

This type of drill pipe joint thread connects the inner and outer thickened drill pipes, forming the inner diameter of the drill pipe joint and the inner diameter of the thickened end are equal, and both are smaller than the diameter of the inner diameter of the drill pipe body. Although the specifications of this type of thread are few, it is used.V-0.065, V-0.050 (tooth bottom is circular arc, tooth top width is 0.050 inches, 1.27mm) and V-0.040 (tooth bottom is circular arc, tooth top width is 0.040 inches, 1.02mm) have been widely used in faucets, kelly, drill pipes, drill collars and drill bits. At present, except 51/2FH and 65/8FH, which use V-0.050 profile and 1:6 taper, the rest are among the API eliminated threads. The 4FH, which only uses the V-0.065 profile, is replaced by the digital thread NC40 as well as the internal flat thread.

（3）regular thread

The thread of this type of drill pipe joint was used to connect the inner thickened drill pipe, forming the inner diameter of the drill pipe joint smaller than the thickened inner diameter, and the inner diameter of the thickened end of the drill pipe smaller than the inner diameter of the pipe body.The main purpose of API's design of regular threads is to apply them to drill connections. Since the drill bit is located at the end of the drill string, the existence of stress concentration at the bottom of the thread is irrelevant, so API retains all their specifications. This type uses V-0.050 and V-0.040 tooth types. In the 40th edition of API SPEC 7, 1REG and 11/2REG threads with V-0.055 tooth types (flat tooth bottom, tooth top width of 0.055 inches, 1.397mm) are newly added.

（4）Digital thread

This is a thread expressed as a numerical value in inches and tenths of an inch of the pitch diameter of the thread base. Threads of all specificationsV-0.038R a flat-topped, round-bottomed triangular tooth type, which is characterized by a round tooth base radius of 0.038 inches (0.965mm). Digital thread is the preferred thread recommended by API,This type of thread also has1:6 and 1:4 two types, used in Kelly, drill pipe, drill collars, drill stabilizers and other drilling tools, in Daqing Oilfield NC50 thread is also used on the drill bit.

Thread typeV-0.038R refers to a V-thread with a radius of 0.038in(0.965mm) at the bottom of the thread, I .e. a digital thread. The respective numbers in the V-0.040, V-0.050, V-0.065, and V-0.055 represent the number of inches of the V-thread crest width. The bottom shapes of these four threads are completely different. The radius of the V-0.040 thread bottom arc is 0.020in(0.51mm), the radius of the V-0.050 thread bottom arc is 0.025in(0.635mm), and the V-0.065 and V-0.055 thread bottom are flat.