Date: July 2026
Location: China
API drill pipe thread connections have long been the industry standard for ensuring the strength, reliability, and performance of drilling tools. According to API specifications, drill pipe threads are primarily classified into four types:
Internal Flush (IF) – Designed to provide a nearly flush internal bore when connecting upset drill pipe. Most IF thread sizes have been phased out by API due to stress concentration concerns and replaced by the more advanced NC series.
Full Hole (FH) – Commonly used with internal-external upset drill pipe to maximize bore diameter. Today, only the 5½ FH and 6⅝ FH connections remain in the current API standard.
Regular (REG) – Primarily intended for drill bit connections. Because of their proven reliability, REG threads continue to be retained in API specifications and remain widely used for bit applications.
Numbered Connections (NC) – The API-recommended standard for modern drilling operations. NC threads feature a rounded-root thread profile that improves fatigue resistance and reduces stress concentration. They are extensively used on drill pipes, drill collars, stabilizers, and other drilling tools.
Among these thread types, NC (Numbered Connection) has become the preferred industry standard, offering improved durability, interchangeability, and overall drilling performance. Understanding the characteristics of each connection helps operators select the most suitable thread type for different drilling conditions and maximize equipment reliability.
