1. Basic Parameters of API REG Threads

- Thread Type: Buttress Thread (REG thread), compliant with API 5B standard.

- Taper: 1:16 (62.5 mm/m).

- Pitch:

- 2 3/8" size: 6 threads per inch (≈4.24 mm).

- 3 1/2" size: 6 threads per inch (same as 2 3/8").

- Thread Angle: 60° (load flank angle 3°, stab flank angle 10°).

2. Key Dimensional Specifications  

Dimensions are calculated based on nominal outer diameter (OD) and standard thread height (units: inches):

| Thread Size       | Nominal OD       | Root Diameter     | Base Diameter      | Remarks  

| API 2 3/8" REG PIN | 2.375 in (≈60.33 mm) | ≈2.266 in (57.58 mm) | ≈2.240 in (56.90 mm) | Thread height ≈0.054 in (1.37 mm), taper 1:16

| API 3 1/2" REG PIN | 3.500 in (≈88.90 mm) | ≈3.391 in (86.13 mm) | ≈3.365 in (85.47 mm) | Thread height same as 2 3/8", identical taper

- Root Diameter: Minimum diameter at the thread root (minor diameter). Formula: Nominal OD – 2 × Thread Height.

- Base Diameter: Reference diameter at the start of the tapered thread, accounting for taper effect (typically slightly smaller than root diameter).

- Actual values are subject to manufacturing tolerances (e.g., standoff, makeup torque); final verification must use API 5B master gauges.

3. Application Considerations  

- Sealing Mechanism: API REG threads rely solely on thread flank engagement and coupling compression for sealing and lack an independent metal-to-metal seal. For high-pressure gas wells, premium connections (e.g., HYD PH6, BGT series) are recommended.

- Typical Applications:

- Drill string connections (e.g., hammer drill bit shanks, drill pipe tool joints).

- Tungsten carbide drill bits, crossover subs (e.g., double-pin subs).

- Manufacturing Requirements: Precision CNC machining with copper-plated thread surfaces to minimize galling risk.

- API 2 3/8" REG PIN: Root diameter ≈2.266 in, Base diameter ≈2.240 in.

- API 3 1/2" REG PIN: Root diameter ≈3.391 in, Base diameter ≈3.365 in.

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