How to measure BSP threads？

Measurement of BSP threads (including BSPP parallel threads and BSPT tapered threads) needs to be combined with their key parameters (such as size designation, pitch, tooth angle, taper, etc.), and the following are the specific methods and steps:

Preparation before measurement

1. Confirm the thread type:

BSPP (G series): parallel thread, sealing depends on the gasket or O-ring (such as G1/2, G3/4).  

BSPT (R series): tapered thread (1:16 taper), sealing depends on thread taper fit (e.g. R1/2, R3/4).  

The initial judgment is made by observing the appearance of the threads (parallel or tapered) or the product marking (G/R).

2. Clean threads: Remove oil, rust or sealing material (e.g. raw tape) to ensure accurate measurement.

Measurement of key parameters

Measurement of Pitch

Tool: Pitch gauge (thread sample) or calipers.  

Steps:  

① Use a pitch gauge to match the thread tooth type, find a sample plate that matches perfectly, and read the pitch value (e.g., the pitch of G1/2 is 14 teeth/inch, i.e., 1.814mm).  

② If there is no pitch gauge, calipers can be used to measure the total axial length of 10 teeth, and then divided by 10 to get the pitch of a single tooth (to be converted to teeth per inch).

Measurement of large diameter (external threads) or internal diameter (internal threads)

Tool: Vernier caliper or micrometer.
  

Steps:  

  For male threads (e.g., bolts): Measure the largest diameter at the top of the thread (major diameter).  

  Female thread (e.g. nut): Measure the minimum diameter at the bottom of the thread (small diameter).  

Note: The nominal size of the BSP thread (e.g. G1/2) corresponds to the through diameter of the pipe, not the actual major diameter (e.g. G1/2 has a major diameter of 20.955mm).

Measuring the center diameter (critical dimension)

Tool: Three-pin measurement method (high precision) or thread micrometer.

  

Steps for the three-pin method (external thread)  

     ① Select a three-pin of suitable diameter (matching the pitch).  

     ② Place the three pins into the threaded groove and measure the distance between the outer edges of the three pins (M value) with a micrometer.  

     ③ Calculate the center diameter according to the formula:  

     \[

     d_2 = M - 3d + 0.866P

     \]

     (\(d_2\) is the center diameter, \(d\) is the diameter of the three pins, and \(P\) is the pitch)

Measuring Taper (BSPT only)

Tool: Taper gauge or angle gauge.  

Steps:  

     ① Fit the taper gauge to the taper of the thread and observe whether it makes full contact.  

     ② Confirm that the taper is 1:16 (for every 16 units of length, the diameter changes by 1 unit).  

     ③ Or use calipers to measure the large diameter of both ends of the thread to calculate whether the taper is in accordance with the standard.

Tooth Angle Verification

Tool: Thread angle gauge or projector.

  

Standard: BSP threads have a tooth angle of 55°, which needs to be distinguished from NPT (60°).

Specialized Gauges (Recommended Efficient Methods)

1. Thread ring/plug gauge:

  

Use BSP thread gauges (e.g. G1/2 ring gauge, R3/4 plug gauge) directly to check for conformity.  

The “through gauge” should be screwed in smoothly and the “stop gauge” should not enter more than 2~3 teeth. 2.

2. Digital display thread measuring instrument:

  

Automatic measurement of pitch, center diameter, taper and other parameters (for precision testing).

Verification against standard parameter table

According to the measurement results, compare with the BSP thread standard parameter table (e.g. ISO 228, BS 2779) to confirm whether it matches or not.  

 Summarize  

For non-professionals, it is recommended to prioritize the use of thread gauges or quick verification against standard tables;  

Precision measurements should be combined with the three-pin method or specialized instruments, and strictly follow the 55° tooth shape and taper requirements.