Thread Standards

Figure 1: Valve threads

To ensure compatibility and ease of maintenance, threads are standardized. For existing and new applications, it is necessary to identify the thread type to guarantee a proper connection of the joint.

Table of Contents

• Common Thread Standards
  • BSP - British Standard Pipe
  • NP – National (American) Pipe Thread
  • M – Metric Thread (ISO)
• How to identify an unknown thread type?
• Thread Compatibility
• Additional Information

Common Thread Standards

BSP – British Standard Pipe

The BSP, or Whitworth thread, is a family of thread standards that has been adopted internationally, except in the United States. This thread form is based on a 55° V-thread with rounded roots and crests, as seen in Figure 2. For a thread that conforms to BSP, the major diameter of the pipe thread is slightly smaller than the actual OD of the pipe, and the minor diameter will be very close to (smaller than) the inside diameter of the female thread. There are two types of BSP threads:

1. BSPP: Both the male and female threads are parallel. BSPP connections are widely used in UK, Europe, Asia, Australia, New Zealand and South Africa. Sizes can be seen in Table 1.
2. BSPT: The male threads are tapered and the female threads are commonly parallel. BSPT connections are especially popular in China and Japan. Sizes can be seen in Table 2.

Figure 2: A BSPP male parallel thread profile (left) and a BSPT taperd male thread profile (right)

BSP threads are identified with letters each of which represents the type of thread and their associated standards¹:

• G: external and internal parallel (ISO 228, DIN 259) - BSPP
• R: external taper (ISO 7, EN 10226, BS 21, JIS B 0203) - BSPT
• Rp: Internal parallel (ISO 7-1, EN 10226) - BSPT
• Rc: internal taper (ISO 7) - BSPT
• Rs: external parallel (BS 21) - BSPT- Obsolete

¹ISO 7: Pipe threads where pressure-tight joints are made on the threads. ISO 228: Pipe threads where pressure-tight joints are not made on the threads.

Labeling Example: EN 10226 Rp 2 ½
This refers to a British Standard Pipe thread tapered (EN 10226) with an internal parallel form (Rp) and a nominal size of 2 ½.

The actual sizes of the most commonly used BSP threads are listed in Table 1 and Table 2 for BSPP and BSPT threads respectively. Table 3 provides data on pipe sizes associated with these threads.

Note: Each thread size is identified with a number which has little to do with the actual size of the thread. This discrepancy originates from amendments in industrial practices and standardisation throughout the history of standardisation of pipe threads.  Therefore, always compare measurements with actual sizes listed in the tables.

Figure 3: NPT thread profile mating with the top being a fitting internal thread and the bototm being a pipe external thread.

NP – National (American) Pipe Thread

The American national pipe thread was created based on a 60° V-thread with flattened peaks and valleys (Figure 3) and is widely used in the US and Canada. There are two types of NP threads:

1. NPS: Straight threads, meaning the male and female threads are parallel.
2. NPT: Tapered threads, meaning the male and female threads are tapered. NPT threads are more commonly used. NPT Thread specifications are based on ANSI B1.20.1 and the threads sizes can be seen in Table 4.

Labeling Example: 3/8 - 18 NPT

National pipe threads are designated with their nominal size (3/8) followed by number of threads per inch (18) and the symbol for the thread series (NPT).

Note: NPS (National Pipe Straight) is not to be confused with NPS (Nominal Pipe Size) which is an American set of standards for pipes. For a given outside diameter, NPS (Nominal Pipe Size) provides multiple pipe schedules (wall thicknesses) while the pipe thread profile remains the same among them.

M – Metric Thread (ISO)

Figure 4: Metric Thread profile

The Metric Thread is one of the first internationally agreed general-purpose thread type. The V-shaped thread form has a 60° flank angle and male and female threads are both parallel (Figure 4). Metric Threads come in different pitch sizes for a given diameter: coarse pitch and fine pitches. Coarse threads have the default pitch size whereas fine threads have smaller pitch sizes and are used less often. As a result, coarse threads are identified by diameter only while fine threads are recognized by diameter as well as pitch size.

Metric threads come in two different pitch sizes for a given diameter:

1. Coarse Pitch: These have the default pitch size according to Table 5 and are the most commonly used. They adhere to ISO 724 (DIN 13-1).
2. Fine Pitch: These have a smaller pitch size and in the labeling they require the diameter of the pitch size. They adhere to ISO 724 (DIN 13-2 to 11).

Labeling Example: M8
This refers to a metric coarse thread with a diameter of 8 mm (which with reference  Table 5, corresponds to a pitch size of 1 mm)

Labeling Example: M4 x 0.5
This indicates a fine thread with a diameter of 4 mm and a pitch size of 0.5 mm. 

M 10M 10

How to identify an unknown thread type?

Figure 5: Pitch gauge

To identify an unknown thread, verify the following:

• Gender: Male or female.
• Taper: Parallel or tapered. This can be determined whether by visual inspection or by measuring the diameter of the thread at the beginning and the end via a caliper.
• Thread Diameter: For male threads, measure the major diameter and for female threads measure the minor diameter. Note that this diameter will always be different from the nominal size of the thread.
• Thread Pitch: For this purpose, a pitch gauge can be used (Figure 5). Make sure to measure multiple samples to confirm accurate pitch. If a pitch gauge is not available, count the number of threads in a 1-inch span (TPI) and the reciprocal is your thread pitch in inches. If you are skeptical that the thread is metric, count the number of threads in a 10-mm span. The reciprocal of this number is your pitch size in mm.

Check Tables 1-5 to find a matching actual diameter corresponding to a nominal thread size. Confirm thread pitch or TPI with the values corresponding to that diameter.

Thread Compatibility

Although some BSP and NPT threads might be tightened and engaged together fairly well, pitch and thread angle differences between them will allow spiral leakage. Specifically speaking, at 1/16”, 1/8”, 1/4”and 3/8” sizes, pitch difference results in misalignment of the threads. This misalignment might result in the threads to be fastened only partially. However, at 1/2” and 3/4” sizes, NPT and BSP have the same pitch which will allow them to engage well. Nevertheless, since there are differences in thread forms and thread angles, there will still be issues with this connection. For low end applications, if an appropriate thread sealant is used, these threads might be used together effectively. Table 6 shows the compatibility of threads and the required seal type to be used as a reference for your application.

Additional Information

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