Screws that are used to connect and join pipes and pipe parts such as water pipes are called pipe screws. However, the types and names of pipe threads are rather complicated and often confusing. Using screws with different specifications may cause leakage or damage, so it is important to confirm the types and names carefully. In this article, we discuss the types and names of pipe threads in detail.

 

Types of Pipe Threads

There are two main types of pipe threads: tapered pipe threads and parallel pipe threads. Each type has a male and female thread, and is used in combination with a male thread (threaded on the outside) and female thread (threaded on the inside). Let's take a look at the roles and standards of each type.

 

 

What is a tapered pipe thread?

This is a conical screw that tapers toward the tip. The standard number is JIS B0203, and it is used for piping such as water pipes and vacuum pipes because it is watertight, airtight, and fits tightly when screwed in with the proper torque.

 

The male threads are denoted by "R" in the ISO standard (PT in the former JIS standard) and the female threads by "Rc" in the ISO standard (PT in the former JIS standard), and the size is added to this symbol and called "R1/8" and so on.

 

Normally, "R" and "Rc" are used in combination, but if parallel female threads are to be used with tapered male threads, the ISO standard for parallel female threads is "Rp" (PS in the former JIS standard). Note that "Rp" has different dimensional tolerances from those of parallel threads for pipe, and is treated as a separate item.

 

What is a Parallel Pipe Thread?

This is a cylindrical screw with a uniform thickness (parallel) all the way to the tip. The standard number is JIS B 0202, and it is mainly used for joining machine parts.

 

Both male and female screws are denoted by "G" (PF in the former JIS standard) in the ISO standard, and the size is added. In the case of male screws, a grade "A" or "B" is added after the size, depending on the effective diameter tolerance, so they are called "G1/4B," etc.

 



Type of screw

ISO standard

Old JIS standard

Standard number

Taper screw for pipes

(parts that require water tightness and airtightness)

Taper male screw

R

PT

JIS B0203

Tapered female thread

Rc

PT

Parallel female thread

Rp

PS

Parallel threads for pipes

(parts whose main purpose is mechanical joining)

Parallel male thread for pipes

G(A)(B)

PF

JIS B 0202

Parallel female thread for pipes

G

PF

 

Inch Designation and Typical Dimensions

Pipe thread sizes are generally expressed in inches. Starting with the smallest size, the sizes increase to "1/8", "1/4", and so on, but there are some cases where a unique nomenclature is used. Therefore, we have summarized the nomenclature of inches and typical dimensions. Male threads measure the outside diameter of the screw, and female threads measure the inside diameter of the screw, and male and female threads of the same dimensions will be the ones that fit exactly.

 

Taper Threads for Pipe

For tapered threads, measure the male thread at the midpoint of the thread and the female thread at the thread's throat.

Nominal size

Number of threads

Male thread outer diameter

(φmm)

Female thread inner diameter

(φmm)

1/8

28 

9.728

8.556

1/4

19 

13.157

11.445

3/8

19 

16.662

14.95

1/2

14

20.955

18.631

3/4

14

26.411

24.117

1

11 

33.249

30.291



Parallel pipe threads

Parallel pipe threads are the same as tapered pipe threads.

Nominal size

Number of threads

Male thread outer diameter

(φmm)

Female thread inner diameter

(φmm)

1/8

28 

9.728

8.556

1/4

19 

13.157

11.445

3/8

19 

16.662

14.95

1/2

14

20.955

18.631

3/4

14

26.411

24.117

1

11 

33.249

30.291

 

 

Unlike tapered pipe threads, parallel pipe threads can be screwed in without stopping because the threads are parallel. For this reason, they are not often used for piping. However, it can be used to create a stopping position by using О-rings, packing, sealing tape, etc. If the female thread is longer than the male thread, seal at the base of the male thread; if the male thread is longer than the female thread, seal at the tip of the male thread. Be flexible and respond according to the site.

 

Summary

The types and names of pipe threads are surprisingly complicated, but if the thread dimensions are not correct, malfunctions or leaks may occur, so be sure to master them. The terminology may vary depending on the site or person, but it should be easy to understand if you have the basics down. Remember the screw standards.