Name That Thread

Published on Machine Design (http://machinedesign.com)

Name that thread

by Stephen J. Mraz

Created 10/21/2004 - 02:00

Four basic steps identify threads on industrial hydraulic fittings.

Burleigh Bailey

Parker Hannifin Corp.

Tube Fittings Div.

Columbus, Ohio

Hydraulic fittings have either tapered or parallel threads. But tapered ones are not always pipe threads,

and pipe threads can be parallel.

Using a thread-pitch gage is the easiest way to determine pitch.

Use calipers measure nonpipe size.

15.11.2011 Name that thread

http://machinedesign.com/print/77515 1/8

Determine pipe-thread size by comparing threads against nominal-size profiles.

Determining the correct thread for a hydraulic fitting sounds simple. But a burgeoning global marketplace

for hydraulic equipment has produced a proliferation of fittings and connectors. This makes correct

identification of threads an increasingly difficult and frustrating task, even for the most-experienced

technician and engineer.

Most people in the fluid-power industry are aware of American pipe threads (NPT/NPTF) and what are

often called SAE or Unified threads (UN/UNF). But there seems to be an endless number of other

unfamiliar threads, usually classified as "metric," regardless of their actual form. In reality, there are true

metric threads as well as BSP (British Standard Pipe) threads.

For identification purposes, hydraulic tube fittings and connectors can be divided into six different thread

types: UN/UNF, NPT/NPTF, BSPP (BSP, parallel), BSPT (BSP, tapered), metric parallel, and metric

tapered. Three are parallel (UN/UNF, BSPP, metric parallel) and three are tapered (NPT/NPTF, BSPT,

metric tapered). Three are pipe threads (NPT/NPTF, BSPT, BSPP) and three are not (UN/UNF, metric

parallel, metric tapered). Keep in mind that tapered does not necessarily mean it is pipe thread (for

example, metric tapered). And pipe threads can be parallel (for instance, BSPP).

Determining the correct type of thread is critical for selecting the proper mating or replacement fitting. First

and foremost it is a safety issue. Correct assembly maintains pressure

and reliably seals a fitting or adapter. It also prevents costly and time-consuming thread stripping and

damage. Regardless of thread type, using a few simple tools and the following steps will result in proper

identification.

1. Determine if the thread is tapered or parallel. Sometimes visual inspection is sufficient. Tapered

threads get smaller in diameter toward the end of the fitting while parallel threads have the same diameter

from end to end. If this is not obvious just by looking at the fitting, use the parallel jaws of a caliper, for

example, to make a comparison. An O-ring or tube nut usually indicates parallel threads.

2. Determine pitch. Pitch is typically defined in terms of threads per inch (TPI) or distance between

threads. For metric threads, pitch is the distance between threads in millimeters. Use a pitch gage to

compare threads, or accurately measure and calculate the number of threads within a given length. It is

much easier to compare threads against a lighted background with a pitch gage. Because some thread

pitches are nearly identical, try several gages before deciding on the one that fits best. (Possible pitches

are shown in the second column of How to identify threads.)

3. Determine size. The two methods for sizing threads depend on whether the threads are pipe or non-

pipe. For pipe threads, determine nominal size by comparing the part against a size profile, (as shown in

the accompanying image). A good rule of thumb for pipe sizes up to 2-in. nominal is to measure the OD

and subtract 1 /4 in. Round off for the nominal pipe size.

For other threads, determine actual size by measuring the OD (major diameter) with a caliper.

4. Designate the thread. Technically, this final step is not part of the identification process but, rather, a

method of designating thread types in a standard format. It usually indicates thread size (nominal or

actual), type and, in some cases, pitch.

Grab a handful of "mystery threads" and practice the four-step ID process. It's kind of like learning how to



change a tire — a useful skill to have when the need arises.

Helpful hints for thread ID

British taper pipe threads (BSPT) and American pipe threads (NPT/NPTF) have some similarities and

some differences, but there are two sizes, 3 /4, that are difficult to distinguish with the naked eye. The two

have the same nominal size and pitch, and sometimes can be differentiated only through deductive

reasoning.

Standard thread diameters are normally no larger than their stated sizes. Gauged accurately, with a

caliper for example, the measured diameter is normally slightly smaller than the actual thread size.

Internal threads should not be identified using this method because it is difficult to verify the proper fit of

the thread pitch gage. If possible, find the mating external thread and follow the four-step process to

identify it.

It is not unusual to have a tapered external thread connected to a parallel internal thread.

NPSM threads are actually parallel but have the same pitch as NPT/NPTF. These threads are typically

used inside swivel nuts for holding only. Contact between the 30° internal chamfer of the male pipe thread

and the seat inside the swivel provides the seal.

HOW TO IDENTIFY THREADS

STEP 1 STEP 2 STEP 3 STEP 4

The chart summarizes the four-step thread

ID process. *Note that JIS (Japanese

Industrial Standards) threads are

identified similar to BSPP and BSPT but

defined with PF and PT, respectively. For

example, PF 1 /8 and PT 1 /2.

Determine

if tapered

or parallel

Determine

pitch

Determine

size

Thread

type

Thread

designation

Parallel

12, 14,

16, 18,

20, 24

Measure

with

caliper

UN/UNF

(SAE)

Size-pitch,

type (for

example, 3

/4-16

UN/UNF)

Tapered11 1/2, 14,

18, 27

Compare

with

profile

NPT/NPTF

(American

Pipe)

Size-pitch,

type ( 1 /4-

18 NPT)

Parallel11, 14,

19, 28

Compare

with

profile

BSPP

(British

Pipe)

G, size* (G1/8)



Tapered 11, 14,

19, 28

Compare

with

profile

BSPT

(British

pipe)

R, size* (R1/2)

Parallel1.0, 1.5,

2.0

Measure

with

caliper

Metric

parallel

M, size X

pitch (M14

X 1.5)

Tapered1.0, 1.5,

2.0

Measure

with

caliper

Metric

tapered

M, size X

pitch, keg

or taper

(M10 X 1

keg or

taper)

BRITISH STANDARD THREADS

Thread

60° cone

(BSPP)

BS

60° cone

(BSPP)

JIS

30° flare

(BSPP)

JIS

JIS

B2351

(BSPP)

BSPT

DIN 3852-2

Type E

(BSPP)

ISO 1179-3

Type G

(BSPP)

DIN 3852-2

Type B

(BSPP)

1 /8-28 G 1 /8 - - - R 1 /8 G 1 /8 G 1 /8 G 1 /8

1 /4-19 G 1 /4 PF 1 /4 PF 1 /4 PF 1 /4 R 1 /4 G 1 /4 G 1 /4 G 1 /4

3 /8-19 G 3 /8 PF 3 /8 PF 3 /8 PF 3 /8 R 3 /8 G 3 /8 G 3 /8 G 3 /8

1 /2-14 G 1 /2 PF 1 /2 PF 1 /2 PF 1 /2 R 1 /2 G 1 /2 G 1 /2 G 1 /2

5 /8-14 G 5 /8 - - - - - - -

3 /4-14 G 3 /4 PF 3 /4 PF 3 /4 PF 3 /4 R 3 /4 G 3 /4 G 3 /4 G 3 /4



1-11 G1 PF1 PF1 PF1 R1 G1 G1 G1

1 1 /4-

11G1 1 /4 PF1 1 /4 PF1 1 /4 PF1 1 /4

R1 1

/4G1 1 /4 G1 1 /4 G1 1 /4

1 1 /2 -

11G1 1 /2 PF1 1 /2 PF1 1 /2 PF1 1 /2

R1 1

/2G1 1 /2 G1 1 /2 G1 1 /2

2-11 G2 PF2 PF2 PF2 R2 G2 G2 G2

UN/UNF, NPT/NPTF, AND NPSM THREADS

Dash

size

O-ring

face

seal

SAE 37° flare

(Formally JIC)

24°

flareless

(inch)

SAE straight

thread (ORB)

SAE 45°

flare (SAE

flare)

Taper pipe

(NPT/NPTF)

Pipe

swivel

(NPSM)

2 - 5 /16-24 5 /16-24 5 /16-24 5 /16-24 1 /8-27 1 /8-27

3 - 3 /8-24 3 /8-24 3 /8-24 3 /8-24 1 /8-27 1 /8-27

4 9 /16-18 7 /16-20 7 /16-20 7 /16-20 7 /16-20 1 /8-27 1 /8-27

5 - 1 /2-20 1 /2-20 1 /2-20 1 /2-20 1 /8-27 1 /8-27

611 /16-

169 /16-18 9 /16-18 9 /16-18 5 /8-18 1 /4-18 1 /4-18

813 /16-

163 /4-16 3 /4-16 3 /4-16 3 /4-16 3 /8-18 3 /8-18

10 1-14 7 /8-14 7 /8-14 7 /8-14 7 /8-14 1 /2-14 1 /2-14



12 1 3 /16-

12

1 1 /16-12 1 1 /16-12 1 1 /16-12 1 1 /16-14 3 /4-14 3 /4-14

14 1 5 /16-

121 3 /16-12 1 3 /16-12 1 3 /16-12 - 3 /4-14 3 /4-14

16 1 7 /16-

121 5 /16-12 1 5 /16-12 1 5 /16-12 - 1-11 1 /2 1-11 1 /2

20 1 11 /16-

121 5 /8-12 1 5 /8-12 1 5 /8-12 - 1 1 /4-11 1 /2

1 1 /4-11 1

/2

24 2-12 1 7 /8-12 1 7 /8-12 1 1 /2-11 1 /2 - 1 1 /2-11 1 /21 1 /2-11 1

/2

32 2 1 /2-12 2 1 /2-12 2 1 /2-12 2 1 /2-12 - 2-11 1 /2 2-11 1 /2

METRIC THREADS

Tube

size,

mm

24°

flareless

(LL

Series)

24°

flareless

(L

Series)

24°

flareless

(S

Series)

60°

cone

30°

flare

24°

flareless

(JIS)

ISO

6149

Metric

taper

DIN

3852-

1

Type

E

DIN

3852-1

Port with

O-ring

and

retaining-

ring seal

DIN

3852-

1

Type

B

4 M8 X 1 M12

X 1.5

M8 X

1

M8 X

1keg

M8 X

1M8 X 1

M8 X

1

5 M10

X 1

M10 X

1keg

M10

X 1M10 X 1

M10

X 1

6 M10 X 1M12 X

1.5

M14 X

1.5

M14

X 1.5

M14

X 1.5

M12

X 1.5

M12 X

1.5keg

M12

X 1.5M12 X 1.5

M12

X 1.5



8 M12 X 1M14 X

1.5

M16 X

1.5

M16

X 1.5

M14 X

1.5

M14

X 1.5

M14 X

1.5keg

M14

X 1.5 M14 X 1.5M14

X 1.5

10 M14 X 1M16 X

1.5

M18 X

1.5

M18

X 1.5

M16 X

1.5

M16

X 1.5

M16 X

1.5keg

M16

X 1.5M16 X 1.5

M16

X 1.5

12 M16 X 1M18 X

1.5

M20 X

1.5

M22

X 1.5

M22

X 1.5

M18 X

1.5

M18

X 1.5

M18 X

1.5keg

M18

X 1.5M18 X 1.5

M18

X 1.5

14 M22 X

1.5

M22

X 1.5

M22 X

1.5keg

M20

X 1.5M20 X 1.5

M20

X 1.5

15 M22 X

1.5

M24

X 1.5

M22 X

1.5

M22

X 1.5M22 X 1.5

M22

X 1.5

16 M24 X

1.5

M26

X 1.5

M22

X 1.5

M22 X

1.5keg

M24

X 1.5M24 X 1.5

M24

X 1.5

18 M26 X

1.5

M26 X

1.5

M26

X 1.5M26 X 1.5

M26

X 1.5

20 M30 X 2 M27

X 2

M27

X 2M27 X 2

M27

X 2

22 M30 X 2 M30 X

1.5

25 M36 X 2M38

X 1.5

M33

X 1.5

M33

X 2

M33

X 2M33 X 2

M33

X 2

28 M36 X 2 M36 X

1.5

M36

X 2M36 X 2

M36

X 2

30 M42 X 2 M42

X 2

M42

X 2M42 X 2

M42

X 2

32 M45

X 1.5

M36

X 1.5



35 M45 X 2 M45 X

1.5

M45

X 2M45 X 2

M45

X 2

38 M52 X 2 M42

X 1.5

M48

X 2

M48

X 2M48 X 2

M48

X 2

40 M52

X 1.5

42 M52 X 2

50 M60

X 2

MAKE CONTACT

Parker Hannifin Corp., Tube Fittings Div.,

(614) 279-7070, parker.com [1]

Source URL: http://machinedesign.com/article/name-that-thread-1021

Links:

[1] http://www.parker.com



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