A thread is a surface whose axis is contained in the plane and around it describes a cylindrical helical path (this is the name given to any curved line whose tangents form a constant angle (α), following a fixed direction in space).

There are five fundamental types of threads:

1. Metric
2. Whitworth
3. NPT: The most well-known and widely used fitting where the pipe thread provides both mechanical and hydraulic seal
4. UNC: Unified simple thread (common)
5. UNF: Unified Thread thin way

General rules

Metric system

• BS 3643; ISO Metric Threads
• BS 4827: ISO Miniature or thin threads
• BS 4846: ISO Trapezium or trapezoid threads
• BS 21: Threads for connections and thin wall pipes

English System

• BS84: Whitworth threads
• BS93: British Association (BA) threads
• DIN 477

American National Conical Threads, NPT

• BS 949/79
• AINSI B1.20.3 1976

Unified simple thread (common), or UNC

• ANSI/ASME B1. 1 1989

Unified thread thin walkway, or UNF

• ANSI/ASME B1. 1 1989

Main differences

Metric

The angles of the coils are 60º, although in screws the bottom of the thread is rounded, and the tips are flat.

On metric threads, the pitch is indicated by the feed in millimeters for each turn

Whitworth

The angles of the coils are 55º

In Whitworth threads the pitch is indicated by the number of wires per inch

Another difference between metric and Whitworth is that while metric threads are sharp in the external part in a height t1=0,64595h and rounded in the intern part with r = 0.1082h, in Whitworth as much the external point as the intern part are rounded with a height oft1 = 0.64033h and r = 0.13733h.

NPT

It has a male and female tapered thread that seals with Teflon tape or a bonding compound.

UNC

Coarse thread recommended for general engineering use

UNF

Fine thread recommended for general use in automobiles, airplanes, and other applications where wall thickness requires thin thread

Designation

The designation of the thread in the identification of the main elements involved in the manufacture of a given thread is done by means of its representative letter and indicating the dimension of the outer diameter and the pitch.

For example:

Thread M 3.5 x 0.6

Where:

• M is metric
• 3.5 outer diameter in mm
• 0.6 pitch in mm

Another example:

W Thread 3/4"-10

Where:

• W is Whitworth
• 3/4" outside diameter in inches
• 10 wires per inch

There are two things to keep in mind:

1. To indicate the direction of the turn, the designation will indicate "L" if it is a left-hand thread, and if it is right-handed, nothing will be indicated
2. If the thread has more than one inlet, it will indicate "2 ent" or "3 ent" and if it is from one inlet, nothing will be indicated.

It is possible to create a thread with non-standard dimensions, but it is always advisable to use standard threads to acquire them in hardware stores and facilitate the location of the spare parts.

Only metric and Whitworth threads have been highlighted as the most widely used, but there are many important threads for special uses.

Below are detailed tables for the fine and coarse series of these families:

	Denomination	Other names
UNF	Unified American Thread thin pitches	NF, SAE
NPT	American conical thread for pipe	ASTP
Ba	British Association
ISO	International Standarads Organisation
C	Thread for bycicles
BSP	Whitworth thread	R
Rd	Rounded-crest thread
Tr	Trapezoid-crest thread
NPSF	American Cylindric “dryseal” thread for pipes
BSW	Whitworth thread of simple pitch	W
BSF	Whitworth thread of thin pitch
BSPT	Cylindric Whitworth thread for pipes	KR
NPTF	American Conical “dryseal” thread for pipes
PG	Thread for armoured pipes	Pr
M	Metric Thread of thin pitch	SIF
UNC	Unified American thread of simple pitch	NC, USS
M	Metric Thread simple pitch	SI
UNEF	Unified American thread superfine pitch	NEF
NPS	American cylindric thread for pipes
E	Edison Thread
UNS	American Thread Special pitch	NS
API	American Petroleum Institute

As we have seen, metric threads and Whitworth threads have different sizes/measurements, but, even so, there are sizes that are equivalent. These are shown in the following table where you can see the equivalences between metric and Whitworth wrenches and the common use of the nut:

Dif.	Mouth size	Mouth size	Wrench	Wrench	Wrench	Nut or Bolt
(mm)	Mm	Inch	Metric	AF= Across Flats	British	(* = not recommended)
	2.97	0.117			10BA
0.23	3.2	0.126	3,2 mm			M1.6
0.13	3.33	0.131			9BA
0.53	3.86	0.152			8BA
0.11	3.97	0.156		5/32 AF		No.0 UNC
0.03	4	0.157	4mm			M2
0.37	4.37	0.172			7BA
0,39	4.76	0.188		3/16 AF		No. 2 UNC
0.14	4.9	0.193			6BA
0.1	5	0.197	5mm			M2.5
0.5	5.5	0.217	5,5 mm			M3
0.06	5.56	0.219		7/32 AF
0.03	5.59	0.22			5BA
0.37	5.95	0.234		15/64 AF
0.05	6	0.236	6mm			M3.5 *
0.3	6.3	0.248			4BA
0.05	6.35	0.25		1/4 AF		No. 4 UNC
0.65	7	0.276	7mm			M4
0.14	7.14	0.281		9/32 AF
0.02	7.16	0.282			3BA
0.77	7.94	0.313		5/16 AF		No. 6 UNC y No 10 UNC (pin)
0.06	8	0.315	8mm			M5
0.23	8.23	0.324			2BA
0.36	8.59	0.338			1/8 W; 3/16 BSF
0.15	8.73	0.344		11/32 AF		No. 8 UNC
0.27	9	0.354	9mm
0.27	9.27	0.365			1BA
0.25	9.53	0.375		3/8 AF		N º 10 UNC (nut)
0.48	10	0.394	10mm			M6
0.49	10.49	0.413			0BA
0.51	11	0.433	11mm			M7 *
0.11	11.11	0.438		7/16 AF		1/4 UNF
0.19	11.3	0.445			3/16 W, 1/4 BSF
0.7	12	0.472	12mm
0.7	12.7	0.5		1/2 AF		5/16 UNF
0.3	13	0.512	13mm			M8
0.33	13.34	0.525			1/4 W; 5/16 BSF
0.67	14	0.551	14mm
0.29	14.29	0.563		9/16 AF		3/8 UNF
0.71	15	0.591	15mm
0.24	15.24	0.6			5/16 W; 3/8
					BSF
0.64	15.88	0.625		5/8 AF		7/16 UNF (pin)
0.13	16	0.63	16mm
1	17	0,669	17mm			M10
0.46	17.46	0.688		11/16 AF		7/16 UNF (nut)
0.54	18	0.709	18 mm
0.03	18.03	0.71			3/8 W; 7/16 BSF
0.97	19	0.748	19mm			M12
0.05	19.05	0.75		3/4 AF		1/2 UNF
0.95	20	0.787	20mm
0.64	20.64	0.813		13/16 AF		9/16 UNF (pin)
0.19	20.83	0.82			7/16 W, 1/2 BSF
0.17	21	0.827	21mm
1	22	0.866	22mm			M14 *
0.22	22.23	0.875		7/8 AF		9/16 UNF (nut)
0.78	23	0.906	23mm
0.37	23.37	0.92			1/2 W, 9/16 BSF
0.44	23.81	0.938		15/16 AF		5/8 UNF
0.19	24	0.945	24mm			M16
1	25	0.984	25mm
0.4	25.4	1		1 AF
0.25	25.65	1.01			9/16 W; 5/8 BSF
0.35	26	1.024	26mm
0.99	26.99	1.063		1-1/16 AF
0.01	27	1.063	27mm			M18 *
0.94	27.94	1.1			5/8 W, 11/16 BSF
0.06	28	1.102	28mm
0.57	28.58	1.125		1-1/8 AF		3/4 UNF
1.43	30	1.181	30mm			M20
0.48	30.48	1.2			11/16 W, 3/4 BSF
1.27	31.75	1.25		1-1/4 AF
0.25	32	1.26	32mm			M22 *
1.02	33.02	1.3			3/4 W; 7/8 FBS
0.32	33.34	1.313		1-5/16 AF		7/8 UNF
0.66	34	1.339	34mm
1.31	35.31	1.39			13/16 W
0.69	36	1.417	36mm			M24
0.51	36.51	1.438		1-7/16 AF
1.08	37.59	1.48			7/8 W, 1 BSF
0.41	38	1.496	38mm
0.1	38.1	1.5		1-1/2 AF		1 UNF
2.03	40.13	1.58			15/16 watts
0.87	41	1.614	41mm			M27 *
0.27	41.28	1.625		1-5/8 AF
1.14	42.42	1.67			1 W; 1-1/8 BSF
0.44	42.86	1.688		1-11/16 AF		1-1/8 UNF
3.14	46	1.811	46mm			M30
0.04	46.04	1.813		1-13/16 AF
1.21	47.24	1.86			1-1/8 W; 1-1/4 BSF
0.38	47.63	1.875		1-7/8 AF		1-1/4 UNF
2.38	50	1.969	50 mm			M33 *
0.8	50.8	2		2 AF
1.27	52.07	2.05			1-1/4 W; 1-3/8 BSF
0.32	52.39	2.063		2-1/16 AF		1-3/8 UNF
2.61	55	2.165	55mm			M36
0.56	55.56	2.188		2-3/16 AF
0.83	56.39	2.22			1-3/8 W; 1-1/2 BSF
0.76	57.15	2.25		2-1/4 AF		1-1/2 UNF
2.85	60	2.362	60mm			M39 *

As can be seen in the table, for a metric wrench, there are several equivalent measurements.

For example, the equivalent of a 3.2 mm metric wrench would be a 0.131-inch (3.33 mm) wrench.

However, the following should be noted: The size of the mouth is often the maximum dimension of the nut, so the given wrench mouth is likely to be slightly larger to allow for an acceptable fit.

Thread profiles

Trapezoidal thread

It is recommended when you want to direct the force in a single direction, for example, in jacks and pipe bolts.

Square Thread

The square thread differs in that it can transmit all forces in the direction almost parallel to the shaft.

Acme Thread

It allows the use of a split or release nut. It has generally replaced the truncated fillet thread.

It is sturdy and easy to carve and allows the use of a split nut or clutch release that cannot be used with a square fillet thread.

There are three kinds of Acme:

• 2G
• 3G
• 4G

Each one has clearances in all dimensions to allow free movement.

Acme Thread of Truncated Fillet

If there are space constraints, it is advisable to use this type of nut as it is stronger for power transmission applications

Whitworth Thread

The Whitworth thread is equivalent to the British Standard Pipe (BSP) thread explained below.

Rounded Thread

It is recommended when threads have to be rolled or molded in sheet metal. For example, in caps for bottles.

Sharp V-thread

This thread is very uncommon and is usually applied in precision instruments where the most important thing is adjustment or friction clamping.

Metric

Its advantages include tensile strength, due to the large angle of the thread.

Defects? Depending on the position of the thread threads, it may lose effectiveness.

A bit of history

The Whitworth thread shape was selected for connecting pipe threads, as it became self-sealing by cutting at least one of the strands in a cone. This became known as the Standard Thread for British Pipe (BSP – Tapered BSP Thread or Parallel BSP). The Whitworth thread is now used internationally as a standard thread for joining lightweight carbon steel pipes.

BSP thread is a family of standard thread types that has been adopted for interconnection and sealing ends of pipes by combining an external (male) with an internal (female) thread.

• BSPP (Parallel): The diameter of this type of thread is constant along the length of the thread.         
• BSPT (Tapered): The diameter of this type of thread increases or decreases along the length of the thread.      

The thread is the same in both, the difference is that one is conical and the other parallel.

On the other hand, in 1864 in North America, William Sellers established the standard for nuts, bolts, and screws that he made the National Tapered Pipe Thread (NPT).

NPT (American National Pipe Thead; Conical): Refers to an American standard. This is the most well-known and widely used connection where the pipe thread provides both mechanical bonding and hydraulic sealing. The NPT thread has a male and female tapered thread that seals with Teflon tape or a bonding compound.

The NPS thread and the NPT thread are the same, the difference is, as we have said, that NPT is conical and NPS parallel.

Warning! Good threading performance is obtained when the male and female are both of the same type, either NPT with NPT, BSPT with BSPT and BSPP with BSPP. As long as the thread is done well, this combination will not cause any leakage.