Threads are divided into: fastening threads, pipe threads, transmission threads and special threads. Fastening threads include: common threads; wide-gap threads (hot-dip galvanized threads, wide-gap threads for high-temperature and high-pressure applications); self-tapping threads, self-extruding threads (triangular self-tapping threads); wood threads; transition fit and interference fit threads; and specialty threads for instruments.
Parts that are fastened by threads are called threaded fasteners. Common threaded fasteners include bolts, studs, machine screws, tapping screws, self-drilling screws, and nuts.
What Are Fastener Threads?
Fastener threads refer to continuous protrusions with a specified tooth shape formed along a spiral line on a cylindrical or conical surface. The function of threads is to form a mechanical fastening connection by combining internal and external thread products to transmit force and movement.
Fastener threads are divided into three types according to the measurement unit and the country of invention: metric, American, and imperial threads, which are further divided into coarse and fine thread standards.
Fastener Thread Terminology
- External thread: A thread formed on the outer surface of a fastener (such as a bolt, screw, or stud).
- Internal thread: An internal thread formed on the inner surface of a fastener (such as a nut or threaded hole).
- Major diameter: The diameter of the external thread crest; the diameter of the internal thread root.
- Minor diameter: The diameter of the external thread root; the diameter of the internal thread crest.
- Pitch diameter: The diameter of a cylinder or cone whose generating line passes through the point where the groove and projection on the thread profile are equal in width.
- Right-hand thread: A thread that rotates clockwise.
- Left-hand thread: A thread that rotates counterclockwise.
- Coarse thread: wide and deep pitch.
- Fine thread: dense and shallow pitch.
- Tooth angle: The angle between two adjacent tooth sides on the thread tooth profile.
- Pitch: The axial distance between corresponding points on the pitch line of two adjacent threads.
Metric (ISO) Threads
Metric thread (ISO standard thread) is the most commonly used mechanical fastening thread. The International Organization for Standardization and industrially developed countries have all developed their own metric thread standards.
The basic tooth profile of the metric thread is an equilateral triangle with a tooth angle of 60°. The thread is marked with the letter M (for metric thread). The nominal diameter (D or d) and pitch (P) are expressed in millimeters, representing the metric thread.
- D – Nominal diameter
- D2 – Pitch diameter
- D1 – Minor diameter
- P – Pitch
- Thread angle 60°
- H – Initial triangle height
- 5/8H – Basic thread height
Metric Thread Fit Grades
There are three thread tolerance grades for metric external threads: 5h (6h), 6g and 8g; there are three thread tolerance grades for metric internal threads: 5H, 6H and 7H.
The G thread has a deviation to facilitate assembly and allow for plating and coating. The numbers 5, 6, and 8 refer to the thread tolerance and tightness. The external thread grade of commercial fasteners is 6G, and the internal thread grade is 6H.
Metric Common Thread Standard Table
| ISO | GB | DIN | JIS | ASME | |
| Thread type | ISO 68-1 | GB/T 192 | DIN ISO 68-1 DIN 13~19 | JIS B0205-1 | ASME B1.13M |
| Thread pitch (normal) | ISO 261 | GB/T 193 | DIN ISO 261 | JIS B0205-2 | |
| Thread pitch (preferred) | ISO 262 | GB/T 9144 | DIN ISO 262 | JIS B0205-3 | |
| Basic dimensions | ISO 724 | GB/T 196 | DIN ISO 724 DIN 13-1 DIN 13-11 | JIS B0205-4 | |
| Thread tolerance | ISO 965-1 | GB/T 197 | DIN ISO 965-1 DIN 13-52 | JIS B0209-1 | |
| Limit deviation | ISO 965-3 | GB/T 2516 | DIN ISO 965-3 | JIS B0209-3 | |
| Limit dimension | ISO 965-2 | GB/T 9145 | DIN ISO 965-2 | JIS B0209-2 | |
| Thread inspection | ISO 1502 | GB/T 3934 | DIN ISO 1502 DIN ISO 2241 DIN ISO 2279 DIN ISO 2284 DIN ISO 2285 DIN 2299-1 | JIS B0251 | ASME B1.3M ASME B1.16M |
American (Inch) UN Unified Thread
American thread is the “unified thread”, a standard jointly developed by the United Kingdom, the United States and Canada.
The basic tooth pattern of the U.S. uniform thread is the truncated profile of the top and bottom of an equilateral triangle, with a tooth angle of 60°. Thread specifications are marked by the nominal diameter (D or d) and the number of teeth per inch (n), and the unit of measurement is inches.
- P = pitch n – indicates the number of threads in 1 inch; P = 1/n in
American Un Thread Matching Code
â– Code for American internal and external threads
Unified thread tolerances for external and internal threads are indicated by letters:
- “A” indicates external thread, such as “2A” or “3A”
- “B” indicates internal thread, such as “2B” or “3B”
â– Application of American thread series
The American Unified Coarse Thread (UNC) is widely used in mass-produced fasteners. It’s generally used for low-strength materials (i.e., low-performance grades) and offers easy assembly. Coarse threads have higher resistance to stripping, less wear and less stress.
Commercial fasteners often use the coarse UNC series thread.
Steel structural bolts generally use 8UN or UNC threads (a matching of A194 and A193).
The American unified fine thread UNF is generally used for fasteners in high-strength joints. Compared with coarse threads, fine threads have a lower profile height, a larger minor diameter, and a larger tensile stress resistance area.
â– UNR Thread
UNR thread tooth top is flat or arc connection, but the tooth root must be arc-shaped, and the arc radius is not less than 0.108P. Only applicable to external threads.　
The UN thread root is flat, which causes stress concentration. In order to reduce stress concentration when the bolt is under stress, the arc radius of the UNR thread bottom cannot be too small.
UNR threads have higher fatigue strength than UN threads and are better suited for notch-sensitive materials.
The US IFI standard also stipulates that bolts and screws with a strength grade of 5 or higher must use UNR threads. (A574 high-strength hexagon socket screws with a thread diameter of 1 inch or less should use UNRC/UNRF threads.)
The metric standard stipulates that the external thread higher than or equal to grade 10.9 must have an arc curve at the bottom of the thread and must withstand a certain fatigue impact load.
Types of Unified Threads
- UNC Unified Coarse Pitch Thread
- UNRC Coarse Pitch Thread with a Radius Root for External Threads
- UNF Unified Fine Pitch Thread
- UNRF Fine Pitch Thread with a Radius Root for External Threads
- UNEF Unified Extra Fine Pitch Thread
- UNREF Extra Fine Pitch Thread with a Radius Root for External Threads
- UN Unified Constant Pitch Thread
- UNR Constant Pitch Thread with a Radius Root for External Threads
- UNS Special Series Unified Threads
British Thread (inch)
The basic tooth profile of the British thread is an isosceles triangle, with a tangent profile of the tooth top and tooth bottom arc transition, a tooth angle of 55°, and the thread is marked with the nominal diameter and the number of teeth per inch. The unit of measurement is inches.
Inch thread types
- BSW Whitworth Coarse Thread
- BSF Whitworth Fine Thread
- Whitworth Non-Standard Series Whitworth Thread
We usually refer to British threads and American threads as British threads, and those using millimeters as units are also called ordinary threads.
Coarse Thread vs. Fine Thread
Coarse threads have large pitch, large tooth height, poor self-locking properties, are easy to fall off, require more effort to tighten, have poor fine-tuning properties, poor vibration resistance, and inferior sealing effects to fine threads. However, they have high screw-on efficiency, are easy to process, and the coating has little effect on the use of coarse threads, and are low in cost. If the thread is not damaged, the tensile and shear strengths are inferior to those of fine threads, but the threads have good impact resistance, are not easily damaged, and have a long fatigue life. It is the most common thread type, and coarse threads are generally preferred.
Fine threads have a small pitch and low thread height, offer excellent self-locking properties, are less prone to falling out, require less effort to tighten, offer excellent fine-tuning capabilities, are vibration-resistant, and offer better sealing than coarse threads. However, they have lower threading efficiency, are difficult to machine, and have significant impacts on the performance of plating, leading to higher costs. Due to its small tooth height, it can be used in thin-walled scenarios. If the thread is not damaged, its tensile and shear strengths are better than coarse teeth. However, the teeth have poor impact resistance, are easily damaged, and have a short fatigue life. Fine threads should not be used for load-bearing connections but can be used for non-load-bearing connections requiring precise adjustment.
| Coarse Thread vs. Fine Thread | ||
| Feature | Coarse Thread | Fine Thread |
| Pitch | Large (standard value) | Small |
| Thread profile | Less, Deep | Dense, Shallow |
| Assembly speed | Fast | Slow |
| Anti-loosening | Poor | Better |
| Strength | Larger | Smaller |
| Marking | No thread pitch (e.g. M10) | Pitch must be marked (e.g. M10 x 1.25) |
| Uses | General fastening connection | Thin-walled, vibration-resistant, fine-tuned, sealed |
Thread Marking
ISO Thread
Complete metric thread markings include:
Thread type code for thread characteristics.
The size of thread, generally should be composed of diameter and pitch, for multi-threaded threads, should also include the lead and the number of threads.
Thread precision, the precision of most threads is determined by the tolerance zone of each diameter (including the position and size of the tolerance zone) and the screwing length.
Examples of markings for metric threads:
- Coarse threads with a nominal diameter of 8mm and a pitch of 1.25mm – M8
- Fine thread with a nominal diameter of 8mm and a pitch of 1mm – M8×1
Example of marking code for tolerance zone of middle or major diameter of metric external thread:
- Coarse external threads with a tolerance zone of 6g for the center and top diameters–M10-6g
- Fine threads with a tolerance zone of 5g for the center diameter and 6g for the top diameter–M10×1-5g6g
Example of marking code for tolerance zone of metric internal thread pitch or major diameter:
- Coarse pitch internal threads with a 6H tolerance zone for the pitch diameter and the top diameter are M10–6H.
- Fine pitch internal threads with a 5H tolerance zone for the pitch diameter and a 6H tolerance zone for the major diameter are M10×1–5H6H.
- Metric internal threads with a 6H tolerance zone and a 6g tolerance zone are M20×2-6H/6g.
- Metric internal threads with a 6H tolerance zone and a 5g6g tolerance zone are M20×2-6H/5g6g.
Unified thread
- Indicates the thread diameter or basic major diameter
- The number of threads (pitch) and thread designation
- Thread accuracy
Marking of British thread
Differences in Threads
| Item | Metric (DIN Example) | US | UK |
| Unit | Metric | Inch | Inch |
| Thread profile | 60° thread profile angle Flat or rounded root | 60° thread profile angle UN – Flat root UNR – Rounded root | 55° profile angle Fully rounded tip and root |
| Series | Coarse thread ：MC(d) Fine thread：MF(d×P) | UNC, UNRC, UNF, UNRF, 4UN, 6UN, 8UN, 12UN. | Coarse thread：BSW Fine thread：BSF |
| Level | External thread：5g, 6g, 8g Internal thread：5H, 6H, 8H | External thread：5g, 6g, 8g Internal thread：5H, 6H, 8H | External thread：C, m, F Internal thread：M, N |
| Thread standards | DIN 13-1~15 | ANSI B1.1 | BS 84 |
| Gauge Standards | ISO 1502 DIN 13-1~15 | ANSI B1.1 | BS 919-2 |
| Thread inspection | ISO 1502 DIN 13-1~18 | ANSI B1.3M | BS 919 |
