Left-handed and right-handed fastener threads: Everything you need to know
Left-handed vs. right-handed fastener threads. Fastener thread sizes are either left or right handed. So what are the differences? We’ve designed this guide to answer that question. We’ll also explain fastener thread engagement and show you different fastener thread shapes. In this guide, we’ll cover:
- What is screw thread?
- What is screw handedness?
- Left-hand thread
- Right-hand thread
- Types of threads
- Common uses and applications of fastener threads
- Uses and applications of left-handed fastener threads
- Uses and applications of right-handed fastener threads
- The differences between right-handed and left-handed fastener threads
What is screw thread?
Screw threads are the continuous helical ridges or grooves that wrap around the cylindrical or conical surface of a screw – see the illustration below. It’s the part of the screw responsible for creating a mechanical connection between two objects by turning and engaging with a complementary thread on a nut, tapped hole or another screw. In essence, screw threads convert energy between rotational and linear movement and force.
Screw threads are usually designed to have a specific pitch, which is the distance between adjacent threads, and a specific profile, which describes the shape of the thread's cross-section. To learn more about screw sizes, read our article: What are screw threaded sizes?
Also, you may check our guide: A guide to threaded rods
What is screw handedness?
Screw handedness refers to the orientation of a screw's threads along its length, also known as thread direction. There are two types of screw handedness: right-handed and left-handed.
Left-handed vs. right-handed fastener threads
Right-handed screws have threads that slope upward to the right, while left-handed screws have threads that slope upward to the left. The handedness of a screw is critical, because it determines how it behaves when rotated. When a right-handed screw is turned clockwise, it moves downward into the material being screwed into.
Conversely, a left-handed screw rotated clockwise moves upward, potentially causing it to loosen. It’s essential to match the screw's handedness to the corresponding nut or hole it will be used in to avoid issues such as loosening or stripping of threads. Let’s take a closer look.
Left-hand thread
Left-handed threads, also known as reverse threads, are screw threads that slope upward to the left. When a left-handed screw is rotated clockwise, it moves away from the user instead of towards them, unlike a right-handed screw.
Left-handed threads are not as common as right-handed threads and are usually used in specific applications where they provide advantages over standard threads. For example, take rotating machinery, such as engine and turbine parts, where the rotation can cause right-handed threads to loosen over time. Left-handed threads provide a more secure fastener thread engagement that resists loosening.
For this reason, left-hand thread is not interchangeable with right-hand thread. Using the wrong thread can weaken or make the connection ineffective and may even cause damage to the parts being fastened together.
Right-hand thread
Screw threads that have a helix sloping upwards to the right when viewed from the screw's end are referred to as a right-handed thread. Clockwise rotation of right-handed screw threads move it into the material it’s being screwed into.
Right-hand thread is the most vast majority of screw thread, used in a wide range of applications from screws and bolts to intricate machinery and equipment.
In plumbing and piping systems, right-hand threads create leak-tight seals. They’re designed to provide a secure connection that resists loosening when exposed to external forces. The friction between the threads and the material in which it's screwed into helps prevent the screw from backing out when tightened in a clockwise direction.
Types of threads
There are four main types of thread. Standard thread, which are either Unified Thread Standard (UTS) or metric. UTS is the primary standard used in the United States and encompasses both coarse and fine thread versions. The metric standard is used worldwide.
Other types include acme, buttress and square, which have different fastener thread shapes, as seen below. You can learn more in our guide, What are screw heads, drives and threads?
Common uses and applications of fastener threads
Standard, acme, buttress and square fastener thread shapes can be either right- or left-handed screw threads. Typical applications for these threads include:
Standard
Typical applications: The most common fastener thread. Used across all industries, including automotive, machinery and construction.
Acme
Typical applications: Power transmission and precision movement, such as machine tools, lead screws, lathes, milling machines and jacks.
Square
Typical applications: When high accuracy and load-carrying capacity is required, such as in machine tools and presses.
Buttress
Typical applications: Suitable for heavy-lifting equipment, machines with high loads, and pipe thread designed to provide a tight hydraulic seal.
Uses and applications of left-handed fastener threads
Left-handed vs. right-handed fastener threads implies that the two are interchangeable, but they’re not, as we’ve already stated. While right-handed threads dominate the market and most applications, left-handed threads have a more specialized use.
Left-handed threads are typically used in applications where a right-handed thread could loosen over time due to the direction of rotation or other external forces. This isn’t limited to screws and other fasteners but threaded caps, for example. Some examples of left-handed thread applications include:
Left-handed threads
| Propane tanks | Prevent tanks from being accidentally attached to a regulator designed for other gas types. |
|---|---|
| Gas supply valves |
Extra layer of safety to prevent loosening due to vibration. Normal operation of the valve will actually tighten the valve, rather than loosen it, due to the direction of the thread. |
| Automotive | Ideal in tie rods or ball joints to prevent them from loosening due to the rotation of the wheels. |
| Machinery parts | Suitable for rotating shafts to prevent them from unscrewing due to rotational forces acting on them. |
| Heating and plumbing | Sometimes used to provide extra defense against leaks. |
Uses and applications of right-handed fastener threads
Right-handed threads are the most common type of thread used in fasteners and have a wide range of applications. It’s impossible to list everything here, so here are some of the most common industrial uses and applications of right-handed thread:
Right-handed threads
| General fastening | From simple screws and bolts to more complex machinery and equipment. |
|---|---|
| Repair and maintenance | An example includes replacing a worn-out bolt or screw in a piece of machinery. Readily available in a variety of sizes and materials, making them easy to source and replace when needed. |
| Automotive | Ensures that parts remain securely fastened together, even under high stress and extreme conditions. |
| Electrical and electronics | Secures components, such as circuit boards and connectors. Ensures that components remain securely in place and depending on material, can help provide a secure electrical connection or electrical insulation. |
| Construction | From simple drywall screws to more complex structural applications. Used to ensure that components remain securely fastened together, even under heavy loads and vibrations. |
| Medical | From fasteners used in orthopedic surgeries and surgical tools to medical devices and equipment. |
The differences between right-handed and left-handed fastener threads
Their main difference is their direction of rotation. Remember, when viewed from the end of the fastener, a right-handed thread rotates clockwise to tighten and anti-clockwise to loosen.
A left-handed thread rotates anti-clockwise to tighten and clockwise to loosen. The differences between the two types of threads include:
Direction of torque
Right-handed threads have a clockwise direction of torque, while left-handed threads have an anti-clockwise direction of torque. This means that they are designed to resist loosening in different directions.
Compatibility
They cannot be used interchangeably and are not compatible threads. They can't match up and fastener thread engagement will fail.
Applications
Right-handed thread is more common and is used in most fastener applications. Left-handed thread is less common and is used in specific applications to prevent loosening due to torque.
Identification
Manufacturers will sometimes mark their left-handed fastener thread sizes. For example, in production drawings, a standard metric right-handed thread might be identified as M8 x 1.25, while a left-handed thread might be identified as M8 x 1.25LH, or 1.25L. Sometimes they’ll even mark the head with an “L”.
Left-handed fastener thread sizes in the Unified Thread Standard (UTS) can be identified in the same way. To further identify whether or not the fastener is fine or coarse thread, left-handed UTS threads are also identified by the UNC or UNF thread series. Designations on UTD fastener thread measurements are followed by the letters "LH". For example: 1/4-20 UNC-LH or 5/16-24 UNF-LH.
Some manufacturers, whether of metric or UTS fastener thread sizes, use arrows to indicate left-handed thread. In these instances, the arrow will typically appear either on the surface of the head or on the crest of the thread end.
Marking is important. It prevents accidental removal or damage by maintenance personnel who might assume all threads are right-handed.
You can learn more in our guide, What’s the difference between metric and standard fasteners?
Cost
Manufacturing left-handed threads requires special tools and processes, which can make these threads more expensive to produce than right-handed threads.
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You might also find it helpful to read our guide, What are screw thread sizes?
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