Left-Handed Threads

Left-handed threads are a less common thread direction used on certain fasteners and rotating components. With a left-hand thread, you tighten the fastener by turning it counterclockwise and loosen it by turning it clockwise. If you look at the fastener from the end, the thread “ramps” upward to the left as it moves away from you, which is why it’s called left-handed. Left-hand threads are often identified with an “LH” marking or a clear callout on drawings and specifications.

Left-handed threads are typically used when normal operation could cause a standard right-hand fastener to loosen due to rotation, vibration, or repeated motion. For example, they may be used on some rotating shafts, special nuts, certain mechanical linkages, and specific automotive or industrial components where the direction of rotation would tend to back off a right-hand thread. Because left-hand threads are nonstandard, it’s important to verify thread direction during maintenance and sourcing, and to ensure the mating nut, bolt, or tapped hole is also left-hand so the assembly is compatible.