A reverse thread, also called a left-hand thread, is a screw thread that tightens when turned counterclockwise instead of the usual clockwise direction. It’s the opposite of the standard right-hand thread found on most bolts, screws, and nuts. If you’ve ever heard the phrase “righty-tighty, lefty-loosey,” a reverse thread flips that rule entirely: lefty-tighty, righty-loosey.
How Reverse Threads Work
The vast majority of screws, bolts, and threaded components use right-hand threads. When you look at a right-hand threaded fastener from the top and rotate it clockwise, it drives forward and tightens. Turn it counterclockwise, and it loosens. This has been the universal default in engineering for so long that most people never think about it.
A reverse thread spirals in the opposite direction. The grooves wind the other way, so the fastener advances when turned counterclockwise and backs out when turned clockwise. On engineering drawings, left-hand threads are typically marked with the letters “LH” to make sure nobody tries to force them the wrong way.
You can sometimes identify a reverse thread by looking at the angle of the thread ridges. Right-hand threads slope upward to the right, while left-hand threads slope upward to the left. This visual cue is subtle but reliable once you know to look for it.
Why Reverse Threads Exist
Reverse threads aren’t just a curiosity. They solve specific mechanical problems that standard threads can’t handle. The most common reason is preventing a fastener from loosening during rotation. When a component spins in one direction during normal use, vibration and rotational forces can gradually unscrew a standard threaded fastener. A reverse thread resists those forces because the rotation that would loosen a standard bolt actually tightens a left-hand one.
This principle, sometimes called self-loosening, was famously studied by a German engineer named Junker in 1969, who demonstrated that cyclic vibration could cause bolted joints to lose their clamping force rapidly. In rotating machinery, choosing the correct thread direction for the direction of spin is one of the simplest ways to prevent that from happening.
Common Places You’ll Find Reverse Threads
Bicycle Pedals
The left pedal on every bicycle uses a left-hand thread. As you pedal, the rotation of the crank would gradually unscrew a standard right-hand thread on that side. The reverse thread ensures the pedal stays tight during riding. The right pedal uses a normal right-hand thread for the same reason: the rotation on that side naturally tightens a standard thread. If you’re removing the left pedal, remember to turn the wrench clockwise to loosen it.
Power Tool Blades and Arbor Nuts
Circular saws, miter saws, and angle grinders often use reverse-threaded arbor nuts or locking nuts. The general principle is straightforward: the nut holding the blade or disc in place threads in the opposite direction of the blade’s spin. That way, the spinning force pushes the nut tighter rather than loosening it during use. On some saws, this means the blade nut is left-hand threaded. Always check before forcing a nut that won’t budge, because you may be turning it the wrong way.
Gas Cylinder Connections
Fittings for flammable gas cylinders (like acetylene or propane) use left-hand threads as a safety measure. This prevents someone from accidentally connecting a flammable gas regulator to an oxygen or inert gas cylinder, which could be catastrophic. The Compressed Gas Association sets standards for these connections, and the mismatched threads act as a physical safeguard. If a regulator won’t screw onto a tank, that’s by design: the threads are telling you the connection is wrong.
Turnbuckles
A turnbuckle uses one left-hand thread and one right-hand thread on opposite ends of its body. When you rotate the central barrel, both end pieces move toward each other (tightening) or away from each other (loosening) simultaneously. This lets you adjust the tension on a cable, rope, or rod without twisting the attached lines. It’s a clever use of opposing threads that shows up in everything from fence tensioners to aircraft rigging.
Plumbing Unions
When two sections of pipe are fixed in place and can’t be rotated, a connecting piece with both left-hand and right-hand threads lets a plumber join them. Turning the fitting screws it into both pipe ends at once, solving the problem of threading into something that can’t spin.
Older Vehicle Lug Nuts
Before 1975, Chrysler vehicles in the United States used left-hand threaded lug nuts on one side of the car and right-hand threaded nuts on the other. The idea was that wheel rotation would tighten the nuts rather than loosen them. Most Buicks, Pontiacs, and Oldsmobiles did the same thing prior to 1965. Engineers eventually realized that the tapered seat design on modern lug nuts accomplished the same goal, so virtually all vehicles today use right-hand threads on every wheel. But if you’re working on a classic car, check before you wrench.
How to Tell If a Fastener Is Reverse Threaded
Look for markings first. Many left-hand threaded bolts and nuts have a notch, groove, or the letters “LH” stamped on them. Gas fittings for flammable gases often have a notched hex nut as a visual indicator.
If there are no markings, examine the thread angle. Hold the fastener horizontally and look at the direction the threads spiral. If they rise to the left, it’s a left-hand thread. If they rise to the right, it’s standard.
Context is also a strong clue. If a fastener is on the left pedal of a bike, the arbor of a circular saw, or a propane fitting, assume it’s reverse threaded until you confirm otherwise. Forcing a reverse-threaded fastener in the wrong direction is one of the fastest ways to strip threads or round off a nut, and it happens constantly to people who don’t know to check.
Working With Reverse Threads
The most important thing is simply knowing they exist. Once you’re aware that a fastener might be left-hand threaded, you’ll pause and check before applying force in the wrong direction. Beyond that, reverse threads follow the same rules as standard threads: they need proper torque, they benefit from anti-seize compound in corrosive environments, and they can strip or cross-thread just like anything else.
When buying replacement fasteners, always confirm the thread direction. A right-hand bolt will not work in a left-hand threaded hole, no matter how hard you try. Hardware stores carry left-hand threaded bolts, nuts, and taps, but they’re specialty items and may need to be ordered. The thread pitch and diameter must also match, just as with any threaded connection.