At their invention, nuts and bolts were cut by hand. In 1568, Jacques Besson industrialized the process with a semi-automatic screw-cutting lathe. At first, there were no standards outside any single manufacturer — i.e., a Besson bolt would not fit a Hindley nut. Eventually, the first cross-threaded bolt forced the industry to standardize. Over 450 years later, cross-threaded nuts and bolts are still the bane of shops and garages everywhere.
What Is Cross-Threading?
The threads on a nut or bolt are essentially wedges wrapped around a cylinder. As a wedge forces something apart, the wrapped wedges pull things together, like a wheel and hub. The nut and bolt diameter and thread pitch need to match and thread together at the right angle, or they cross threads. Here are three ways that can happen:
- Mismatched Thread Pitch: Typical lug nuts and wheel studs, 12 millimeters by 1.25 inches or one-and-a-half inch by 20 millimeters, are not interchangeable. Mismatched pieces will cut new threads into each other.
- Wrong Insertion Angle: A nut or bolt needs to be threaded to its mate straight along the axis of rotation. Otherwise, the resulting interference will cut new threads along the new angle, damaging one or both pieces.
- Debris and Damage: Debris, such as rust or dirt, and damage, from impact or abrasion, can cause issues. The damaged or dirty portion of the threads will cut new threads in the opposite piece.
Cross-threading nuts and bolts is problematic. For example, cross-threaded wheel nuts won’t hold the proper torque, creating extra noise and vibration. At worst, it could cause a wheel stud to break or a wheel to fall off.
How to Prevent a Cross-Threaded Fastener
- Clean the threads. Clean fasteners inside and out. Most automotive torque specifications are based on dry assembly. Use a wire brush and spray solvent, then allow it to dry. At most, apply light engine oil to ease installation.
- Repair threads. Thread damage does not always require replacement. In many cases, threads can be repaired with special tools such as a thread file or tap and die set.
- Replace parts. Sometimes replacement is simpler than repair, such as with wheel studs and lug nuts, which don’t require special tools.
- Align the pieces. Before starting bolts in blind holes, align the pieces using a drift punch or dowel pins.
- Start by hand. Always start nuts and bolts by hand, threading them down using your fingers. Any difficulty indicates potential problems.
- Thread all the bolts. Some pieces, such as torque converters and some wheel hubs, use several bolts in blind-threaded holes. In this case, thread all bolts by hand until they’re flush. Tightening just one bolt may misalign the piece for the rest.
- Go backwards. Using your fingers, turn the fastener counterclockwise until you feel or hear a click, indicating the first thread has made contact. Then, turn the nut or bolt clockwise, engaging the threads and spinning it on by hand.
A Word on Power Tools
Power tools can speed up many service and repair operations, saving time, but they should never be used to start nuts and bolts. The power of an impact gun or cordless ratchet can easily overcome crossed threads, damaging the fastener. Save the assault ratchet for removing lug nuts, and use your fingers and a torque wrench to tighten your wheels.
Check out all the cutting and grinding tools available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on cross-threaded nuts and bolts, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.
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Source: NAPA Know How