You can lock a bolt in place using chemical threadlockers, locking washers, or mechanical pins, and the best method depends on whether the joint needs to stay permanent, resist vibration, or be disassembled later. Most bolted joints loosen over time because vibration gradually reduces the clamping force between the bolt and the mating surface. Each locking method addresses this problem differently.
Chemical Threadlockers
Threadlockers are liquid adhesives you apply to bolt threads before assembly. They fill the microscopic gaps between male and female threads and cure into a solid plastic once sealed from air. This bonds the threads together and prevents the bolt from rotating loose. Threadlockers are color-coded by strength, and picking the right one matters because it determines whether you can ever remove the bolt with basic tools.
Purple (low strength): The lightest option, designed for small fasteners and soft metals like aluminum and brass. Good for electronics enclosures, instrument screws, and any situation where you need easy disassembly with normal hand tools and minimal force.
Blue (medium strength): The most common choice for general maintenance. Blue threadlocker holds firmly against vibration but can still be disassembled with standard hand tools. It’s the go-to for automotive bolts, machinery fasteners, and anything you expect to service in the future.
Green (medium to high strength, wicking grade): Unlike other threadlockers, green is thin enough to wick into already-assembled joints by capillary action. You apply it around the exposed threads of a bolt that’s already tightened, and it draws itself into the gap. This makes it useful for locking set screws, electrical connectors, and preassembled fasteners you can’t easily take apart to apply adhesive. Removal requires heat plus hand tools.
Red (high strength): Essentially a permanent bond. Red threadlocker is meant for critical fasteners that should never come loose, like head bolts or structural connections. You cannot remove a red-locked bolt with hand tools alone. Disassembly requires applying localized heat above 250°C (550°F) to break down the adhesive before the bolt will turn. A propane or MAP gas torch aimed directly at the bolt for 30 to 60 seconds typically does the job.
All standard threadlockers resist temperatures from about -65°F to 300°F, with some high-performance formulas rated up to 650°F. To apply any threadlocker properly, clean the threads with a degreaser first, since oil and grease prevent curing. Apply a few drops around the bolt threads, then thread the bolt in and torque to spec. Full cure takes about 24 hours, though most products reach handling strength within an hour.
Locking Washers
Locking washers sit between the bolt head (or nut) and the joint surface, adding friction or mechanical resistance to keep the fastener from backing out. Not all locking washers perform equally, and the differences are dramatic under vibration.
Split Lock Washers
The helical split washer is the most common type you’ll find in hardware stores. It’s a single coil of hardened steel that compresses flat when the bolt is tightened. The theory is that the spring tension and sharp edges bite into the bolt head and the joint surface to resist rotation. In practice, split lock washers perform poorly. Vibration testing consistently shows they lose clamping force at roughly the same rate as a plain washer or no washer at all. They flatten permanently under load and stop providing any meaningful spring action. For low-vibration applications like furniture or light fixtures, they’re fine. For anything subject to real vibration, such as engines, heavy equipment, or structural steel, they’re unreliable.
Wedge-Locking Washers
Wedge-locking washers (sold under brand names like Nord-Lock) use a completely different principle. They come as a pair of washers with angled cams on one face and radial teeth on the other. The teeth grip the bolt head and joint surface, while the cam faces lock against each other. For the bolt to rotate loose, it would have to climb the cam angle, which actually increases clamping force instead of releasing it. In standardized vibration testing (the Junker test, which shakes bolted joints sideways to simulate real-world conditions), wedge-locking washers maintain nearly all their clamping force through thousands of cycles. Plain washers, split washers, nylon insert nuts, and even double nuts all show significant clamp loss under the same conditions. If vibration resistance is your priority and you don’t want to use a chemical threadlocker, wedge-locking washers are the most effective mechanical option.
Castle Nuts and Cotter Pins
A castle nut (also called a castellated nut) has slots cut into its top that align with a hole drilled through the bolt shank. Once the nut is tightened, you slide a cotter pin through the bolt hole and the nut slot, then bend the pin’s legs apart. The pin physically prevents the nut from rotating in either direction, regardless of vibration. This method is standard in safety-critical applications like wheel bearings, steering linkages, and aircraft components where a loose bolt could be catastrophic.
Getting this right means matching your cotter pin diameter to the hole in your bolt. The pin needs to slide through without excessive play. For a bolt with a 1/8-inch cross hole, you’d use a 1/8-inch nominal cotter pin, which has an actual shank diameter of about 0.120 inches and fits a gauge hole of 0.141 inches. For a 3/16-inch hole, the pin’s shank measures about 0.176 inches and fits a 0.203-inch gauge hole. Always size the pin to the hole, not to the bolt diameter.
Once the pin is through, bend one leg forward over the end of the bolt and the other back against the nut. Some specifications call for bending both legs to the same side, wrapping them around the nut. Either way, trim excess length so the bent legs don’t interfere with nearby components. Never reuse a cotter pin. The bending weakens the metal, and a used pin can snap during installation, leaving you with a false sense of security.
Choosing the Right Method
Your choice comes down to three factors: how much vibration the joint faces, whether you need to disassemble it later, and how critical the consequences of failure are.
- Low vibration, easy disassembly needed: Blue threadlocker or a nylon insert lock nut. Both hold adequately for general maintenance and come apart with basic tools.
- High vibration, periodic disassembly: Wedge-locking washers. They outperform every other reusable mechanical method in vibration testing and don’t require chemicals or special removal procedures.
- Permanent or semi-permanent joints: Red threadlocker. You’ll need a torch to remove it, but nothing short of a mechanical pin provides more security against loosening.
- Safety-critical, zero tolerance for failure: Castle nut with a cotter pin. The physical pin makes accidental loosening impossible. This is why it’s the standard for suspension, steering, and aviation fasteners.
For many home and shop projects, combining methods provides extra insurance. A bolt torqued to spec with blue threadlocker and a lock washer covers both chemical and mechanical retention. On the other end, a high-vibration industrial joint might use a wedge-locking washer paired with proper torque monitoring on a maintenance schedule. Match your approach to what’s actually at stake if the bolt comes loose.