Lock Nut: What It Is and How to Use It

What is a lock nut?

A lock nut is a type of nut that stays tight under vibration and repeated use. It includes a built-in locking feature—such as a nylon ring, deformed metal threads, serrations, or a thin “jam” profile—that increases resistance to turning. By adding this extra friction or a mechanical stop, lock nuts help prevent fasteners from backing off in projects that see movement, temperature changes, or regular handling.

How does it work?

Standard nuts rely on clean threads and proper torque to hold. Under vibration or alternating loads, they can slowly rotate loose. Lock nuts add an extra element that resists that rotation:

Nylon-insert nuts use a nylon ring that grips the bolt threads.
All-metal prevailing-torque nuts have slightly out-of-round or slotted top threads that pinch the bolt.
Serrated flange lock nuts bite into the surface beneath the nut to resist turning.
Jam nuts use two nuts tightened against each other to lock the joint.

Each style creates resistance so the nut is less likely to spin free once tightened to the correct clamping force.

Common DIY and home uses

Lock nuts show up across the house and yard wherever movement or vibration happens:

Outdoor furniture and playsets that shift as people sit or climb
Deck rail brackets and gate hardware that see repeated opening and closing
Lawn equipment and wheel assemblies (e.g., mower handles, cart axles)
Garage organization systems, shelving, and ladder racks
Appliances and fans where vibration is common
Bicycle and scooter accessories, trailer lights, and license plate brackets

They’re also handy for any project you don’t want to keep retightening throughout the season.

Types of lock nuts

Nylon-insert (nyloc) nuts: A metal nut with a nylon ring at the top. The nylon deforms over the bolt threads, adding friction. Common, affordable, and great for general use. Avoid high-heat areas because nylon softens around 250°F/120°C.
All-metal prevailing-torque nuts: Sometimes called stover, top-lock, or distorted-thread nuts. The top threads are intentionally out-of-round or slotted, so they grip the bolt. These handle heat and chemicals better than nylon-insert types.
Serrated flange lock nuts: A wide, built-in washer (flange) with serrations on the bearing surface. The serrations dig into the mating surface to resist rotation. Good for sheet metal and brackets; not ideal on soft or finished surfaces you don’t want marred.
Jam nuts: Thin nuts used in pairs. Tighten the main nut to clamp the joint, then tighten a jam nut against it to lock the threads. Useful where space is tight or when you want reusable locking without special parts.
Kep (keps) nuts: A nut with a captive, external-tooth lock washer. It speeds assembly and adds bite under the nut. Good for light-duty, indoor tasks.
Castellated nuts (with cotter pin): Not a friction lock nut, but often grouped with locking hardware. A slotted nut used with a drilled bolt and cotter pin to mechanically prevent rotation. Common on axles and pivot points.

Choosing the right lock nut

Environment: Use nylon-insert for indoor and outdoor general use. Choose all-metal for hot areas (engines, near exhaust, grills) or where chemicals attack plastics.
Surface protection: Pick smooth flange or standard nylon-insert nuts for painted or soft wood. Avoid serrated flanges on finished surfaces.
Strength and grade: Match the nut’s grade to the bolt. For steel bolts: Grade 2/5/8 (US) or 8.8/10.9/12.9 (metric) have matching nut grades. For stainless, 18-8 (A2) fits most home projects; A4 for marine/corrosive settings.
Size and thread pitch: Match diameter and pitch exactly (e.g., 1/4"-20 vs 1/4"-28, or M8-1.25 vs M8-1.0). Mixing coarse and fine threads will damage parts and won’t hold.
Reuse expectations: Need frequent disassembly? Consider jam nuts or all-metal types. Nylon-insert nuts lose grip after a few uses.

How to install a lock nut (step-by-step)

Verify size: Confirm the nut matches the bolt’s diameter and thread pitch. Test by threading a standard nut first if unsure.
Prepare the joint: Align parts, add washers as needed. Use a flat washer to protect wood or paint; use a lock washer only if the design calls for it.
Orient correctly: For nylon-insert nuts, the nylon ring faces away from the surface so it engages last. Serrated flanges face the mating surface. All-metal top-lock nuts are directional—install the distorted end away from the surface.
Start by hand: Spin the nut by hand several turns to avoid cross-threading. If it binds immediately, back off and realign.
Tighten: Use the correct wrench or socket. With prevailing-torque nuts, expect resistance before the nut seats. Apply final torque based on bolt size and material. A torque wrench is helpful if strength is critical.
Check thread protrusion: Aim for 2–3 full threads showing past the nut. If the bolt is too short, the locking feature may never engage.
Recheck after use: After the first few cycles of use or vibration, inspect and retorque if needed.

Maintenance and reuse

Nylon-insert nuts: Limit reuse. If the nylon looks worn, glossy, cut, or the nut spins on too easily, replace it.
All-metal lock nuts: Can be reused a few times, but replace in high-vibration or safety-critical spots.
Jam nuts: Reusable. If threads are damaged or rounded, replace both nuts.
Stainless fasteners: Use a small amount of anti-seize to prevent galling (thread binding), especially on larger sizes.

Common mistakes to avoid

Using nylon-insert nuts near heat sources (grills, engines, dryer vents). Choose all-metal instead.
Cross-threading by forcing the nut with a wrench from the start. Always begin by hand.
Mismatched threads (metric vs SAE, coarse vs fine). Double-check markings.
Too little thread engagement. Ensure 2–3 threads past the nut after tightening.
Over-torquing and stripping threads, especially in soft materials or with small bolts.
Relying on serrated flange nuts on painted or soft wood where the serrations will damage the surface.
Mixing stainless and plain steel outdoors without thought. Use compatible materials or coatings to reduce corrosion.

Related terms

Bolt, machine screw, washer, flat washer, lock washer
Thread pitch, coarse thread, fine thread
Threadlocker (liquid thread-locking compound)
Torque wrench, anti-seize, galling

Practical examples

Deck rail bracket: Use a stainless bolt with a nylon-insert lock nut and a flat washer to keep the bracket tight through seasonal movement.
Lawn cart axle: Choose an all-metal prevailing-torque lock nut where vibration and occasional heat are present. Ensure the nut doesn’t rub on the wheel hub.
Garage shelving: A serrated flange lock nut can speed assembly and resist loosening on steel framing, but add a flat washer if you want to protect painted surfaces and choose a smooth flange instead.
Kids’ playset: Nylon-insert nuts are great here. After the first week of use, recheck tightness and replace any nylock that feels loose during reassembly.

By picking the right style, matching size and grade, and installing with care, lock nuts give you a reliable way to keep projects tight without constant retightening.