Lock Nut Comparison: Nylock vs Stover vs All-Metal
There is no single “lock nut” — there are several types, each resisting loosening a different way, with real tradeoffs in temperature limit, reusability, and cost. This guide compares the common ones: nylon-insert (nylock), all-metal prevailing-torque (often called a Stover nut), serrated flange, K-lock (Keps), and the humble jam nut, so you can pick the right lock for the joint.
Side-by-Side Comparison
| Type | How it locks | Temp limit | Reuse | Best for |
|---|---|---|---|---|
| Nylon-insert (Nylock) | Nylon collar grips bolt threads | ~250°F | A few times | General assembly, moderate vibration |
| All-metal prevailing-torque (Stover) | Distorted top thread grips by friction | High (all steel) | Good | Heat, heavy-duty, reuse |
| Serrated flange | Teeth on flange bite the surface | High | Marks surface | Built-in washer + bite, soft/thin materials |
| K-lock (Keps) | Captive star washer bites surface | High | Marks surface | Fast electrical / sheet-metal |
| Jam nut (double-nut) | Two nuts wedge on the threads | High | Yes | Setting position, locking a primary nut |
Nylon-Insert (Nylock)
A nylon collar inside the top of the nut deforms around the bolt threads, creating friction that resists back-off. It is the everyday lock nut — inexpensive, effective against moderate vibration, and re-usable a few times before the nylon wears. The catch is heat: above roughly 250°F the nylon softens and the lock degrades, so it is not for engines, exhaust, or other hot spots.
All-Metal Prevailing-Torque (Stover)
A one-piece steel nut with a slightly distorted top thread that grips the bolt by metal-to-metal friction — no nylon. Commonly called a Stover nut, it is the choice for high temperature, heavy-duty service, and applications that need more reuse cycles than a nylock allows. For Grade 8 / Grade 9 bolt assemblies, use the Grade 9 all-metal version. Flexloc collet-style nuts are a related all-metal option for high-vibration and aerospace-type work.
Serrated Flange & K-Lock (Keps)
Both lock by biting the bearing surface. A serrated flange nut combines a hex nut, a built-in washer face, and teeth that dig in — it spreads load and locks in one part, good on soft or thin materials. A K-lock (Keps) nut has a free-spinning captive star washer that speeds electrical and sheet-metal assembly. Both mar the surface, so they are not for finished faces.
Jam Nut (Double-Nut Method)
Not a dedicated lock nut, but a thin nut used to lock a primary nut or set a position on threaded rod. Thread on both nuts and tighten them against each other so they wedge on the threads. See our hex jam nuts for sizes and grades.
Which Lock Nut Should You Use?
- General assembly, may remove later: nylon-insert (nylock).
- Heat (over ~250°F), heavy-duty, or frequent reuse: all-metal Stover (Grade 9 for high-strength bolts).
- Soft/thin material or want a built-in washer: serrated flange.
- Fast electrical / panel assembly: K-lock (Keps).
- Set a position or back up another nut: jam nut.
- Safety-critical, must not loosen and must stay serviceable: a castle/slotted nut with a cotter pin — see that comparison.
Frequently Asked Questions
What is a Stover nut?
A Stover nut is an all-metal prevailing-torque lock nut — a one-piece steel nut with a distorted top thread that grips the bolt by friction, with no nylon insert. It is used where heat, heavy load, or reuse rule out a nylon-insert nut.
Nylock vs all-metal lock nut: which is better?
Neither is universally better. Nylon-insert is cheaper and fine for general, moderate-temperature assembly. All-metal (Stover) handles high heat and heavy-duty reuse where nylon would fail. Choose by temperature and duty: under ~250°F and general use, nylock; hot or heavy, all-metal.
What temperature can a nylon-insert lock nut handle?
About 250°F (roughly 120°C). Above that the nylon softens and the locking grip degrades. Use an all-metal prevailing-torque or Grade 9 lock nut for higher temperatures.
Can lock nuts be reused?
Nylon-insert nuts can be reused a few times but lose grip as the nylon wears. All-metal Stover nuts tolerate more cycles. Serrated flange and K-lock nuts mar the surface, so reuse is fine but the bite is best on first install. For safety-critical joints, install a fresh lock nut.
What is the strongest way to keep a nut from loosening?
For a positive, non-friction lock that cannot back off, use a castle or slotted nut with a cotter pin through the bolt. Among friction-type lock nuts, all-metal prevailing-torque (Stover) and serrated flange give the most reliable hold under vibration.
Need Lock Nuts?
Eugene Fastener stocks nylock, all-metal (Stover), Grade 9, serrated flange, K-lock, and Flexloc lock nuts in steel, stainless, and brass.