Overview: Lock nuts are fasteners designed to stay tight under vibration and stress. Two common types are nylon-insert (Nylock) nuts and all-metal prevailing-torque nuts. This report compares these types in terms of their materials, how they lock, environmental performance (temperature and vibration), reusability, cost, and sourcing factors. The goal is to give sourcing managers a clear technical comparison and help guide component selection in different applications.

Material Composition: Nylon Insert vs All-Metal

• Nylock nuts are typically made of steel or brass with a polymer (nylon) insert molded into one end of the nut. The nut’s body provides strength, while the nylon ring provides locking action. The metal portion is usually carbon steel (often zinc-plated) or stainless steel for corrosion resistance, and the nylon insert is usually a nylon or other polymer material.
• All-metal lock nuts (sometimes called prevailing-torque nuts) are made entirely of metal. Common materials include carbon steel, stainless steel, or alloy steel. They have no polymer parts. The locking function comes from a deliberate deformation or serration in the metal threads or body. All-metal nuts may include types like distorted-thread hex nuts or spring-type slotted nuts.

Key difference: Nylock nuts combine metal and polymer (nylon), while all-metal lock nuts rely solely on metal construction. This affects their performance under heat, reusability, and environmental exposure.

Locking Mechanisms Explained

• Nylock (nylon-insert) nuts use a deformed nylon collar that presses against the bolt threads. When the nut is tightened, the bolt cuts slightly into the nylon, creating high friction and “prevailing torque.” The nylon insert essentially grips the threads and resists loosening. This locking action is largely independent of the bolt’s clamping force – even if the assembly is vibrating, the nylon’s friction holds the nut tight.
• All-metal lock nuts achieve locking by metal-to-metal interference. Common designs include:
  • Distorted-thread nuts where the top threads are slightly oval or out-of-round, causing a spring action that grips the bolt.
  • Split or slotted nuts where a cut or slot makes the top threads act like a spring or clip, squeezing the bolt threads.
  • Serrated flange nuts (a subtype) use serrations under the flange to bite into the mating surface, preventing rotation.
  • In all cases, the metal deformation (elastic strain in the threads or body) generates friction.

Key difference: Nylock nuts rely on a polymer deforming to lock, while all-metal nuts rely on the elastic deformation of metal. Nylock tends to provide a consistent grip initially (until the nylon wears), whereas all-metal nuts depend on the metal spring action each time. All-metal nuts may require less torque to start turning (once installed) than Nylocks, but both create a prevailing torque to resist vibration.

Temperature and Environmental Resistance

• Temperature Range: Nylock nuts are limited by the nylon insert’s thermal properties. Typical nylon inserts lose effectiveness or melt above about 80–120°C (176–250°F). Special high-temperature inserts (e.g. using Vespel or other polymers) can extend this limit to roughly 350°F (177°C), but even these are much lower than metal’s limits. In contrast, all-metal lock nuts can tolerate much higher temperatures, well above 300°C (572°F) for common steels, and higher for high-grade alloys. This makes all-metal nuts the choice for high-temperature environments.
• Low-temperature behavior: Nylon can become brittle in very cold conditions (below -40°C), which may reduce its locking grip or cause cracking. All-metal nuts do not suffer this effect (metal remains ductile at low temperatures) and can be used in cryogenic or very cold environments without loss of function.
• Environmental exposure: The nylon in Nylock nuts can degrade under prolonged UV exposure, extreme humidity, or salt spray. Over long periods outdoors, nylon can absorb moisture or weaken, potentially reducing the lock force. All-metal nuts (especially stainless steel) are generally more resistant to UV, moisture, and chemical exposure. However, any metal nut must be matched to the environment: for example, stainless steel nuts (nylock or all-metal) are preferred for corrosion resistance in marine or outdoor applications.
• Chemical resistance: Nylon is attacked by certain chemicals (strong acids, bases, some solvents), so Nylocks should be avoided in chemically harsh environments unless specified. All-metal nuts only need to consider corrosion resistance of the metal (e.g. stainless grade).

Key difference: All-metal lock nuts greatly outlast nylon-insert nuts in extreme temperatures and harsh environments. Nylock nuts are suitable for ordinary temperature ranges (below ~250°F/120°C) but are not recommended for continuous high-heat or certain corrosive exposures that can degrade the nylon.

Reusability and Lifespan

• Nylock nuts: The locking action of a Nylock nut depends on the integrity of the nylon insert. Each time the nut is tightened and removed, the nylon is slightly deformed and worn. In practice, many engineers recommend using a new Nylock nut after a few assembly cycles to maintain specified locking performance. In non-critical applications, Nylocks may be reused several times (often 3–5 cycles) if the insert still exerts sufficient grip. However, the prevailing torque (resistance to turning) will gradually decrease with each use. After many uses, a Nylock nut will behave much like a regular nut, losing its locking benefit. For safety-critical or high-vibration applications, it is common to replace Nylock nuts whenever the joint is disassembled to guarantee performance.
• All-metal lock nuts: These nuts can generally be reused many more times than nylon-insert nuts. The metal thread deformation that creates the locking action tends to spring back each time, so an all-metal nut can be installed and removed repeatedly without dramatically losing its ability to apply prevailing torque—provided the threads (of both nut and bolt) are not damaged. Over very many uses, the metal interference can also wear down, and plating or thread coatings may be rubbed off, which slightly reduces friction. In most normal applications, however, all-metal nuts remain functional through dozens or even hundreds of assembly cycles. Some high-reliability specifications still call for replacing certain locknuts after a set number of uses to ensure safety, but this is much more a critical maintenance practice than a general limitation.
• Thread wear and galling: A noted issue with all-metal lock nuts is that the metal-to-metal friction can potentially wear the bolt threads over time, especially if the threads are dissimilar metals or poorly lubricated. Nylon, by contrast, tends to cut into the nylon rather than the bolt, leaving the bolt threads intact. In corrosive or galling-prone combinations (e.g. titanium bolt with steel nut), a nylon insert can protect against galling. But in most industrial settings with proper material pairing, this is a minor concern.

Key difference: Nylock nuts have a more limited reusable life (a few uses in typical cases) because the nylon insert wears. All-metal nuts are typically reusable many more times, making them preferable for frequent disassembly. For critical joints, new lock nuts may be specified each time in either case.

Vibration Resistance and Performance

• Nylock nuts: These are widely used to resist loosening under vibration. The nylon insert provides a high coefficient of friction against the bolt threads, so even strong vibrations must overcome this friction before the nut can turn. Nylock nuts are effective in automotive, machinery, and equipment applications where vibration is present but temperatures and other conditions are moderate. The locking effect is largely independent of any lock washer or thread tension, so if the joint is properly torqued, the nylon’s grip persists even if the bolt stretches slightly under load.
• All-metal lock nuts: Prevailing-torque nuts also perform well under vibration. The metal interference provides a steady locking torque that resists rotation. In fact, all-metal lock nuts can offer more consistent performance in environments where temperature or chemical conditions might soften nylon. They are commonly used in heavy machinery, aerospace, and engines where both vibration and extreme conditions are factors. One limitation is that if the bolt threads and nut threads become lubricated (e.g. from oil or grease), the prevailing torque may be reduced somewhat, whereas a Nylock’s nylon insert tends to be less affected by lubrication.
• Comparison: In typical vibration scenarios (engines, motors, chassis), both types will generally keep a joint secure. Nylock nuts often yield higher initial resistance (require more torque to turn) due to the tight nylon grip, which is excellent for preventing self-loosening. All-metal nuts provide a somewhat lower but still significant prevailing torque. For very high-frequency or impact vibration, any lock nut should be chosen carefully, but both designs are considered reliable locking methods in general. Some engineers prefer all-metal nuts for extremely high vibration and temperature (e.g. turbocharger studs) because their performance is not diminished by heat. Nylocks are often adequate for moderate vibration and are popular for their simplicity.

Key difference: Both nut types resist vibration well, but Nylock nuts rely on polymer friction (best in standard conditions), while all-metal nuts rely on metal interference (better for harsh conditions). Sourcing managers should pair the nut type with the expected vibration severity and operational environment.

Cost and Procurement Factors

• Cost considerations: In large volumes, Nylock and all-metal lock nuts are both inexpensive commodity items. The cost difference between a standard Nylock nut and a comparable all-metal prevailing torque nut is usually small. Nylock nuts may have a slightly higher unit cost due to the additional manufacturing step of inserting the nylon. However, cost can depend more on material grade (stainless vs. plated carbon steel), size, and quantity than on the locking style. Bulk ordering and common sizes generally keep both types low-cost per piece. For high-end applications (aerospace-grade, specialty alloys, or very small metric sizes), costs can rise, but again the difference is driven by material and spec, not solely by Nylock vs all-metal.
• Specification and standards: Both Nylock and all-metal nuts come in standard sizes (metric ISO, UNC/UNF, etc.) and grades (e.g. grade 8 or 10 for steel, A2/A4 for stainless). There are industry standards (DIN, ISO, MIL, etc.) for lock nuts. Nylock nuts are often designated as “Nyloc” (a common brand name) or “nylon insert lock nut” in catalogs, whereas all-metal nuts may be called “prevailing torque nut,” “locknut,” or by specific design names (Stover nut, etc.). Sourcing managers should ensure the correct type is specified. For example, if an assembly drawing calls for an MS (mil-spec) lock nut, that usually means an all-metal prevailing torque nut rather than a polymer-insert.
• Availability: Both types are widely available from industrial suppliers. Nylock nuts are extremely common in automotive and consumer markets, so they can often be sourced from multiple vendors. All-metal prevailing torque nuts are also common in industrial and aerospace supply chains. Any reputable fastener supplier will carry both, though specialized variants (e.g. high-temperature all-metal nuts in exotic alloys, or high-performance nylon inserts) might require specialized vendors or higher lead times.
• Shelf life and inventory: Nylon inserts can last many years in storage, but extreme heat or UV in storage should be avoided. Metal nuts have virtually unlimited shelf life if kept dry. Packaging and handling are similar. Inventory planning might consider that Nylock nuts may need replacement over time in use, whereas all-metal nuts are more “one and done.”
• Regulatory and environmental factors: If there are regulations on materials (e.g. prohibition of certain plastics or polymers, or restrictions on materials in regulated industries), all-metal nuts may have an advantage as they avoid plastics altogether. Conversely, if an application mandates polymer components (for example, to avoid metal-to-metal contact), Nylock might be specified. In general, compliance issues are minimal for standard lock nuts.

Key difference: Cost and sourcing challenges are generally minimal for both. The decision often hinges on performance specs rather than price. However, procurement teams should verify exact part numbers and material grades (nylon insert vs prevailing torque style) to ensure the correct component is ordered, as the choice has significant functional implications.

Applications and Use-Case Comparison

Below are example scenarios to illustrate when a sourcing manager might choose a Nylock (nylon-insert) nut or an all-metal lock nut:

• Moderate-temperature, high-vibration assemblies: Nylock nuts are commonly used in automotive suspension components, consumer machinery, and general industrial equipment. For example, in car subframes or motor mount brackets, Nylocks provide secure fastening under vibration while assembly temperatures remain well below the nylon’s limit. All-metal nuts could also work here, but Nylocks are often preferred for their high initial lock and lower propensity to cause thread galling on service.
• High-temperature or continuous operation (exhaust systems, engines, HVAC): All-metal prevailing torque nuts are recommended. In an engine exhaust manifold or a furnace assembly, temperatures can exceed nylon limits. All-metal nuts will retain their locking action when heated, whereas Nylocks could fail or melt. For example, racing engine components or industrial turbines almost always use all-metal nuts (often in alloy steel or high-temp stainless) to withstand heat cycles.
• Frequent disassembly (maintenance-heavy systems): All-metal nuts may be favored for their greater reusability. In equipment that is routinely serviced, such as machinery that is taken apart for cleaning or inspection, using all-metal lock nuts avoids the need to replace the nut at every service. Nylocks in such scenarios might lock effectively at first but would need replacement after some cycles to ensure reliability.
• Sensitive or precision equipment (aerospace, medical devices): All-metal lock nuts are often mandated by specification (e.g. MIL-N-25027 for aircraft) because they avoid any material (polymer) that could creep or degrade under extreme conditions. For example, aircraft structural bolts and engine assemblies typically use metal lock nuts (often self-locking metal nuts per military specs) to ensure predictable behavior. Nylocks are rarely used in primary structural aerospace applications.
• Outdoor/marine environments: Corrosion resistance is a priority. Stainless steel Nylock nuts exist and are used (for example, in marine hardware below the waterline where moderate heat is not an issue), but care is taken that the nylon insert does not weather too quickly. All-metal stainless lock nuts are also common in marine settings, offering corrosion resistance and not being affected by sunlight. For example, stainless steel Nylocks are popular on boats for railing assemblies (mild vibration and splash exposure), whereas heavier structural joints on ships may use metal lock nuts.
• Budget or commodity projects: If the main concern is cost or availability and the environment is unchallenging (e.g. indoor equipment, static loads), either type can be chosen. Nylock nuts are popular because they provide good lock at a reasonable price. All-metal nuts might be chosen simply for standardization (if a plant wants to use one type for all lock-nutting needs).

Application Summary:

• Choose Nylock nuts when: operating temperature is below ~250°F (120°C), vibration is present, joint disassembly is infrequent or acceptable, and cost is a factor. Common in automotive, general machinery, appliances.
• Choose all-metal lock nuts when: high temperatures or chemical exposure are expected, reusability is important, or industry standards require them (e.g. aerospace, heavy industry). Also use them if nylon’s longevity under environment is a concern.

Conclusion

Both Nylock (nylon-insert) and all-metal lock nuts provide reliable anti-loosening performance, but they suit different needs. Nylock nuts combine a metal body with a polymer insert, yielding excellent vibration resistance at moderate temperatures and typically lower assembly torque. All-metal prevailing-torque nuts use only metal, offering superior high-temperature tolerance and durability over repeated use.

Key takeaways for procurement:

• Material & Mechanism: Nylock = metal + nylon (polymer), locking by nylon friction. All-metal = entirely metal, locking by deformed threads or serrations.
• Temperature: Nylocks limit ~120°C (250°F). All-metal nuts can handle much higher temperatures.
• Reusability: Nylocks lose locking force after a few uses (often treated as single-use in critical joints). All-metal nuts can be reused many times with less degradation, though extreme reuse can wear threads.
• Vibration: Both types resist vibration; Nylocks excel in everyday vibro environments, while all-metal handle extreme conditions (heat/vibration) without polymer concerns.
• Cost & Availability: Both are widely available commodity items; cost differences are small relative to performance needs. Verify specifications (metric/imperial, grade, insert vs prevailing type) carefully to meet application requirements.
• Applications: Use Nylocks in general machinery, vehicles, and consumer products where moderate conditions apply. Use all-metal nuts in high-heat, high-reliability, or service-intense applications (aerospace, industrial engines, etc.).

By matching lock nut type to the operating environment and maintenance regime, sourcing managers can ensure secure fastenings and avoid performance issues. In summary, specify Nylock nuts for general-purpose, cost-sensitive, or standard-vibration applications, and all-metal lock nuts when facing extreme temperatures, stringent reuse cycles, or regulated specifications.