Threadlocker Strength Grades Explained: Low, Medium and High

What a threadlocker does

A threadlocker is an anaerobic adhesive applied to threaded fasteners. It cures in the absence of air between close-fitting metal threads, locking the fastener against vibration and sealing the thread against leakage. It is not a glue for general assembly: it is designed for threads, and it works best on clean metal threads with a close fit.

Threadlockers are usually described in three broad strength bands: low, medium and high. These are general descriptions, not specifications for any particular product code. The exact strength, temperature range and removability of a product depend on its code, so confirm the code's published description before use.

Low strength: adjustment and small fasteners

Low strength threadlockers suit small fasteners, adjustment screws and instruments that must be re-adjusted or removed with hand tools. They resist vibration without making the fastener difficult to remove, which matters for calibration screws, small electronics fasteners and optical mounts.

Low strength products are also used where the fastener or the part would be damaged by removal force. If a screw must come out regularly for maintenance, a low strength product is usually the safer choice.

Medium strength: the general purpose band

Medium strength threadlockers are the general purpose choice for most machinery fasteners. They hold against vibration and can still be removed with hand tools, usually with some extra force. This band covers a large share of maintenance and assembly work on motors, gearboxes, pumps and vehicles.

For a maintenance team, medium strength is often the default stock item, with low and high strength products kept for special cases. That keeps the shelf simple and reduces the risk of using the wrong grade on a serviceable joint.

High strength: permanent joints

High strength threadlockers are for joints that should not come apart in service: studs, structural fasteners and safety-critical assemblies. Removing a high strength joint usually needs heat or special tooling, and sometimes the fastener is sacrificed.

Do not use a high strength product on a fastener that will need routine removal. The cost of a seized fastener, a broken bolt or a damaged thread is far higher than the cost of choosing the right grade at assembly.

When not to use a threadlocker

Threadlockers need clean metal threads and a close fit. They do not fill large gaps, they do not bond plastics reliably, and they do not replace a mechanical lock where one is specified. Contaminated threads — oil, rust, plating residues — reduce curing and holding power, so clean the threads first.

Plated fasteners, stainless steel and inactive metals can cure more slowly with anaerobic products. Some products need an activator or primer on such surfaces. Confirm the combination — product code, fastener material and plating — before assembly, especially for production runs.

How anaerobic curing works

Anaerobic adhesives cure when air is excluded and metal ions are present. Between close-fitting metal threads, air is squeezed out and the metal surface triggers curing. On its own in a bottle, with air present, the adhesive stays liquid — which is why it has a shelf life and why an opened bottle eventually thickens.

Gap size matters. Anaerobic products cure fastest in small gaps and more slowly in large ones. A loose thread, a worn nut or a large clearance slows curing and weakens the lock. For large gaps, a higher viscosity grade or a different locking method may be needed.

Inactive metals — stainless steel, plated surfaces, aluminium and some alloys — cure more slowly because they supply fewer metal ions. Activators or primers speed curing on such surfaces. If your fasteners are stainless or plated, confirm the combination before a production run.

Temperature and chemical resistance

Threadlockers have temperature limits that belong to the exact product code. General purpose grades suit ordinary machinery temperatures; high temperature grades exist for engines and exhaust-adjacent parts. Exceeding the limit softens the adhesive and the joint loosens.

Chemical exposure also matters. Oils, fuels, coolants and cleaning solvents attack some grades and not others. A threadlocker that holds in a dry gearbox may fail in a fuel-wetted joint. State the fluids the joint will meet when asking for a product recommendation.

Removal and service

Removal method depends on strength. Low and medium strength joints usually come apart with hand tools. High strength joints may need localised heat to soften the adhesive, plus hand tools. Heat must be applied carefully near seals, plastics and wiring.

After removal, clean the threads before reassembly. Old adhesive residue blocks curing of the new application and changes the torque-tension relationship, which matters for torque-critical fasteners.

Torque, preload and locking

A threadlocker changes the torque-tension relationship of a fastener: a lubricated thread reaches a given preload at a lower torque than a dry thread. For torque-critical joints, the assembly specification should state whether torque values assume a threadlocker, and which one.

A threadlocker is not a substitute for correct preload. It prevents loosening from vibration; it does not create clamping force. Under-torqued fasteners still fail, locked or not.

Choosing by fastener size

Fastener size guides the grade. Small screws — below about 6 millimetres — usually suit low strength products, because removal force must stay low to avoid breaking the screw. Medium fasteners suit medium strength products. Large studs and structural bolts suit high strength products, where the joint is not expected to come apart.

These are rules of thumb, not specifications. The product code's published description governs, and a small fastener in a safety-critical joint may still need a higher grade. When in doubt, describe the fastener size, material and service interval in your inquiry.

Thread engagement also matters. A short engagement gives the adhesive less area to work on, so a higher grade may be needed for the same holding power. Conversely, a long engagement with a high strength product can make removal extremely difficult.

Production versus maintenance use

Production assembly and maintenance repair use threadlockers differently. Production lines need consistent dispensing, predictable cure and a grade that matches the assembly torque specification. Maintenance teams need a grade that holds in service but still allows disassembly with the tools at hand.

A common maintenance mistake is reaching for the strongest product on the shelf. On a serviceable joint, that choice creates the next failure: a seized fastener, a broken bolt or a stripped thread during the next service. Stock the grades by application, label them clearly, and train fitters on which grade goes where.

For production, validate the grade on the actual fastener, material and plating, cured under production conditions and tested to the required torque and vibration standard. A grade that works on a test bench may behave differently on a plated production fastener.

A note on product selection

Guides on this site describe product types in general terms. They do not promise that any particular product suits a particular job. Before ordering, match the product code, the manufacturer's published description and your application's requirements, and confirm anything not stated in writing with the supplier.