Every machine that vibrates is slowly trying to unscrew itself. Threadlocker is how you stop it, and the strength you pick decides how easy that fastener is to get off later.
Pick too weak, and the bolt backs out. Pick too strong, and the next tech spends an afternoon with a heat gun. This guide covers each strength grade, how to match one to your fastener, and how to apply and remove it correctly.
The Quick Answer
| If your fastener is... | Use this grade | Comes off with |
|---|---|---|
| Small (1/4 in and under) or in soft metal | Low strength (often purple) | Hand tools |
| Taken apart during routine maintenance | Medium strength (often blue) | Hand tools |
| Never supposed to come apart | High strength (often red) | Local heat + hand tools |
| Already assembled | Wicking grade (often green) | Hand tools, sometimes heat |
When in doubt, choose the lowest strength that will hold the joint. You can always step up; stepping down means a fight at the next service.
What Is an Anaerobic Threadlocker?
It's a one-part liquid adhesive, usually methacrylate-based, that locks and seals threaded metal fasteners. "Anaerobic" simply means it cures without air.
Here's how it works:
- In the bottle: oxygen keeps it liquid.
- Between tight threads: oxygen gets shut out.
- On the metal: ions on the thread surface kick off the cure, turning the liquid into a tough, solid plastic.
The metal matters more than most people expect. In peer-reviewed curing research, the same adhesive cured faster and more completely on copper than on iron. That's why some metals need an activator (more on that below).
Why Not Just Use a Lock Washer?
A tightened bolt only touches its nut along part of each thread, which leaves tiny gaps. Side-to-side vibration uses those gaps to rotate the fastener loose, a mechanism Gerhard Junker's research identified in the late 1960s.
Threadlocker fills the gaps, so the threads have no room to move. It also seals out moisture and corrosion, which a lock washer can't do.
Threadlocker Strength Grades, Explained
Strength comes down to one thing: how much torque it takes to break the cured joint loose. Here's what each grade is for, plus the options Perigee stocks.
Low Strength: For Small and Delicate Fasteners
- Best for: set screws, adjustment screws, and small fasteners, typically 1/4 in and under.
- Why it exists: On soft metals like aluminum and brass, a stronger grade can strip the threads or shear the screw during removal.
- Removal: standard hand tools.
Medium Strength: The Everyday Workhorse
This is the general-purpose, removable grade and the most common pick in industrial maintenance. It holds against vibration but still comes apart with hand tools.
- Loctite 242: removable, medium-strength threadlocker for fasteners up to 3/4 in.
- Permabond MM115: non-drip medium-strength threadlocker; handling strength in about 10 minutes, full strength in 24 hours on M10 steel.
- Permabond MM115 PURE: clear version of MM115, certified to NSF/ANSI 61 for drinking water systems.
High Strength: For Set-It-and-Forget-It Joints
Use it on fasteners that should never loosen in normal service, like studs and heavy machinery mounts. Removal generally takes localized heat, around 250°C (482°F), plus hand tools.
Size tip: bigger threads mean bigger gaps, so larger fasteners need a thicker (higher-viscosity) product that fills the space and stays put.
- Permabond HM128: permanent, general-purpose, thin (500 cP); replaces lock washers on bolts, studs, and cap screws.
- Permabond HM129: permanent, high strength, for fasteners up to 1 in.
- Permabond HM118: non-drip, high strength, for vibrating machinery and off-road equipment.
- Permabond HH120: thick (7,000 cP), for coarse threads, fills gaps up to 0.25 mm.
- Permabond HH131: high strength and high-temperature resistance, for fasteners up to 1-1/2 in.
- Loctite 277: thick, high strength, for large bolts and studs, M25 (1 in) and up.
Wicking Grade: For Fasteners Already in Place
Wicking grades are thin enough to seep into assembled threads on their own, by capillary action. No teardown needed.
- Loctite 290: very thin (20–55 cP), medium-to-high strength; also seals porosity in welds, castings, and powdered-metal parts.
Heads Up: Color Isn't a Spec
Color is a habit, not a rule. Permabond's anaerobic selector guide lists a red threadlocker that's low strength, and MM115 PURE is clear. Check the strength and size range on the data sheet, not the bottle.
How to Choose: 5 Questions to Ask
Run through these before you grab a bottle. Each answer narrows the field.
- How big is the fastener? Small screws (1/4 in and under) point to low strength. Large bolts and studs (1 in and up) require a thick, high-strength grade, such as Loctite 277 or Permabond HH120.
- Will anyone need to take it apart? If a tech will remove it in the field without heat, stay at medium strength, like Loctite 242 or Permabond MM115. If it should never move, go with a high-strength option, like Permabond HM128 or HM129.
- What metal is it? Stainless steel, aluminum, and plated fasteners cure slowly and may require an activator (see next section).
- How hot does it run? For elevated service temperatures, consider a high-temperature grade like Permabond HH131 and confirm the max in the TDS.
- Is it already assembled? Use a wicking grade like Loctite 290 instead of taking it apart.
Two special cases worth knowing:
- Drinking water systems: use an NSF/ANSI 61 certified grade, such as Permabond MM115 PURE.
- Vertical or overhead fasteners: non-drip (thixotropic) grades stay where you put them.
Stainless, Aluminum, or Plated? You May Need an Activator
Threadlocker needs metal ions to cure, and some metals give off more than others.
- Active metals (cure easily): steel, iron, copper, brass.
- Passive metals (cure slowly or incompletely): stainless steel, aluminum, and zinc- or cadmium-plated fasteners.
An activator gives passive surfaces a head start. It also helps with larger gaps and cold shops, generally below about 5°C (41°F).
- Permabond ASC10: fast-drying activator for plated, passivated stainless, and other inactive surfaces; parts can be prepped up to 30 days ahead.
- Loctite SF 7649: primer and accelerator for Loctite threadlockers; speeds set-up and increases breakaway torque on most fastener surfaces.
Pair the activator with the threadlocker brand it's made for, and let it dry before assembly.
What about plastic? Anaerobic threadlockers generally aren't recommended for plastics, especially thermoplastics that can stress-crack. Check the TDS and test on a real part first.
How to Apply Threadlocker in 6 Steps
Most threadlocker failures come from dirty threads or product in the wrong spot, not the wrong chemistry.
- Clean the threads. Remove oil, grease, and cutting fluid from both male and female threads with a cleaning solvent. Let them dry.
- Activate if needed. Passive metal, big gap, or cold shop? Apply the activator and let it dry.
- Through-holes: put a few drops on the bolt where the nut will land, not on the tip.
- Blind holes: put drops down the inside threads, toward the bottom. Per Henkel's anaerobic adhesive guide, coating only the bolt lets trapped air push the liquid out as you tighten.
- Assemble and torque to spec. Keep the bottle tip off the metal, or the product can cure in the nozzle.
- Give it time. Most grades handle in minutes and reach full strength in about 24 hours at room temperature. Passive metals and cold slow that down.
One more tip: never pour unused product back into the bottle. Contamination shortens its shelf life.
How to Remove a Threadlocked Fastener
How hard this is depends entirely on the grade you picked.
- Low- and medium-strength: standard hand tools.
- High strength: heat the nut or bolt locally to roughly 250°C (482°F), then remove it while it's still hot.
- Wicking grade: try hand tools first; add heat if it won't budge.
When heat is involved, wear heat-resistant gloves and eye protection, and work in a ventilated area.
Before you reapply, clear out the old cured threadlocker. Permabond's training guide flags this as an important step; a solvent soak plus a wire brush usually does it. For more cleanup tips, see Removing Cured & Uncured Epoxy & Glues.
Working From a Spec Callout?
If a drawing or manual names a specific grade, match the callout, not the color.
- ASTM D5363 classifies one-part anaerobic adhesives and replaced the older MIL-S-22473, MIL-S-46163, and MIL-R-46082 specs. ASTM notes it's a classification system, not a design standard.
- Many products list their callouts on the data sheet. Permabond HH120 lists MIL-S-46163A Type I Grade L, and Permabond HM118 lists ASTM D5363 AN0331.
Safety note: most anaerobic threadlockers aren't recommended for pure-oxygen or oxygen-rich systems, or as sealants for chlorine and other strong oxidizers. Check the current SDS before use.
Continue Learning with Perigee Direct
Choosing a threadlocker is one piece of a broader question about how to lock, seal, and bond industrial assemblies. For more on related chemistries, explore these guides from the Perigee Direct blog:
- How to Choose the Right Two-Part Epoxy Adhesive for Industry
- Best Adhesive for Plastic: What Actually Works and Why
Shop the full range of threadlockers, including Loctite 242, Permabond MM115, and Loctite 290, plus primers and accelerators for passive metals, at Perigee Direct.