Thread locking and thread sealing are not the same thing — they solve different problems with different products, and using the wrong one for the job either fails to work or actively causes problems. A threadlocker prevents a fastener loosening under vibration; it does not seal a fluid leak. A thread sealant prevents leakage through a threaded connection; it is not designed to resist fastener back-out under vibration.
This guide covers the full range of anaerobic adhesive products used in industrial and maintenance applications: Loctite threadlockers by colour and strength, thread sealants for pipe and hydraulic fittings, PTFE tape selection including the critical white vs yellow distinction for gas applications, gasket eliminators for flanged joints, and retaining compounds for cylindrical assemblies. For each product type it explains what it does, when to use it, and the common failure modes that come from using the wrong product or applying it incorrectly.
Threadlockers vs thread sealants — what each one does
The distinction matters because the two product families are not interchangeable, and each has incompatible applications.
Threadlockers are anaerobic adhesives applied to bolt and screw threads to prevent loosening from vibration, thermal cycling, and dynamic loads. They cure in the absence of oxygen and the presence of metal ions, filling the microscopic gaps between mating threads and creating a bond that resists rotation. They do not reliably seal fluids or gases — they are not formulated for that purpose and most thread forms leave sufficient helical path for fluid migration even with a threadlocker in place.
Thread sealants are anaerobic or PTFE-based products applied to tapered or parallel pipe threads to prevent fluid or gas leakage through the threaded joint. They fill the thread form completely, sealing the leak path. They also provide some resistance to loosening but are not the correct primary choice when vibration resistance is the main requirement.
Gasket eliminators are anaerobic flange sealants applied to machined flat faces in place of cut gaskets. They work on a different joint geometry — flat face to flat face — rather than thread-to-thread.
Retaining compounds are high-strength anaerobic adhesives for bonding close-fitting cylindrical parts: bearings to housings, bushings to bores, sleeves to shafts. They are applied to the cylindrical mating surfaces, not threads.
Loctite threadlocker colour guide
Loctite threadlockers are colour-coded by strength. The colour system is consistent across the Loctite range and is the fastest way to select the right product for an application.
Purple — Loctite 222 (Low strength)
The lowest-strength threadlocker in the Loctite range. Suitable for small fasteners M2 to M8 and fine-thread screws — instruments, electronics, optical equipment, set screws, adjustment screws. Fasteners can be removed with standard hand tools. The low-strength formulation means it will not over-constrain small or fragile fasteners and will not damage plastic housings during removal. Also used as a temporary thread sealant for components that are frequently serviced.
Blue — Loctite 243 (Medium strength)
The most widely used threadlocker in industrial and maintenance applications globally. Medium strength — designed for fasteners M6 to M36 that may need to be disassembled for servicing. Fasteners can be removed with standard hand tools after cure. Loctite 243 (the current formulation, which replaced 242) is oil-tolerant — it can be applied to lightly oiled threads without requiring pre-cleaning in most cases — and is primerless on most metals. It resists vibration, prevents corrosion in the threaded joint, and seals the thread against moisture ingress. The correct default choice for most bolted joints in machinery, equipment, and vehicles where future disassembly is anticipated.
Red — Loctite 263 / 271 (High strength)
High-strength threadlocker for fasteners M6 to M36 where permanent or near-permanent retention is required. Fasteners cannot be removed with hand tools after full cure — removal requires localised heat (approximately 250°C) applied with a heat gun or torch to break the adhesive bond before attempting to turn the fastener. Loctite 263 is the standard high-strength formulation; Loctite 271 is functionally equivalent. Use red for stud installations, wheel bolts, engine mounting studs, and any fastener that should never work loose and is acceptable to disassemble only with heat. Do not use red on fasteners in assemblies where hand-tool disassembly will be required for maintenance.
Green — Loctite 290 (Wicking grade)
Loctite 290 is a low-viscosity wicking grade threadlocker with a unique application method: it is applied after assembly, not before. The very low viscosity allows it to wick into the thread engagement by capillary action. Use Loctite 290 when you cannot disassemble a joint for conventional threadlocker pre-application — for example, to lock pre-installed studs, to retrofit-lock bolts on equipment in service, or to treat assembled fasteners found to be working loose. Medium-to-high strength; not removable with hand tools.
Loctite 242 vs 243 — what changed and which to use
This is one of the most consistently asked questions in technical forums, and it has a straightforward answer: use 243. Loctite 243 is the direct upgraded replacement for 242. Both are medium-strength blue threadlockers, but 243 improved on 242 in two specific ways:
- Oil tolerance: Loctite 243 can be applied to lightly oiled fasteners without requiring solvent cleaning first. Loctite 242 is not reliably oil-tolerant and requires clean, dry threads for consistent cure.
- Primerless performance: Loctite 243 cures without primer on most active metals. Loctite 242 requires primer on some metal types to achieve consistent cure.
Loctite 242 is still available and still works on clean, active metal threads — it was not a bad product. If you have 242 on the shelf and the threads are clean steel, it will perform adequately. For new purchases, specify 243 — it is strictly better in everyday conditions and there is no performance trade-off on applications where 242 would have worked. The break-away torque of 243 is marginally higher (approximately 10–15%), which is not a meaningful practical difference.
When to use each threadlocker strength
The correct strength is determined by two factors: the fastener size and whether the joint needs to be disassembled for maintenance.
| Strength | Product | Fastener size | Removal method | Typical applications |
|---|---|---|---|---|
| Low | Loctite 222 (Purple) | M2–M8, fine-pitch screws | Hand tools | Instrument screws, set screws, adjustment points, electronics |
| Medium | Loctite 243 (Blue) | M6–M36, standard fasteners | Hand tools | General machinery, equipment, vehicles — any fastener requiring future servicing |
| High | Loctite 263 / 271 (Red) | M6–M36, large fasteners M20+ | Heat then hand tools | Studs, permanent installations, high-vibration without disassembly |
| Wicking | Loctite 290 (Green) | Any assembled fastener | Not removable by hand | Post-assembly locking, retrofit locking, assembled studs |
For large fasteners (M20 and above): Blue 243 may not develop adequate break-away torque on large-diameter, coarse-thread fasteners — the thread engagement area is large and the adhesive bond per unit area is the same as for small fasteners, but the leverage available to remove it is much greater. For very large fasteners where vibration resistance is the primary concern, Loctite 272 (medium-high strength, rated to 150°C) or 263/271 red is the more reliable choice. Loctite also offers 248 and 268 in stick form for applications where liquid is impractical.
Threadlocker on stainless steel and passive metals
Anaerobic adhesives cure via two conditions: absence of oxygen and the presence of metal ions. Most common metals — mild steel, cast iron, copper, brass — release sufficient ions to trigger curing reliably. A specific class of metals does not: passive metals, where the surface has a natural or applied oxide layer that suppresses ion release.
The following metals produce significantly delayed or reduced-strength cure with standard anaerobic threadlockers applied without primer:
Stainless steel — all grades | Zinc plating (galvanised fasteners) | Chrome plating | Anodised aluminium | Black oxide coating | Cadmium plating | Passivated titanium
The current Loctite 243 formulation is rated primerless on passive metals — meaning it will cure without activator — but cure is substantially slower (48–72 hours rather than 24) and initial bond strength is lower than on active metals. For non-critical joints assembled at room temperature, this may be acceptable. For critical joints, cold-workshop conditions (below 15°C), or where you need reliable cure within 24 hours, activator is the correct call.
Recommended: Apply Loctite 7649 (Primer N) or Loctite 7471 (Primer T) to both mating surfaces before applying the threadlocker. Allow to dry (30–60 seconds), then apply threadlocker as normal. The activator pre-treats the passive surface to trigger reliable, rapid curing — the cost is trivial relative to a joint failure.
This is the most common cause of threadlocker failures in stainless steel fastener applications and is responsible for most of the complaints about Loctite "not working" on stainless. It is not a quality issue — it is a surface chemistry issue with a simple fix.
Removing Loctite threadlocker
Blue (243) and purple (222): Standard hand tools after full cure. If the fastener is stuck beyond hand-tool torque (which can happen if blue was applied heavily to a small fastener or the engagement is very long), brief application of heat (100–150°C) softens the adhesive sufficiently for removal without damaging the fastener or component.
Red (263/271) and green (290): Heat is required. Apply localised heat of approximately 250°C to the fastener head or the joint using a heat gun or gas torch. Allow the metal to reach temperature, then attempt removal with a wrench while the adhesive is still hot. Do not let it cool before attempting to turn — the window of reduced adhesive strength is while it is hot. On aluminium housings, be careful not to exceed the alloy's limits (most aluminium alloys begin to lose strength above 150–200°C). In aluminium assemblies, impact tools applied to a slightly warmed fastener are often more effective than high heat.
Wicking grade (290): Treat as high-strength for removal purposes.
Loctite thread sealants — 567, 577, and 569
Thread sealants are applied to the male threads of pipe and hydraulic fittings to prevent leakage of fluids and gases through the threaded joint. Unlike PTFE tape, anaerobic thread sealants cure to a solid within the thread, giving a defined joint that can be disassembled and resealed without residue from the first application degrading the new seal. They also provide thread locking as a secondary function — vibration will not loosen a joint sealed with anaerobic sealant. For a dedicated guide to Loctite 577, including cure times, pressure ratings and application technique, see our Loctite 577 Thread Sealant Guide.
Loctite 567 — standard metal pipe threads
Medium-strength anaerobic thread sealant for metal pipe threads up to ¾ inch BSP. Seals against pressures up to 690 bar on close-fitting metal threads. Suitable for hydraulic and pneumatic systems. Resists petroleum oils, fuels, and many process fluids. Full cure at room temperature in 24 hours; handling strength for low-pressure applications in 1 hour. Can be disassembled with standard hand tools after cure — the joint can be re-sealed after cleaning. The standard choice for metal-to-metal tapered BSP fittings in hydraulic and pneumatic systems.
Loctite 577 — wider thread fittings, higher pressure
Medium-to-high strength anaerobic thread sealant rated for pipe threads up to 2-inch BSP. Seals pressures to 690 bar like Loctite 567 but is formulated for larger bore fittings where 567 may not fully fill the larger thread gaps. Also more viscous than 567, which helps it stay in place on vertical or overhead assemblies. Use 577 for larger fittings (¾ inch and above) and wherever higher viscosity is beneficial. Many engineers default to 577 for all industrial anaerobic thread sealing as it covers the 567 application range as well.
Loctite 569 — plastic-compatible, wide-gap thread sealant
Loctite 569 is formulated for sealing parallel (non-tapered) threads including plastic fittings and is safe for use on a wider range of polymers than standard anaerobic products. It also handles larger gap sizes than 567/577. Suitable for water, gas, and low-pressure hydraulic applications on brass and plastic fittings. Not appropriate for very high-pressure hydraulic systems.
PTFE tape — white, yellow, and Loctite 55
PTFE (polytetrafluoroethylene) thread seal tape — commonly called Teflon tape or plumber's tape — is the simplest and most widely used thread sealing method for pipe connections. It is inert, chemically compatible with most fluids and gases, easy to apply, and available everywhere. The critical variable that is consistently misunderstood is that not all PTFE tape is suitable for all applications.
White PTFE tape — water and low-pressure applications only
Standard white PTFE tape (typically 12 mm wide, 0.075 mm thick) is designed for water plumbing applications — cold and hot water, low-pressure air, and general-purpose thread sealing on non-gas connections. It is the most common tape on shelves at hardware stores. White tape is thin, tears easily, and has no resistance to hydrocarbons — it degrades on contact with petroleum-based fuels, oils, and gases.
Yellow PTFE tape — gas applications (AU compliance)
Yellow PTFE tape is the correct and, in Australia, legally required tape for natural gas and LPG pipe connections. Australian Standard AS/NZS 5601.1:2022 (gas installations) permits only gas-rated PTFE tape or AGA-approved thread compounds on gas fittings — white plumber's tape is explicitly not permitted on gas connections.
Using white plumber's PTFE tape on natural gas or LPG connections is a compliance breach under AS/NZS 5601.1:2022 in Australia and can void your insurance if a gas leak occurs.
White tape is thinner (0.075 mm vs approximately 0.1 mm for yellow gas tape), has no hydrocarbon resistance, and shears under the torque of gas fitting assembly, sending fragments downstream. Yellow gas-rated PTFE tape is denser, hydrocarbon-resistant, and rated for natural gas, LPG, and propane service. On any gas connection — residential, commercial, or industrial — use yellow gas-rated PTFE tape only.
Yellow tape is applied using the same technique as white tape but typically requires fewer turns (3–4 vs 4–5 for white) because it is denser and fills the thread form more effectively.
Applying PTFE tape correctly
Correct PTFE tape application is straightforward but has four non-negotiable rules:
- Wrap clockwise — looking at the male thread end-on (the direction you will turn the fitting during assembly). Wrapping anti-clockwise causes the tape to unwind as the fitting is tightened, leaving the thread bare.
- Start one thread back from the end — leave the first thread at the tip of the fitting bare. Starting tape at the very end results in tape entering the system when the fitting is fully made up.
- 3–5 turns for white tape; 3–4 turns for yellow gas tape. Keep the tape taut as you wrap. More wraps do not give a better seal — excessive tape prevents the fitting from achieving full thread engagement and can crack plastic fittings.
- Never use PTFE tape on hydraulic fittings. Tape fragments entering hydraulic fluid damage valves, pumps, and actuators. Hydraulic fittings seal on the seat (flare or face), not the thread. Use anaerobic sealant only where thread sealing is needed on hydraulic port threads.
Loctite 55 — PTFE sealing cord
Loctite 55 is a PTFE-impregnated sealing cord (a wound fibre with a PTFE-saturated core) applied in place of PTFE tape. Compared to tape it is stronger, more resistant to being cut by sharp metal thread peaks, and does not require attention to wrapping direction. It handles pressures up to 60 bar for gas and 80 bar for water and compressed air. Application: wrap the cord tightly around the male threads (3–6 turns depending on thread size), keep tension on the cord, and cut flush at the end. Unlike PTFE tape, the direction of wrap is less critical as the cord's structure means it does not unwind. Loctite 55 is increasingly used in professional plumbing and gas installation as a more robust alternative to PTFE tape.
PTFE tape vs anaerobic sealant: decision guide by application
Both product types seal pipe threads. The choice between them is application-specific:
| Application | Correct sealant | Reason |
|---|---|---|
| Cold and hot water plumbing (domestic) | White PTFE tape or Loctite 55 | Simple, clean, serviceable |
| Natural gas and LPG connections | Yellow gas-rated PTFE tape or AGA-approved compound | AS/NZS 5601.1 compliance; white tape not permitted |
| Compressed air lines (low pressure) | PTFE tape, Loctite 55, or Loctite 567 | Any of the three works; 567 gives vibration resistance |
| Hydraulic fittings (BSP, JIC) | Loctite 567 or 577 (on port threads only) | No tape on hydraulic systems — contamination risk |
| High-pressure pneumatic (>40 bar) | Loctite 567 or 577 | Tape may extrude at high pressure; anaerobic is retained |
| Petroleum fuels and oils | Loctite 567 (check chemical compatibility) | White PTFE tape degrades; yellow tape may be acceptable |
| Plastic fittings | PTFE tape or Loctite 569 | Standard anaerobics attack some polymers; 569 is plastic-safe |
| Stainless fittings with anaerobic sealant | Loctite 567 + activator 7649 | Passive metal — activator required for anaerobic cure |
Gasket eliminators — Loctite 515 vs 518
Gasket eliminators are anaerobic flange sealants applied to machined flat faces in place of a cut gasket. They fill the microscopic surface imperfections on machined faces and cure to a tough, resilient seal. The joint can be disassembled, the faces cleaned, and the sealant reapplied — the joint is serviceable, unlike a welded or permanently bonded assembly.
Gasket eliminators are appropriate for rigid metal-to-metal flanged joints: gearbox covers, pump bodies, engine sump covers, valve bodies, transmission housings. They are not appropriate for rough-machined or cast faces with deep surface irregularities, or for joints involving rubber, paper, or composite gasket materials — anaerobic adhesives do not bond well to these surfaces.
Loctite 515 — rigid flanges, standard gap
Loctite 515 is a medium-strength anaerobic flange sealant for close-fitting rigid metal flanges with surface gaps up to 0.25 mm. It remains slightly flexible after cure, which accommodates minor deflection in the flanged joint. It is not fully oil-tolerant — heavily contaminated faces should be cleaned with Loctite 7063 or acetone before application. Suitable for pump bodies, compressor housings, gearbox covers, and transmission casings. Squeeze-out that is not trapped between the faces remains liquid and washes away with the system fluid — it does not cure without the confined metal-to-metal contact.
Loctite 518 — aluminium flanges, wider gaps
Loctite 518 is the preferred choice for aluminium flanges and for joint faces with gaps up to 0.5 mm (slightly larger than 515 can handle). It cures faster than 515 and is formulated specifically to perform well on aluminium surfaces, where standard anaerobics can cure slowly. Loctite 518 is widely used in engine and gearbox assembly for aluminium components. Less forgiving of heavy oil contamination than 515 — clean aluminium faces thoroughly before applying 518.
| Property | Loctite 515 | Loctite 518 |
|---|---|---|
| Maximum gap fill | 0.25 mm | 0.5 mm |
| Best for | Steel and iron flanges, pump bodies, gearboxes | Aluminium flanges, engine/gearbox covers |
| Oil tolerance | Moderate — clean heavily oiled surfaces | Lower — clean thoroughly before applying |
| Cure speed | Moderate | Faster |
| Handling strength | 1–2 hours | 30–60 minutes |
| Full cure | 24 hours | 24 hours |
Retaining compounds — Loctite 638 and 648
Retaining compounds are high-strength anaerobic adhesives designed for cylindrical assemblies — bonding bearings to housings, bushings to bores, and sleeves to shafts. They are a distinct product family from threadlockers, but work on the same anaerobic curing principle. Retaining compounds fill the microscopic clearance between two concentric metal surfaces and cure to a bond stronger than conventional mechanical interference fits. They eliminate fretting corrosion (micro-movement between surfaces that creates abrasive oxide debris) and can restore worn bores to service without machining.
Loctite 638 — high-strength retaining (standard clearance)
Loctite 638 is the standard general-purpose retaining compound for cylindrical assemblies with diametrical clearances up to 0.25 mm. High strength — not typically removable without heat after full cure. Correct for new-build assemblies, bearing installation in housings, and bushing-to-bore retention where the design intent is a permanent bond. Apply a thin, even film to one or both mating surfaces; assemble immediately; allow to cure with the assembly in position. Use Loctite 7649 activator on stainless or passive metal surfaces.
Loctite 648 — high-temperature retaining (heat-resistant)
Loctite 648 is formulated for the same cylindrical bonding applications as 638 but retains its bond strength at elevated temperatures — rated to 200°C vs 150°C for standard 638. Correct for electric motor bearing seats, engine-related components, and any assembly that will see sustained high operating temperatures. Also more resistant to solvents and process chemicals at temperature. If the assembly will be exposed to heat cycles above 150°C, 648 is the correct product.
Retaining compound vs interference fit: A Loctite-bonded assembly should be designed with a clearance fit (H7/g6 or looser), not an interference fit — the adhesive provides the retention, and interference fit plus adhesive can cause hydraulic lock during assembly and component distortion. Standard practice is to specify an H7/g6 or H7/f7 clearance fit for retaining compound applications.
Application by use case
| Application | Problem to solve | Correct product | Notes |
|---|---|---|---|
| General machinery bolts | Vibration loosening | Loctite 243 (Blue) | Oil tolerant; use activator on stainless |
| Small instrument screws | Vibration loosening on small fasteners | Loctite 222 (Purple) | Low strength; safe for plastic housings |
| Stud installation | Permanent stud retention | Loctite 263 / 271 (Red) | Heat to remove; activator on stainless |
| Already-assembled loose bolts | Retrofit locking without disassembly | Loctite 290 (Green — wicking) | Applied post-assembly; wicks into threads |
| BSP pipe fittings (hydraulic) | Fluid leakage through threaded joint | Loctite 567 (small bore) or 577 (large bore) | Not PTFE tape on hydraulics |
| Water plumbing fittings | Water leakage | White PTFE tape or Loctite 55 | Simple and serviceable |
| Gas pipe fittings | Gas leakage | Yellow gas-rated PTFE tape or AGA compound | AS/NZS 5601.1 — white tape not compliant |
| Gearbox cover (steel) | Oil seepage through flange face | Loctite 515 | Gap <0.25 mm; clean faces first |
| Aluminium engine/gearbox covers | Oil seepage through flange face | Loctite 518 | Gap up to 0.5 mm; clean aluminium faces thoroughly |
| Bearing-to-housing fit | Outer race spinning, fretting | Loctite 638 | Clearance fit H7/g6; activator on stainless |
| Bearing retention at high temperature | Outer race spinning in hot application | Loctite 648 | Rated to 200°C; correct for motor bearing seats |
| Stainless steel fasteners | Vibration loosening — passive metal cure issue | Loctite 243 + Activator 7649 | Activator strongly recommended; 243 primerless per current TDS but cures slower on passive metals |
Common mistakes
Using PTFE tape on hydraulic fittings. This is the most consequential error in this product category. PTFE tape fragments in hydraulic fluid damage valves, seals, and pumps. Hydraulic fittings seal on the seat, not the thread. The correct product for BSP hydraulic port threads where sealing is needed is Loctite 567 or 577 — not tape.
Using white PTFE tape on gas connections. White tape is not gas-rated and is non-compliant with AS/NZS 5601.1 in Australia. Use yellow gas-rated tape on all gas connections. This is both a safety and insurance issue.
Applying threadlocker to oily threads without considering oil tolerance. While Loctite 243 handles lightly oiled threads, it is not formulated to cure through heavy oil, grease, or anti-seize contamination. Clean heavily contaminated threads with Loctite 7063 or acetone before applying threadlocker. Do not apply threadlocker over anti-seize — these two products are mutually exclusive on the same joint.
Applying red Loctite to fasteners that will need disassembly. Red threadlocker creates a near-permanent bond. If you need to service the fastener later and did not plan for heat-tool removal, you have a problem. Establish a policy: blue for anything that will be serviced; red only for studs and permanent installations.
Not using activator on passive metal fasteners. Stainless steel, zinc-plated, anodised aluminium, and chrome-plated fasteners cure anaerobic adhesives significantly more slowly than active metals, and bond strength without activator is reduced. The current Loctite 243 formulation is rated primerless on passive metals, but for any critical joint, cold-workshop application (below 15°C), or where full 24-hour cure is required, activator is the correct choice. Apply Loctite 7649 (Primer N) to both mating surfaces, allow to dry (30–60 seconds), then apply adhesive. Skipping activator on passive metals in demanding conditions is a leading cause of threadlocker underperformance.
Applying threadlocker and then leaving the joint un-assembled. Anaerobic adhesives begin skinning on exposure to air. Apply adhesive immediately before assembly. If you apply it and then delay assembly, the surface skin that forms inhibits further curing and reduces bond strength significantly.
Using gasket eliminator on unmachined or rough-cast faces. Gasket eliminators (515, 518) require machined surfaces — the faces must be flat and smooth enough for the thin film of adhesive to bridge the gap consistently. On rough-cast or poorly finished faces, use a cut gasket or a formed-in-place silicone gasket sealant instead.
Frequently asked questions
What is the difference between blue and red Loctite?
Blue Loctite (243) is medium-strength — fasteners can be removed with standard hand tools after cure. Use blue for any fastener you may need to disassemble for servicing. Red Loctite (263/271) is high-strength and creates a near-permanent bond — removal requires heating the fastener to approximately 250°C with a heat gun or torch to break the adhesive before applying hand tools. Use red for stud installations, permanent fasteners, and high-vibration applications where disassembly is not anticipated.
What is Loctite 243 used for?
Loctite 243 is the standard medium-strength threadlocker for metric fasteners M6 to M36. It prevents loosening from vibration and thermal cycling, seals the thread against moisture and corrosion, and is the correct default choice for most industrial and maintenance bolted joints. It is oil-tolerant and primerless on most active metals, which makes it practical in field applications where fasteners cannot always be cleaned before assembly. It is the most widely used threadlocker in Australian industry.
What is the difference between Loctite 242 and Loctite 243?
Loctite 243 is the improved replacement for 242. Both are medium-strength blue threadlockers, but 243 adds oil tolerance (it works on lightly oiled fasteners without pre-cleaning, which 242 does not) and primerless performance on a broader range of metals. For new purchases, always specify 243. If you have 242 on the shelf and are working with clean, active metal threads, it will work. On oily threads or where primerless performance is needed, 243 is the correct choice.
What does green Loctite (290) do?
Loctite 290 is a wicking grade threadlocker with very low viscosity — it is applied after the fastener is already assembled and wicks into the thread engagement by capillary action. Use it when disassembly for pre-application of threadlocker is not practical: to lock already-installed studs, to retrofit-lock a joint found to be loosening in service, or to treat a batch of assembled fasteners on equipment that is running. Medium-to-high strength after cure — not removable by hand.
What is the difference between Loctite 567 and 577?
Both are anaerobic pipe thread sealants, but they target different fitting sizes. Loctite 567 is optimised for metal pipe threads up to ¾ inch BSP — lower viscosity, easier to apply on small fittings. Loctite 577 is formulated for fittings up to 2-inch BSP — higher viscosity handles the larger thread gap of bigger fittings and stays in place better on vertical assemblies. Both seal to 690 bar. Many engineers default to 577 for all sizes as it covers the 567 application range as well.
Can I use PTFE tape on hydraulic fittings?
No. PTFE tape must not be used on hydraulic fittings — tape fragments can enter the hydraulic fluid and cause damage to valves, pumps, and actuators. Hydraulic fittings (BSP, JIC, ORFS) seal on the seat or face, not on the thread, so tape on the threads does not improve sealing and creates contamination risk. Where thread sealing is required on BSP hydraulic port threads, use Loctite 567 or 577. Never use tape in hydraulic systems.
What is the difference between white and yellow PTFE tape?
White PTFE tape is for water plumbing and low-pressure applications. Yellow PTFE tape is gas-rated — it is denser, thicker, and chemically resistant to hydrocarbons (LPG, natural gas). In Australia, AS/NZS 5601.1:2022 (gas installations standard) only permits gas-rated PTFE tape or AGA-approved compounds on gas connections — white tape is explicitly not compliant. Using white tape on gas fittings is a safety risk, a compliance breach, and can void insurance. For any gas fitting, use yellow gas-rated tape only.
How do I apply PTFE tape correctly?
Wrap clockwise when looking end-on at the male thread (in the direction of assembly, so tightening does not unwind the tape). Start one thread back from the tip of the fitting — leave the first thread bare to prevent tape entering the system. Apply 3–5 turns for white tape, 3–4 turns for yellow gas tape, keeping the tape taut. Do not overwrap — too much tape prevents full thread engagement and can crack plastic fittings. The tape should deform into the thread profile as you wrap, not sit on top of it loosely.
What is a gasket eliminator and when should I use one?
A gasket eliminator (Loctite 515 or 518) is an anaerobic flange sealant applied to machined flat faces in place of a cut gasket. It fills micro-surface imperfections and cures to create a leak-free joint. Use it on gearbox covers, pump bodies, valve bodies, and engine components where the mating faces are machined flat. Loctite 515 suits steel and iron flanges with gaps up to 0.25 mm; Loctite 518 is the choice for aluminium flanges and gaps up to 0.5 mm. Not suitable for rough-machined or cast faces — the surfaces must be reasonably flat for the thin sealant film to span the gap consistently.
Does Loctite work on stainless steel?
Yes, but with an important qualification. The current Loctite 243 formulation is rated primerless on passive metals including stainless steel — meaning it will cure without activator — but cure is substantially slower (48–72 hours rather than 24) and bond strength is reduced compared to active metal surfaces. Stainless is passive because its surface oxide layer suppresses the metal ion release that triggers anaerobic curing. For non-critical joints at room temperature, this may be acceptable. For critical joints, any application below 15°C, or where you need reliable cure within 24 hours, activator is the correct choice. Apply Loctite 7649 (Primer N) or Loctite 7471 (Primer T) to both mating surfaces, allow to dry for 30–60 seconds, then apply threadlocker and assemble. This also applies to zinc plating, chrome, anodised aluminium, and black oxide — all passive surfaces benefit from activator in demanding conditions.
Can I use Loctite threadlocker on plastic threads?
Standard anaerobic Loctite products are designed for metal-to-metal applications and can attack some thermoplastics. For plastic threads, use Loctite 222 (purple, low-strength, low-viscosity) which is formulated to be safe for a broader range of thermoplastics than blue or red. Always check plastic compatibility before applying any anaerobic product to a plastic component — the Loctite compatibility guide lists specific plastic types. For plastic thread sealing rather than locking, PTFE tape is the safer and simpler choice.
How long does Loctite threadlocker take to cure?
Handling strength — sufficient to resist movement and light loads — is typically reached in 10–20 minutes at room temperature on active metals. Full cure strength develops in 24 hours. Curing is temperature-dependent: below 15°C, cure time extends significantly; above 25°C, it accelerates. On passive metals (stainless, zinc plate), curing without activator may not be complete even after 24 hours — use activator to ensure reliable, timely cure. Do not test or load the joint until handling strength is reached.
AIMS Industrial stocks Loctite threadlockers, thread sealants, gasket eliminators, retaining compounds, and PTFE tape including yellow gas-rated tape and Loctite 55 sealing cord. For product selection assistance or technical data sheets, contact our team.
