Product details
| Grade | Loctite 638 |
| Strength | High strength |
| Colour | Green |
| Family | Retaining compounds |
| Brand | Loctite (Henkel) |
| Stock | Held in Dubai — buy one or bulk |
Pack sizes
Prices on request. Tell us the quantity you need.
Check stockWhat Loctite 638 is for
The high-strength retaining compound — it bonds cylindrical parts together so a worn or slipping fit holds again. At 29 N/mm² compressive shear it is the strongest of the common grades, it fills gaps approaching 0.25 mm, and it reaches 13.5 N/mm² in fifteen minutes. It cures on passive substrates including stainless and plated surfaces, and tolerates the cutting and anti-corrosion oils left on a shaft.
Key features
- High strength — 29 N/mm² on steel, 28 on stainless, 17 on aluminium
- 13.5 N/mm² after just 15 minutes
- Gaps approaching 0.25 mm — rescues a worn housing or shaft
- Cures on stainless steel and plated surfaces without activation
- Tolerates cutting, lubricating and anti-corrosion oils
- Gains strength in service: 165% in motor oil at 125°C, 145% in water/glycol
- Tested to GASTEC QA AR-214 for hydrogen service (a regional approval)
- Cold strength tested to −75°C
Technical data
Uncured material
| Technology | Acrylic |
| Chemical type | Urethane methacrylate |
| Appearance | Green liquid |
| Fluorescence | Positive under UV light |
| Components | One component — no mixing |
| Cure | Anaerobic; secondary cure by activator |
| Viscosity, Brookfield RVT 25°C | Spindle 3 at 20 rpm: 2,500 mPa·s |
| Viscosity, cone & plate 25°C | 2,500 mPa·s at 129 s⁻¹ |
| Viscosity class | High |
| Specific gravity at 23°C | 1.1 |
Compressive shear strength (ISO 10123)
Cured at 23°C.
| Substrate and cure | N/mm² (psi) |
|---|
| Steel pins and collars, 15 minutes | 13.5 (1,960) |
| Steel pins and collars, 72 hours | 29 (4,200) |
| Stainless steel pins and collars, 72 hours | 28 (4,000) |
| Aluminium pins and collars, 72 hours | 17 (2,710) |
Torque performance
Cured 72 hours at 23°C, ISO 10964. 638 can also be used for high-strength threadlocking and thread sealing.
| Fastener | Breakaway N·m (lb·in) | Prevail N·m (lb·in) |
|---|
| M10 black oxide bolts, mild steel nuts | 57 (510) | 22 (200) |
| 3/8 × 16 steel nuts and bolts | 25 (220) | 9.4 (85) |
| 3/8 × 16, pre-torqued to 5 N·m | 23 (210) | 12 (110) |
Chemical resistance — steel
Strength rises above its starting value in every one of these.
| Environment | °C | 500 h | 1,000 h | 3,000 h | 5,000 h |
|---|
| Motor oil (5W40 synthetic) | 125 | 175% | 165% | 165% | 165% |
| Water / glycol 50:50 | 87 | 145% | 145% | 145% | 145% |
| Brake fluid | 23 | 120% | 115% | 115% | 115% |
| B100 bio-diesel | 23 | 115% | 115% | 115% | 115% |
| DEF (AdBlue®) | 23 | 115% | 105% | 105% | 105% |
| Ethanol | 23 | 110% | 110% | 100% | 100% |
| Unleaded petrol | 23 | 105% | 105% | 105% | 105% |
| Acetone | 23 | 105% | 105% | 105% | 105% |
Chemical resistance — stainless steel
Phosphoric acid is the one environment where 638 falls away significantly.
| Environment | °C | 500 h | 1,000 h | 3,000 h | 5,000 h |
|---|
| Sodium hydroxide 20% | 23 | 115% | 105% | 95% | 90% |
| Phosphoric acid 10% | 23 | 75% | 60% | 40% | 35% |
Cured material
| Glass transition temperature (ISO 11359-2) | 76°C |
| Coefficient of thermal expansion, below Tg | 96 × 10⁻⁶ K⁻¹ |
| Coefficient of thermal expansion, above Tg | 192 × 10⁻⁶ K⁻¹ |
| Cold strength | Tested to −75°C |
How to use it
- Clean all surfaces, external and internal, with a Loctite cleaning solvent and allow to dry.
- On inactive metals, or where large gaps are present, use an activator and allow to dry.
- Slip fits: apply around the leading edge of the male part and the inside of the female part, and use a rotating motion during assembly to spread it.
- Press fits: apply thoroughly to both surfaces and assemble at high press-on rates.
- Shrink fits: coat to a smooth even film. Heating the female part — coat the male. Cooling the male part — coat the female. Doing both — apply to the cooled part, and avoid condensation on it.
- Do not disturb the parts until handling strength is reached.
Handling and storage
- Not recommended for pure oxygen or oxygen-rich systems.
- Not a sealant for chlorine or other strong oxidising materials.
- Not normally recommended on plastics — thermoplastics can stress-crack.
- Disassembly: hand tools first; if needed apply localised heat to about 250°C and take apart while hot. If that temperature is not possible, heat as far as you can and use mechanical aids.
- Store unopened and dry, ideally 8°C to 21°C; outside 8–28°C the properties can be affected.
- Do not return decanted product to the original container.
Which retaining compound?
638 is the high-strength, gap-filling default: worn housings, slipping bearings, anywhere the fit has opened up. Use a low-viscosity grade such as 601 or 603 for tight slip fits under about 0.1 mm, where a thick product cannot enter the joint. Use 641 where the bearing has to come off again for regular servicing — it is deliberately medium strength. Use 620 where the assembly runs hot.
Always read the current technical and safety data before use —
ask us and we'll send it with your quotation.
