Overview
LOCTITE® 638 is a high-strength, single-component retaining compound designed for bonding cylindrical fitting parts such as shafts, gears, pulleys, and bushings.
It cures in the absence of air between close-fitting metal surfaces, creating a strong, thermally stable bond that resists vibration, shock, and high loads.
With its excellent gap-filling ability (up to 0.25 mm) and tolerance to minor oil contamination, LOCTITE 638 is ideal for maintaining assembly integrity and preventing loosening or leakage under harsh operating conditions.
Key Features & Benefits
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High Strength – Provides exceptional holding power for rigid cylindrical assemblies.
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Temperature Resistant – Performs reliably from –55 °C to +180 °C.
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Gap Filling Capability – Bonds gaps up to 0.25 mm (0.010 in.).
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Fast Curing – Fixture time around 4 minutes on active metals.
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Oil Tolerant – Works on slightly contaminated surfaces.
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Universal Performance – Suitable for active and passive metals, including stainless steel and plated parts.
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Fluorescent – Visible under UV light for quality control.
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Certified Performance – Meets KIWA GASTEC QA AR 214 for gas system applications.
Typical Applications
LOCTITE 638 is ideal for:
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Mounting gears and sprockets on shafts
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Securing pulleys, bushings, and bearings
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Retaining rotors and sleeves in electric motors
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Fixing cylindrical assemblies in automotive and industrial equipment
Technical Data
| Property | Typical Value |
|---|---|
| Color / Appearance | Green |
| Technology | Anaerobic |
| Chemical Type | Dimethacrylate ester |
| Cure Type | Anaerobic (in the absence of air on metal surfaces) |
| Viscosity | 2,000 – 3,000 mPa·s |
| Fixture Time | 4 minutes (steel) |
| Full Cure | 24 hours |
| Gap Fill | Up to 0.25 mm |
| Shear Strength | ~ 31 N/mm² (≈ 4,500 psi) |
| Temperature Range | –55 °C to +180 °C |
| Service Environment | Oil, heat, and vibration resistant |
How to Use LOCTITE 638
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Prepare the surfaces
Clean mating parts with LOCTITE SF 7070 or equivalent solvent cleaner.
Remove old adhesive or contaminants. -
Apply the adhesive
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For slip fits: Apply to the leading edge of the shaft and inside bore; assemble with rotation.
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For press fits: Apply a 360° bead on both parts and press together quickly.
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For shrink fits: Apply to shaft; heat the outer part, assemble, and allow to cool.
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Cure & Handling
Fixture time is typically 4 minutes on steel; full cure after 24 hours.
On passive metals or in cold environments, use LOCTITE SF 7649 Activator to accelerate cure. -
Disassembly
Apply localized heat (≈ 260 °C / 500 °F) to soften the adhesive, then remove parts while hot.
For seized assemblies, use LOCTITE 8040 Freeze & Release.
FAQ
Q1: Can LOCTITE 638 be used on stainless steel or plated parts?
Yes. It bonds passive metals effectively, though cure speed may slow. Use activator 7649 to accelerate curing.
Q2: What is the maximum gap it can fill?
LOCTITE 638 can fill gaps up to 0.25 mm. For larger gaps, use LOCTITE 660.
Q3: How fast does it cure?
Fixture time is around 4 minutes on active metals; full cure in 24 hours at room temperature.
Q4: Can it resist oil exposure?
Yes, it tolerates light surface contamination and remains strong under oil exposure.
Q5: What’s the temperature range?
It performs reliably from –55 °C to +180 °C.
Q6: How can I remove it?
Use localized heat (around 260 °C) and mechanical force while hot.
