Loctite 638

LOCTITE 638 Retaining Compound is a high-strength anaerobic acrylic adhesive formulated with urethane methacrylate chemistry and designed specifically for retaining cylindrical fitting components. This green liquid adhesive exhibits high viscosity (2,500 mPa·s / cP measured by Brookfield RVT at 25°C using Spindle 3 at 20 rpm, and 2,500 mPa·s / cP using Cone & Plate at a shear rate of 129 s⁻¹) and has a specific gravity of 1.1 at 23°C. It cures anaerobically when confined between closely fitting metal surfaces in the absence of air, and can be accelerated using compatible activators such as SF 7471™, SF 7649™, and SF 7091. The product is fluorescent under UV light for inspection purposes and is engineered to bond cylindrical assemblies where bond gaps may reach up to 0.25 mm while delivering maximum strength at room temperature. LOCTITE 638 provides reliable curing performance on active metals such as mild steel and also on passive substrates including stainless steel and plated surfaces. It tolerates minor surface contamination from cutting oils, lubrication fluids, anti-corrosion oils, and protection fluids, and offers excellent resistance to shock, vibration, and leakage. Typical applications include locking bushings and sleeves into housings and onto shafts.

When cured for 15 minutes at 23°C, LOCTITE 638 achieves a compressive shear strength of 13.5 N/mm² (1,960 psi) on steel pins and collars as tested according to ISO 10123. After 72 hours at 23°C, compressive shear strength increases to 29 N/mm² (4,200 psi) on steel, 28 N/mm² (4,000 psi) on stainless steel, and 17 N/mm² (2,710 psi) on aluminum. Physical properties after 24 hours at 23°C include a glass transition temperature (ISO 11359-2) of 76°C and a coefficient of thermal expansion of 96 × 10⁻⁶ K⁻¹ below Tg and 192 × 10⁻⁶ K⁻¹ above Tg. Torque performance according to ISO 10964 shows breakaway torque values of 57 N·m (510 lb·in) for M10 black oxide bolts with mild steel nuts and 25 N·m (220 lb·in) for 3/8 x 16 steel nuts (grade 2) with bolts (grade 5). Prevail torque values are 22 N·m (200 lb·in) for M10 assemblies and 9.4 N·m (85 lb·in) for 3/8 x 16 assemblies. Breakloose torque for 3/8 x 16 assemblies pre-torqued to 5 N·m measures 23 N·m (210 lb·in), while prevail torque under the same pre-torque condition measures 12 N·m (110 lb·in).

Curing speed depends on substrate type, bondline gap, temperature, and activator usage, with shear strength development tested at 23°C according to ISO 10123. The adhesive demonstrates strong hot and cold performance, having been tested down to -75°C (-100°F), and maintains durability under elevated temperature conditions. Environmental resistance testing shows retention of initial strength after exposure up to 5000 hours in various environments including motor oil (5W40 synthetic) at 125°C, unleaded petrol, brake fluid, water/glycol 50/50 mixture, ethanol, acetone, B100 bio-diesel, DEF (AdBlue®), sodium hydroxide 20%, and phosphoric acid 10%, depending on exposure duration and temperature.

For assembly, surfaces should be cleaned with a LOCTITE cleaning solvent and allowed to dry. Activator may be applied to accelerate cure or compensate for larger gaps. For slip-fitted assemblies, adhesive should be applied around the leading edge of the male component and inside the female component with rotational movement during assembly. For press-fitted assemblies, adhesive should be applied thoroughly to both bond surfaces and assembled using high press rates. For shrink-fitted assemblies, the adhesive should be coated evenly on the appropriate component depending on whether heating or cooling is applied, ensuring condensation is avoided. Parts must remain undisturbed until sufficient handling strength is achieved. For disassembly, standard hand tools may be used; if necessary, localized heating to approximately 250°C can assist removal while hot. Cured material may be removed by soaking in LOCTITE solvent combined with mechanical abrasion such as wire brushing.

The product should be stored in its unopened container in a dry environment, with optimal storage temperature between 8°C and 21°C. Storage below 8°C or above 28°C may adversely affect product properties, and contaminated material should not be returned to the original container. The technical data provided represent typical values obtained under controlled conditions of 23±2°C and 50±5% relative humidity, and users are advised to conduct their own suitability testing before production use.