Loctite 638 is a high-strength, general-purpose retaining compound designed for bonding cylindrical fitting parts, particularly where bond gaps can approach 0.25 mm (0.01 in.). This product cures when confined in the absence of air between close-fitting metal surfaces and prevents loosening and leakage due to shock and vibration. It provides robust performance on both active metals (e.g., brass, copper) and passive substrates (e.g., stainless steel, aluminum). The compound offers high-temperature performance and good oil tolerance, making it suitable for a wide range of industrial applications. It is commonly used for retaining gears, pulleys, and similar components onto shafts and in housings.