There are several factors to consider when choosing a threadlocking adhesive including strength, size, and formulation.
Strength. Threadlockers are available in a variety of strengths. Each strength is denoted by color. Low-strength threadlockers are purple or have a purple stripe on their bottle. Medium-strength is typically blue and high-strength is red. These color associations are so strong that longtime threadlocker users will often refer to their desired product by color rather than strength, grade number, or even brand.
Size. Fastener size is an important consideration in relation to the strength and viscosity required for use in an application.
For example:
• Screws that are less than one-quarter inch in diameter, such as calibration screws or gauges, are typically locked by low-strength formulations.
• Fasteners up to three-quarters of an inch in diameter, such as those used in pumps or compressors, are generally fastened by medium-strength formulations.
• High-strength threadlockers are most often used on fasteners between three-quarters of an inch, up to one inch in diameter — and usually used with heavy equipment.
Formulation. The formulations offer different features. Low-strength formulations, for instance, are easy to remove. They provide low-strength bonds that can be disassembled by hand or with basic tools. If disassembly is routinely required, low-strength threadlockers should be used to minimize the wear on the bolts and effort required from an end-user.
Medium-strength grades can be removed using hand tools such as a wrench or vise-grip. High-strength threadlockers are used for permanent bonds, offering the highest bonding capabilities. If users want to disassemble a bonded fastener, high heat and power tools are necessary.