LOCTITE 243 This iconic threadlocker locks and seals metal nuts and bolts to prevent loosening due to shock and vibration. Medium-strength: removable with hand tools. LOCTITE® 243 is a blue, general purpose threadlocker that provides a medium-strength bond. It works on all metals without primer, including passive substrates such as stainless steel, aluminum, and plated surfaces. Suitable for use on most oil-coated, 'as-received' fasteners without cleaning.