There are three types of sealant manufacturers commonly use to assemble their engines. These are RTV or room temperature vulcanizing sealant, anaerobic sealant, and pipe joint compound. Of the three, pliable silicone sealants like RTV are most common when repairing or rebuilding an engine. They absorb engine vibrations and fill imperfect surfaces on engine parts.
Don't use RTV sealant on high-temperature components like oxygen sensors and exhaust manifolds. Always use the appropriate sealer recommended by the engine manufacturer for the specific application.
RTV or room temperature vulcanizing sealant is used throughout the engine, sealing all kinds of leaks. If you've been a technician for any length of time, you've used RTV. An aerobic sealant like RTV cures in the presence of air and forms a flexible rubber gasket.
Always clean the surface before applying the sealant, and try to keep it out of bolt holes. Scrape any old gasket off the surface with a gasket remover, and clean the surface thoroughly for a leak-proof seal. Be careful not to remove any valuable surface from aluminum parts.
Always follow the manufacturer's specifications and procedures when using any sealant around a sensor or actuator. This sealant may be used in place of a conventional gasket only in specific applications.