Silicone RTV (Room Temperature Vulcanizing) is a versatile sealant used from the factory in almost all automotive applications, and remains popular with mechanics for its ability to mate warped or imperfect components. Silicone is most often used to seal water passages, but is just as commonly applied to those that carry oil, exhaust or air.
Silicone is prized for its excellent adhesion to metal surfaces, but all the adhesion in the world isn't going to make it stick to oil or grease. Even if your old gasket comes off without sticking, you'll still need to prep the surface with a brass or steel wire brush and cleaner. A powerful, wire-wheel equipped grinder or buffer is best where space allows, but a small rotary tool will work just as well. Remember, you should never use a steel brush on aluminum components; those hard steel bristles will eat the soft aluminum substrate just as easily as it will any remaining gasket material.
After the old gasket is stripped away, clean the surface with a liberal amount of rubbing alcohol or paint thinner, and avoid touching it afterward. The natural oils produced by your skin will contaminate the surface and cause leaks.
If your engine uses an O-ring style gasket on its oil, air or water passages, then don't use any RTV at all; it will be more likely to cause O-ring leaks than to prevent them. If you need RTV, then apply a thin bead of it (no wider than 1/8 inch) around the port in an unbroken ring, about 1/4 inch away from the port opening. The goal here is to use just enough silicone to seal small imperfections when the gasket is compressed, but not so much that it squishes out from the sides.
Many V-configured engines require that RTV be used to seal the intake manifold's front and rear mounting surfaces. Run a very thin bead (in the same way as you would with passage sealing) along the upper parts of the engine block's front and rear "China Walls" (where the block meets the manifold), and press the front and rear manifold gaskets into place. Run another bead of silicone on top of the rubber gasket, where it will meet the manifold. Some engines require the mechanic to place a small dab of silicone at the block/head/manifold interface and others do not, but it can't hurt.
Silicone RTV uses moisture in the air to vulcanize (harden), so skin-time and curing time will vary with humidity. In general, RTV silicone will form a skin after about 25 minutes of exposure to air with a 50 percent relative humidity; the RTV won't seal as well after this skin is formed, so getting everything installed and torqued down within this time is paramount to sealing. Some RTVs work a little better after allowing them 3-5 minutes of exposure to open air, but personal experiences vary. Allow 30 to 60 minutes before exposing the RTV to pressure or fluid; full curing of the silicone should take place within 24 hours.
