Gasketing and Sealants

Anaerobic gasketing is designed primarily for close fitting (small gap) rigid metal flanges, The anaerobic material remains liquid when exposed to air, but cures when confined between mating flanges. Anaerobic gasketing can fill gaps of 0.050 (primed) and alleviates the need for performed gaskets. It is ideal for flanges that move in service.

One can use anaerobic material as a form-in-place gasket for purposes such as thermostats, compressors, pumps, transmission housings, gearboxes, axle covers.

Does a truly universal gasket sealant/eliminator exist? For some applications, the answer is “Yes.” But for critical areas containing lubes or pressure, indiscriminate use of sealants can harm, or even kill engines. Every experienced technician has his or her own opinion about the proper use of gasketing sealants. Here are a few points/tips/ opinions about popular sealing technologies.

Paper Gaskets

For some reason, paper gaskets are still out there. Thermostat/water outlets are a popular application, and like most, you’ll probably apply a gasket dope of some kind before assembly. All the major manufacturers offer a high tack aerosol or brush-on sealant, and there are the “aviation” products that are high-solids pastes as well as silicones and anaerobics. Less critical gaskets can often be replaced entirely by the anaerobics or silicones. However, one must consider the thickness of the gasket. If the gasket is thick, it’s designed to conform to potentially rough or uneven mating surfaces. Bolt clamping is also designed to account for the gasket thickness. If you choose silicones as a dressing, remember that it acts as an excellent lubricant during assembly, increasing the chance of gasket misalignment, pinching or tearing during reassembly. At the very least, use a very thin coat on both faces of the gasket. “Aviation” sealants — especially the soft-setting kind, might work — but have many of the same limitations as silicones.

Rubber Gaskets

With synthetic rubber gaskets popping up in modern engines, it’s easier now to replace cam covers, oil pans and thermostat covers than ever before. However, holding them in alignment can be difficult. It’s a good idea to stay away from alternate gasketing compounds with rubber unless those gasket compounds are OEM specified. Rubber dressings designed for the purpose work, but shouldn’t be used to restore a hardened and aged gasket. If pinching or tearing is a possibility, a very thin coating of fluid seal (most often oil) is usually specified. Most go on dry, but the same rules about clean, straight and smooth mating surfaces apply.

Cork/Rubber Gaskets

Once used everywhere, cork is now mostly found under valve covers in older domestic engines. On V-type engines this technology needs flat, smooth mating surfaces and consistent clamp loads. Unfortunately, these engines rarely have them resulting in seepage so common many techs regard it as normal. RTV sealants are popular here and have been used to replace the gaskets entirely. With no pressure to seal and little potential damage if a leak appears, it’s probably the most forgiving application. Silicones will promote slippage during installation. A common mistake is to forget to check and correct distortion around the cover’s bolt holes, leading to extrusion around the bolt and too little clamp force between bolts. Over-torquing is another common issue. Just because it’s a non-critical seal doesn’t mean that torque values shouldn’t be followed.

Metal Gaskets

In most cases, keep sealants away from these high-temperature pieces. A possible exception could be cases like embossed intake manifold gaskets where pitting around the sealing region may promote a leak. Exhaust, EGR and head gasket applications are best left with factory-applied coatings only. Add-on chemicals can react with the manufacturer’s sealing compounds or contaminate O2 sensors. They can also simply burn away, causing a leak.

Gasket alternatives

With modern chemical sealant alternatives, many technicians have replaced conventional gaskets in common applications like thermostat housings, oil pan, valve and axle covers. Speed, convenience and tolerance to mating surface imperfections are good reasons to use RTV silicones and anaerobics. But which should you use and when? For temperatures and pressures encountered in automotive cooling systems, a high-temperature RTV product or an anaerobic will work. Anaerobics have the advantage of sealing quickly and cleanly, while silicones are cheap and are available for very high-heat applications. For professional repairers, the method of application is almost as important as the product. The familiar “toothpaste tube” is toolbox-handy, but silicones can be applied with a conventional caulking gun, lowering costs. The ultimate in convenience (and consistency) is the self-pressurized can. While more expensive, self-pressurized cans make a neater job and speeds application. Dressings are mostly aerosol “spray bomb” applied, but some are still available in brush-top bottles.

In every case, using too much can be as or more dangerous than not enough. This is especially true of silicones, which set into rubbery chunks that can fall into oil pans and plug galleries. Anaerobics don’t leave solids behind, as they set in the absence of air, but indiscriminate use can still contaminate engines. Oxygen sensors are another issue. Many shops simply don’t use sealants that are sensor-safe. If you’re not sure about your product, remember that PCV systems mean that the sensor will see any sealant chemistry that’s in the engine with the exception of the cooling system and that’s no place for debris, either. And what about the “aviation” pastes? They’ve been used since the 1930’s, but with modern sealing technologies readily available, why bother? Some types are useful for general shop use like air line fittings or plumbing repair. What about dressings? You can brush it on, but the speed and consistency of sprays make this a better way, despite the apparent waste and mess from overspray. A thin coat is all that’s necessary, but it should be uniform, which the spray provides. It’s faster, too.

There’s much more technology behind general engine sealing than any one article can provide; major sealant manufacturers offer comprehensive training and literature describing applications and use of the many products out there. Check with your jobber, and read the package instructions that come with the products. The aggravation you save will be your own.