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Terminology is one of the hurdles we face day in and day out in the automotive industry. It varies between automakers, parts suppliers, technicians and consumers. Gaskets and seals are some of those terms that are easily mixed up from time to time. So, what’s the difference between a gasket and seal, since they’re both designed to do the same thing?

A gasket is any material installed between two fixed components with flat sealing surfaces, designed to conform to minor surface irregularities and prevent any liquid or non-liquid that passes between the components from going anywhere other than its intended location. In this case, liquid can be oil, coolant, transmission fluid, power-steering fluid, gasoline … you get the idea. Non-liquid refers to air, exhaust or fuel and crankcase vapors. Gaskets can be made of paper, cork, rubber, steel, aluminum, copper or a combination of materials.

A seal, at least in most cases, is made of rubber. The main differentiator, however, is not material but application. Gaskets are compressed tightly between two fixed components, whereas a seal is not tightly sandwiched or compressed in the same manner (at least most of the time), since it must allow movement of one of the components.

It’s easy to get deep in the weeds here, because a seal, by most accepted definitions, is used between a fixed and a moving component. Getting even more “technical,” this is called a dynamic seal, and a gasket can be referred to as a static seal. So, one is the other?! Well, I’ll try to keep the grass as short as possible.

The easiest way to grasp it all is by looking at some examples. Common gaskets are head gaskets, valve-cover gaskets, thermostat-housing gaskets and exhaust-manifold gaskets, just to name a few. The components they seal between are bolted or held firmly to each other.

Gaskets have the advantage of sealing high pressure, such as that built during the compression stroke or in the cooling system, and depending on material, they can handle extreme heat, such as exhaust-manifold gaskets. Seals, on the other hand, can’t handle the same amount of pressure, and rubber can’t handle extreme heat.

Examples of common seals are crankshaft and camshaft seals, transmission input and output shaft seals and axle seals. The common link is the fact that all these components rotate. But get ready to fire up the weed-eater.

If all that’s true, what’s the difference between a thermostat-housing gasket and a thermostat-housing seal? A thermostat gasket is a thin, paper-type material that installs between the housing and the intake manifold, block or wherever the housing is mounted. In most cases, the housing is made of metal. Then, as plastics became more common for use in automotive components, thermostat housings were one of the first things to change over.

Plastic is less expensive and easier to manufacture, and it’s lighter-weight. Plastic thermostat housings, however, required an O-ring seal instead of a gasket, for many reasons. Plastic wasn’t strong enough to handle the same amount of torque as a metal housing, so the lower torque required to prevent cracking the plastic meant a gasket would be less effective.

Rubber O-ring seals compress when tightened, and an advantage of rubber lies in its elastic properties, meaning it always wants to return to its original shape. This causes a rubber O-ring to keep constant tension outward equally in all directions. Another advantage of a rubber O-ring in this case is the expansion rate of metal and plastic is very different. Use of an O-ring allows an increased range of movement while maintaining a positive seal.

Many cooling-system quick-connect hoses and bypass tubes utilize rubber seals. The reason is not only the elasticity of the O-rings, but also the fact that when a rubber seal is used between two components, it allows a certain amount of “float” between them during expansion and contraction, maintaining a positive seal with no stress on the components.

I mentioned earlier that most seals are made of rubber. It’s likely true to say all of them are today, but years ago before we had developed good rubber technology, seals were made of felt, leather and, in some cases, asbestos. It was the only way to bridge the gap between a fixed and moving component and keep it from leaking, at least for the most part.

Prior to the advantages of today’s rubber technology, vehicles often were equipped with two-piece crankshaft seals. These were considered “rope” seals, simply because they looked like a piece of rope. Many of these were made of asbestos. One piece was installed in a groove in the engine block, and the second piece was installed in the bearing cap.

It required very careful work to install these successfully with no leaks, and it proved to be very difficult over the years. If you’re around old cars often, you know that classic-car owners often keep a large piece of cardboard underneath to catch offending drops of oil that in most cases come from a two-piece crankshaft seal. Eventually, auto manufacturers switched over to one-piece crankshaft seals to eliminate this problem, and many old engines can be retrofitted to a one-piece seal.

The bottom line is that seals are used because they allow movement of components while keeping constant tension against them. To aid in sealing, most shaft seals have a small spring on the inside of the sealing lip to assist in keeping tension against the moving component.

O-ring seals are used because they keep constant tension between components while allowing expansion and contraction. This is why O-ring seals are used in air-conditioning systems, and O-ring seals such as this have the ability to handle a higher pressure.

Is there a difference between a gasket and a seal? Absolutely. Is there gray area? Sure. You can dig even deeper with head gaskets that are made of one material yet feature rubber seals around coolant passageways. This is a gasket with seals incorporated in certain areas to take advantage of the benefits of elasticity in the rubber.

Above all, whether your customer asks for a gasket or a seal, you know one thing: They’re trying to stop a leak. That means they need the parts; fluid to replenish what was lost; and shop rags and cleaners to clean up the mess!

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