IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:
GASKETS AND SEALS
GASKETS AND SEALS
In a small engine, gaskets and seals are used in many locations to improve the seal between components. They are designed to prevent the fuel air mixture from leaving the cylinder during combustion and power strokes of the piston. They are also used to keep oil from leaving the crankcase. These are two of the most critical functions gaskets and seals serve. One defective engine gasket or seal can cause a significant loss of horsepower.
A gasket is a flat thin piece of material used mostly for sealing two components with flat surfaces. The gasket is cut to match the shape of the mating surfaces involved. New gaskets should always be used when an engine is reassembled. Gaskets, vital to engine operation, should never be omitted during the reassembly process.
How gaskets are used: small engines will harbor several different types of gaskets. The cylinder head gasket seals the area between the engine block and cylinder head. Cylinder head gaskets are composed of a thin, fibrous material. The cylinder must be tightly sealed so that pressure developed on the piston compression stroke, and rotational force developed on the piston power stroke, will not be compromised by leaks. You will also find gaskets between the crankcase cover and crankcase, between the intake manifold and engine block, and between the carburetor and its engine mount.
Types of gaskets: in most cases, gaskets are made of either a flimsy paperlike material or a tougher fibrous material. Liquid sealant is often used with paper type gaskets to help them seal better. The liquid sealant is brushed onto both sides of a gasket prior to installation. Liquid sealant can be purchased at most auto parts stores. Fiber type gaskets are usually thicker and more rigid; most of the time they do not require a supplemental sealant. Rubber gaskets will also not need a sealant. Engine service manuals will sometimes bear instructions for gasket installation, and recommend that a liquid sealant be used at certain gasket locations. Gasket makers, too, may wield instructions on how to install their product, and if a liquid sealant should be added for increased fastening strength.
On some engines, you may find that surfaces between two components are sealed only with a liquid gasket. This type of liquid gasket turns into a solid material as it dries between the installed parts. These gaskets will often be used to seal two surfaces that fit mega tightly together. Other types of liquid gaskets dry, instead of rigid, somewhat flexible; these flexible liquid gaskets are actually used more frequently than their rigid cousins. The most common flexible liquid gasket is RTV silicone sealant. It is applied in small beads squeezed from a tube to the two mating surfaces, and then the components brought together. Once the components are assembled, the excess sealant is forced from between the surfaces, and should be wiped off.
Most of the time, an engine service manual will advise the type of gasket and the type of sealant required for a given application. When you disassemble an engine, note what sorts of existing gaskets were in place. Setting them aside until they can be matched up with new versions is also not a bad idea. It can be extremely helpful if a service manual does not specifically mention what gaskets are needed.
Unlike gaskets which are used with static components, seals prevent leaks between one or more rotating engine parts. In a small engine, seals are commonly used to prevent leaks where the crankshaft ends exit the crankcase or crankcase cover. The seal must prevent oil from leaking out of the crankcase, and yet not interfere with crankshaft rotation.
How seals are installed: seals are round and flat, constructed of a metal outer ring around a rubber inner surface. The rubber surface is tilted in one direction; this tilt is referred to as the lip of the seal. The lip is intended to capture oil that attempts to sneak out at the seal edges. However, this lip design means that most seals can prevent leaks in only one direction. When you are reassembling an engine, you therefore must take care not to install a seal backwards. An engine service manual will indicate the appropriate direction for a given seal to be installed.
Typically the seal will be press fitted into a hole, then tapped into place with a driving tool of the right circumference and ball peen hammer. Seals rarely demand supplemental sealants. However, it is a good idea to lightly lubricate the rubber part of a seal with oil to help prevent damage to its surface as the crankshaft end is pushed through its opening. Oil can also sometimes help a seal perform better.
Types of seals: one common type of seal is found in a sealed bearing; a sealed bearing is nothing more than a standard ball bearing with a rubber seal surrounding it to prevent leakage and, in some instances, reduce operating noise. The seal of a sealed bearing cannot be removed, or replaced. If the seal becomes faulty, the whole bearing must be replaced instead. Another common seal is an O ring. An O ring is a rubber seal with a round shape letter “O” shape. Once in awhile an O ring will sport a square, rather than a round, cross section. An O ring will often be used between two round components, or other similar applications in which a paper type gasket would be unsuitable. In small engines, O rings are customarily used in oil dipstick and oil filler tubes; like other seals, an O ring will not normally require a supplemental liquid sealant to be used with it.
Seals play a vital role in engine operation. Refer to the applicable engine service manual to determine how to locate and replace seals.