When we repair the car engine, the phenomenon of "three leaks" (water leakage, oil leakage and air leakage) is the most headache for maintenance personnel. "Three leaks" may seem common, but it directly affects the normal use of the car and the cleanliness of the appearance of the car engine. Whether the "three leaks" in important parts of the engine can be strictly controlled is an important issue that maintenance personnel must consider.

1 Types of engine seals and their selection

The quality of engine seal material and its correct selection directly affect the quality of engine seal performance.

① Cork board gasket
Corkboard gaskets are pressed from granular cork with a suitable binder. Commonly used in oil pan, water jacket side cover, water outlet, thermostat housing, water pump and valve cover, etc. In use, such gaskets are no longer the preferred choice for modern cars due to the fact that cork boards are easily broken and inconvenient to install, but they can still be used as a substitute.

② Gasket asbestos plate gasket
Liner asbestos board is a plate-like material made of asbestos fiber and adhesive material, which has the characteristics of heat resistance, pressure resistance, oil resistance, and no deformation. Commonly used in carburetors, gasoline pumps, oil filters, timing gear housings, etc.

③ Oil-resistant rubber pad
The oil-resistant rubber mat is mainly made of nitrile rubber and natural rubber, and asbestos silk is added. It is often used as a molded gasket for sealing of automobile engines, mainly used for oil pans, valve covers, timing gear housings and air filters.

④ Special gasket
a. The front and rear oil seals of the crankshaft are usually special standard parts. Most of them use skeleton rubber oil seals. When installing, pay attention to its directionality. If there is no label indication, the lip with the smaller inner diameter of the oil seal should be installed facing the engine.
b. The cylinder liner is usually made of steel sheet or copper sheet asbestos. At present, most of the automobile engine cylinder gaskets use composite gaskets, that is, an inner metal layer is added in the middle of the asbestos layer to improve its rigidity. Thus, the "washout" resistance of the cylinder head gasket is improved. The installation of the cylinder liner should pay attention to its directionality. If there is an assembly mark "TOP", it should face upward; if there is no assembly mark, the smooth surface of the cylinder head gasket of the general cast iron cylinder block should face the cylinder block, while the cylinder of the aluminum alloy cylinder block should face upward. The smooth side of the gasket should face the cylinder head.
c. The intake and exhaust manifold gaskets are made of steel or copper covered asbestos. When installing, care should be taken that the curled surface (that is, the non-smooth surface) faces the cylinder body.
d. The seal on the side of the last main bearing cap of the crankshaft is usually sealed by soft technique or bamboo. However, when there is no such piece, asbestos rope soaked in lubricating oil can also be used instead, but when filling, the asbestos rope should be smashed with a special gun to prevent oil leakage.
e. The spark plug and exhaust pipe interface gasket should be replaced with a new gasket after disassembly and assembly; the method of adding double gaskets should not be adopted to prevent air leakage. Experience has proved that the sealing performance of double gaskets is worse.

⑤ Sealant
Sealant is a new type of sealing material in the maintenance of modern automobile engines. Its appearance and development provide good conditions for improving sealing technology and solving the "three leaks" of engines. There are many types of sealants, which can be applied to different parts of the car. Automotive engines usually use non-bonded (commonly known as liquid gasket) sealants. It is a viscous liquid substance with a polymer compound as the matrix. After coating, a uniform, stable and continuous adhesive thin layer or peelable film is formed on the joint surface of the parts, and can fully fill the depression and surface of the joint surface. into the gap. The sealant can be used alone or in combination with their gaskets on the engine valve cover, oil pan, valve lifter cover, etc., and can also be used alone under the last bearing cover of the crankshaft, as well as oil hole plugs and oil plugs. and so on.

2 Several issues that should be paid attention to in the maintenance of engine seals

① The old sealing gasket cannot be reused
The sealing gaskets of the engine are installed between the surfaces of the two parts. When the gaskets are compressed, they match the microscopic unevenness of the surface of the parts and play a sealing role. Therefore, every time the engine is maintained, a new gasket should be replaced, otherwise, leakage will definitely occur.

② The joint surface of the parts should be flat and clean
Before installing a new gasket, ensure that the joint surface of the part is clean and free of dirt, and at the same time, check whether the surface of the part is warped, whether there is a convex hull at the connecting screw hole, etc., and should be corrected if necessary. The sealing effect of the gasket can only be fully exerted when the joint surface of the parts is flat, clean and free from warping.

③ The engine gasket should be properly placed and stored
Before use, it should be completely stored in the original box, and must not be stacked arbitrarily to bend and overlap, and it should not be hung on hooks.

④ All connecting threads should be clean and undamaged
The dirt on the threads of the bolts or screw holes should be removed by threading or tapping; the dirt at the bottom of the screw holes should be removed with a tap and compressed air; the threads on the aluminum alloy cylinder head or cylinder body should be filled with sealant , to prevent gas from penetrating into the water jacket.

⑤ The fastening method should be reasonable
For the joint surface connected by multiple bolts, a single bolt or nut should not be screwed into place at one time, but should be tightened several times to prevent the deformation of the parts from affecting the sealing performance. Bolts and nuts on important joint surfaces should be tightened according to the specified order and tightening torque.
a. The tightening sequence of the cylinder head should be correct. When tightening the cylinder head bolts, it must be expanded symmetrically from the center to the four sides, or according to the tightening sequence chart given by the manufacturer.
b. The tightening method of the cylinder head bolts should be correct. Under normal circumstances, the bolt tightening torque value should be tightened to the specified value in 3 times, and the torque distribution of the 3 times is 1/4, 1/2 and the specified torque value. Cylinder head bolts with special requirements shall be carried out in accordance with the manufacturer's regulations. For example, the Hongqi CA 7200 sedan requires a torque value of 61N·m for the first time, 88N·m for the second time, and a 90° rotation for the third time.
c. Aluminum alloy cylinder head, since its expansion coefficient is greater than that of bolts, the bolts should be tightened in the cold state. The cast iron cylinder head bolts should be tightened twice, that is, after the cold car is tightened, and the engine is warmed up and then tightened once.
d. The oil pan screw should be equipped with a flat washer, and the spring washer should not be in direct contact with the oil pan. When tightening the screw, it should be tightened evenly in 2 times from the middle to the two ends, and the tightening torque is generally 2ON·m-3ON·m. Excessive torque will deform the oil pan and deteriorate the sealing performance.

⑥ Correct use of sealant
a. All oil plug plug oil pressure sensor and oil alarm sensor threaded joints should be coated with sealant during installation.
b. Cork board gaskets should not be coated with sealant, otherwise the soft board gaskets will be easily damaged; sealants should not be coated on cylinder gaskets, intake and exhaust manifold gaskets, spark plug gaskets, carburetor gaskets, etc.
c. When applying sealant, it should be applied evenly in a certain direction, and there should be no glue breakage in the middle, otherwise there will be leakage at the broken glue.
d. When sealing the surfaces of the two parts with sealant alone, the maximum gap between the two surfaces should be less than or equal to 0.1mm, otherwise, a gasket should be added.

⑦ After all parts are installed and reassembled as required, if there is still "three leakage" phenomenon, the problem often lies in the quality of the gasket itself.
At this point, the gasket should be re-inspected and replaced with a new one.

As long as the sealing material is selected reasonably and several problems of sealing maintenance are paid attention to, the "three leakage" phenomenon of the automobile engine can be effectively controlled.