Matters Needing Attention When Overhauling an Engine

Issues requiring attention when repairing an engine

In recent years, degree of mechanization of agricultural production in our country has been rapidly increasing, but technology for repairing agricultural machines has not received adequate development, many machines are being repaired and not worn out. For this reason, several issues needing attention in engine repair are re-emphasized in order to help maintenance personnel improve quality of diesel engine repair.

1 Cylinder head gasket thickness cannot be changed at will

The cylinder gasket is installed between cylinder head and cylinder block, its function is to provide a good seal on joint surface and prevent cylinder and water jacket from leaking. The cylinder head gasket is also an integral part of combustion chamber in thickness direction. There are strict rules for cylinder shim thickness, and finished product must be within specified size range. Too thick or too thin will affect compression ratio and cylinder power. Adjusting height of compression chamber actually adjusts compression ratio, and amount of compression ratio directly affects working process of internal combustion engine. For a diesel engine with a standard cylinder head gasket, height of machine's compression chamber at this time may not differ much. However, for 110 series diesel engines, since standard cylinder head gasket is not used, air storage gap is only achieved by increasing or decreasing copper gasket to meet given requirements. pistons, etc., here In this case, it is necessary to correct height of compression chamber.

The way to check compression chamber height is to remove diesel cylinder head fuel injector, unscrew petrol spark plug, and then insert a thick electrical fuse into cylinder through fuel hose. nozzle hole or spark plug seat hole. Shake crankshaft to top dead center, pull out thick fuse and measure it with a caliper to find out its size. The overall height of compression chamber is given in manufacturer's instructions. If height of original compression chamber cannot be verified, you can remove cylinder head of a well-used internal combustion engine, put piston at top dead center and measure its height. As a general rule, difference between height of compression chamber and height specified by manufacturer should not exceed 5%.

2 Pay attention to valve clearance adjustment

Valve clearance is gap between back of valve stem and rocker arm when valve is fully closed with a cold diesel engine. This gap is designed to eliminate thermal expansion of transmission chain of valve mechanism during diesel operation in order to ensure that valve can be tightly closed when diesel is in a hot state. When gap tovalve is adjusted to set value, inlet and outlet valves can open earlier and close later. When valve clearance is too large, valve will open late and close early, so that opening time will be too short and enough air (or mixture) cannot be sucked in during intake process, which will affect combustion characteristics of fuel. engine. During exhaust process, exhaust gases cannot be completely removed, which can easily cause machine to overheat. If gap between intake and exhaust valves is adjusted too small, it is often easy to cause cylinder to not work properly. As engine temperature rises, valve and valve seat will leak and leak. For gasoline engines, this can also lead to carburetor firing or backfiring, which not only reduces power, but also easily burns contact surface between valve and valve seat due to passage of high temperature gas through valve.

The valve clearance standards are different for different engine models, but basically they all fluctuate between 0.25-0.45 mm. During long-term operation of diesel engine, due to wear of parts, valve clearance will change. Therefore, it must be periodically checked and adjusted. Now take 195 type diesel engine as an example to illustrate a specific method for checking and adjusting valve clearance. Inlet and outlet valves are closed (don't forget to turn off pressure relief). Using a wrench and screwdriver, loosen nuts and adjusting screws on rocker, insert a feeler gauge (known as a feeler gauge) between valve and rocker, and turn adjusting screw on rocker until rocker and valve can be in tight contact, but feeler gauge can also be moved. Then tighten nut with a wrench and check again. The valve clearance (cold state) of a 195 diesel is indicated as 0.35 mm for intake valve and 0.3-0.4 mm for exhaust valve. If internal combustion engine has more than two cylinders, after calibrating first cylinder, adjust valve clearances one by one in accordance with firing order of cylinders.

3 Heat aluminum piston when installing piston pin

Aluminum pistons expand more when heated. According to requirements, piston pin must have a certain degree of tightness when installed in piston pin bore at room temperature. If piston is not heated during assembly, but only hit hard with a hammer, then finger hole will be compressed. This will cause loosening during use and result in incorrect operation. In severe cases, piston pin moves in pin bore, breaking circlip, pulling cylinder liner and breaking mechanism. If piston is placed in oil or water and heated to approximately 80°C during assembly, piston pin can be smoothly inserted into piston pin borethumb. Thus, occurrence of above phenomenon can be avoided. If piston pin is still difficult to insert into piston pin seat hole after piston has been heated, piston pin hole size is too small or piston pin diameter is too large. If during assembly it turns out that size of hole for pin is too small, it can be drilled with a reamer to desired size. In addition, when using a hammer to forcibly remove piston or install piston pin, connecting rod is often deformed due to force at this time, which affects normal operation of engine.

4 Valve leak test

A bad seal between valve and valve seat will cause valve to close tightly and air will leak, which will affect normal operation of engine. If intake valve is leaking, then when piston is compressed and mixture ignites on compression stroke, part of high temperature gas will rush into intake manifold, and at this time a “popping” sound may be heard. If exhaust valve is leaking, high temperature and high cylinder pressure gas will come out of it, accompanied by a "squeaky" sound, resulting in a significant drop in cylinder pressure, resulting in diesel engine underpower, black smoke from exhaust pipe, and starting gas into crankcase, oil deterioration and other adverse effects. Therefore, during operation and repair, attention should be paid to checking tightness of valve.

Any severely worn or burnt valve seat and valve will certainly leak air and should be inspected and repaired regularly. Disassembly and assembly of valve should be done with a random special tool. At this time, valve spring can be pressed with a special tool, and valve retainer can be removed or installed, so that valve can also be removed or installed. Here are some commonly used seal test methods: (1) Soft pencil line method: Use a soft pencil to draw several straight lines with almost equal distances on valve head slope, then place valve on valve seat and tap it gently. Secondly, if, after removing it, it is found that marks of each pencil are interrupted at tilt seal line, this means that sealing characteristics of valve and seat are good. (2) Red lead identification method: apply a very thin layer of red lead or other suitable pigments to sloping surface of valve, then insert valve into seat and turn it slightly, and then lift valve for inspection. If seal between valve and seat ring is good, very thin and continuous helix should appear in dark circle we saw earlier, at which time seal is considered good.