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Saturday, May 21, 2011
Dismantling, cleaning and checking (Cylinder-heads)
To remove the valves from the cylinder-head, the valve- springs are compressed so that the collets and keepers can be removed from the grove in the Valve-stem.
A valve-spring compressor is used (figure 3.5), after releasing the compressor, the retainer, a spring seal and the spring seat can be removed.
The valves and associated parts should be kept in the correct order so that they can be reinstalled in their original positions. Parts are usually identified from the front to rear for in-line engines. For a v-type engine, the parts are identified as right hand (RH) or left hand (LH) and also has either intake (IN) or exhaust (EX) as shown in figure 3.6.
Fig. 3.6 (a)
Fig. 3.6 (b)
As well as removing oil and grease with Solvent, a scraper is used to remove the carbon from the combustion chambers and the valve ports. Parts of the cylinder head gasket adhering to the face of the head should be scrapped off with a flat scraper that will not scratch the surface (figure 3.7).
A Shaped wire brush fitted to a small electric drill can be used to finish off the combustion chamber and valve ports (figure 3.8). Carbon and dust should be blown away with compressed air.
The surface of the cylinder block must also be cleaned. Carbon can be removed from the heads of the pistons with the scraper which should be used without scratching the pistons. Turn the crankshaft until the piston to be cleaned is on TDC and place the rags in the other cylinders to keep particles of carbon out. The bolt holes in the cylinder block should be blown with air to remove any particles of carbon.
·Carbon left in the bottom of the bolt holes will affect bolt tightening.
After cleaning, Cylinder-Head should be checked for cracks, wrapping or rough surfaces. Flatness can be checked with a straight edge against the face of the Cylinder-Head. Feeler gauges are used between the straight edge and the surface of the head and the service of the head as shown in figure 3.9. Rough sports and burns can be removed by carefully draw filing with a fine file, but a warped head must be refaced by grinding.
Fig. 3.9 (b)
Fig. 3.9 (a)
Specifications generally allow 0.2mm maximum warping in the overall length of the head, or approximately 0.1mm in any 150mm in any directions. The face of the Cylinder-Head can be restored by grinding it in the surface grinder, but generally not more than 0.05mm of metal should be removed from the surface.
Checking cylinder heads for cracks
Cracks in the Cylinder-heads are hard to find by visual inspection. They often occur in or around the combustion chamber and the valve port areas. Often, cracks can only be located by special crack-detection methods. There are two methods used in automotive workshops – one uses magnetism and the other user’s dye.
The magnetic method of crack detection is suitable for all of ferrous metals that is used on cast iron cylinder heads. The detector magnetizes the area being tested and, if a crack exists, one side of the crack will become a north magnetic pole and other side a south magnetic pole. Metal powder or metallic fluid is applied to the area. The fine particles of the metal will be attracted to the poles and will form a line that follows the crack, making it visible as a fine grey line.
Dye penetrant method
The dye penetrant method is suitable for most materials, including nonferrous metals, that it can be used for both Aluminum alloy and cast iron Cylinder-heads. It can be used around the valve ports where magnetic attraction is difficult.
The suspected area of the part is cleaned with a special solvent to remove all the dirt, Grease and this also cleans any cracks. A red dye penetrant is then sprayed over the surface and this penetrates any cracks (figure 3.10).
After allowing the dye penetrant, the surplus dye is removed and a developer is sprayed over the area. any cracks will then be seen as fine a red lines.