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Friday, May 20, 2011
Dismantling the valve-mechanism
The valve-operating mechanism has to be removed from the top of the cylinder-head before the cylinder-head can be removed from the cylinder-block. There are a number of different arrangements and the procedures that are used will depend largely on whether it is and overhead-camshaft engine or an overhead-valve engine.
With OHC engines, the camshaft (or cam shafts) are removed from the top of the cylinder-head. Variations in the dismantling procedures for different engines will depend on:
1.Whether the engine has rocker arms or bucket tappets.
2.Whether there are hydraulic lash adjusters.
3.Gathered there are one or two camshafts.
4.Whether you camshaft has a belt drive or chain drive.
The camshafts are carried in bearings in the cylinder-head and are retained by the bearing cups. With bucket type tappets, the camshaft can be removed from the cylinder-head and the tappets then removed. With rocker arms, camshaft cannot be removed until the rocker arms or the arm assembly has been removed.
Some rocker arms are mounted individually, but many are mounted on the bearings or bushes on a shaft. These are removed from the cylinder-head as an assembly. Rocker arms of this type are shown in figure 3.2. The camshaft bearing of the rings caps are secured to the cylinder-head with four bolts.
The removing this type of assembly, the bearing bolts are released progressively a little at a time, and in a sequence that involves all the bolts. This is necessary because some parts of the camshaft are under load from the compressed valve-springs. Progress loosening of the bearing cap will prevent distortion of the parts.
·The arrangements of the overhead-camshaft can be seen in a number of the figures in the previous chapter.
Figure 3.3 shows a partly dismantled v-type engine with overhead-valves. Procedures for dismantling the valve-mechanism include removing the valve-co and r were from the top of the cylinder-head and the intake manifold from between the banks of the cylinders.
In the illustration, the valve-covers have been removed and parts of the valve-train can be seen. A single camshaft is mounted in the crankcase and this operates hydraulic valve-lifters. These have rollers that bear to against the cams of the camshaft; the rollers are used to reduce friction and wear.
Push-rods are located between the valve-lifters and the rocker arms. The rocker arms are independently mounted and they pivot about a fulcrum that is retained by the stud. And the stud is threaded into the cylinder-head.
The camshaft for this type of engine is shown in figure 2.27 in the previous post.
Removing and dismantling cylinder heads
With a valve gear removed from the cylinder-head, the cylinder-head itself can be removed from the cylinder-block. Cylinder- had bolts must be released progressively and in the correct sequence (figure 3.4). If the bolts are not all the same length, the location of the different bolts should be noted.
When all the cylinder-head bolts have been removed, the head can be lifted from the block. If they had a stuck, a lever can be used between protrusion on the head and the block. Do not lever between the meeting surfaces and this will cause burns on the surfaces and sealing on reassembly will be difficult.
See Dismantling, cleaning and checking the cylinder-head