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Monday, May 23, 2011
Installing valve mechanisms
Points relating to the installation of valve mechanism include:
1.Camshaft. With OHC engines, the camshaft is installed on top of the head. The valve timing is set to the timing marks the No.1 piston on TDC. The chain or belt is installed and the tensioner is adjusted to provide the correct chain or belt tension.
2.Rocker assembly. When rocker gear is being installed, the bolts are screwed down progressively and then tightened to the correct torque. Screw adjustments on rocker arm can be backed off to relieve the load of the valve springs.
3.Push-rods – On OHV engines, push-rods have to be installed before the rocker mechanism is installed.
4.Valve clearances – These are adjusted with the engine cold, although, in some cases, the clearances can be rechecked when the engine is hot. With hydraulic valve lifters or hydraulic lash adjusters, clearances do no have to be adjusted.
The valve clearance (also Referred to as tappet clearance) is checked and adjusted in different ways for different engines. The valve clearance of engines with hydraulic valve-lifters Hydraulic lash adjusters do not need adjusting, although some hydraulic valve lifters can be checked.
Before checking the valve clearance, the valve must be in the fully closed position. The clearance is checked by using a feeler gauge of the correct thickness.
If the clearance is correct, the feeler gauge will be firm, but not tight, and then moved between the two parts being subject. For a more accurate check, to feeler gauges can be used, with one a little thicker And on a little thinner than the specified clearance.
·Two gauges used in this way are referred to as go and no go gauges.
Some rocker arms have an adjusting screw and locknut (figure 3.15). This is in the valve end of the rocker arm and clearance is checked between the adjusting screw and the tip of the valve stem. Most clearances are adjusted cold, but some clearances are specified hot.
Where clearances are checked hot, engine is to run it reaches operating temperature and then the valve-cover is removed. However, if an engine is being re-assembled, the clearances are set cold and, if specified, rechecked hot after the engine has been assembled and run.
Bucket-type tappet adjustments
Bucket tappets have a spacer or shim between the top of the tappet and the cam on the camshaft (figure 3.16). The clearance can be checked by inserting a feeler gauge between the cam and the top of tappet. Shims are available in a range of sizes, and thicker or thinner shims are fitted to adjust the clearance.
To change a shim, a tool is used to depress the tappet and valve-spring. This gives space between the top of the tappet and the cam so that one shim can be removed and another fitted.
In some engines, the spacers are fitted inside the tappet against the state of the Valve-stem and can only be adjusted with the camshaft removed. This is normally done only when and the Cylinder-Head is dismantled. Clearance is checked prior to dismantling and, if incorrect, is adjusted by fitting a spacer of different thickness during reassembly.
When bucket tappets are removed from the cylinder head, each one should be kept in order with its own spacer or shim.
Sequence of valve adjustments
The valve being checked for a adjustment must be fully closed, with its lifter, rocker arm or tappet on the heel of the cam. This position can be obtained by rotating the crankshaft until the position of the cylinder for the valves being checked is on TDC of the compression stroke. The clearance of both the intake valve and the exhaust-valve for the particular cylinder can be adjusted.
The clearance of the valves of the other cylinders can then be adjusted by rotating the crankshaft to bring each piston to TDC in turn. Following the firing order of the engine will require the least rotation of the crankshaft.
The above a sequence requires the crankshaft to be rotated a number of times before all the valve clearance can be adjusted. However, sequences which enable the clearance to be adjusted with fewer rotations on the crankshaft can be obtained from service manuals.
The sequence of adjusting the valves for a four cylinder engine is shown in figure 3.17. This requires minimum rotation of the crankshaft.
The valves labeled ‘A’ and the illustration are adjusted with piston of No.1 cylinder on TDC at the end of its compression stroke. Crankshaft is then turned one complete turn so that No.4 cylinder is on TDC at the end of its compression stroke. The valves labeled ‘B’ are adjusted.
Label of Fig.3.17 Tappet adjustment sequence A with No.1 piston on TDC, B With No.4 Piston on TDC
Top dead center for No.1 cylinder can be located by setting the zero mark on the crankshaft pulley in-line,
with the pointer on the timing cover (figure 3.18). At the top of the compression stroke, intake and exhaust-valves for the cylinder will be fully closed.
Usually, the engine can be turned with a spanner on the bolt at the end of the crankshaft. Watch the movement of the valves as the engine is being turned. When the intake valves closes, the piston will be at BDC. A half turn of the crankshaft will bring the piston to TDC.
For of four cylinder engine, use the timing marks to locate TDC at the end of the compression stroke. If this is done for No.1 cylinder then the TDC of the other cylinders will occur, in firing order, at half-turn of the crankshaft.
·Timing marks will align with the both No.1 and No.4 cylinders are on TDC, but only one will be on the top of its compression stroke.
For a six cylinder engine, a piston will be on TDC and each 120° of crankshaft rotation and for an eight cylinder engine, a piston will be on TDC eats 90° of engines rotation.
Label of Fig.3.18 Turning the crankshaft to set No.1 cylinder on TDC.