Tuesday, May 24, 2011

Servicing valves and springs

The parts to be checked on a valve are shown in figure 3.19.  The valve face should be inspected for wear, pits, grooves and burn marks to determine if it is suitable for further use.  Minor defects, such as pits and groves, can be removed by re-facing the valve, but a valve that is badly burnt, warped or cracked should be discarded.
The margin or thickness of the edge of the valve face should be about 1 mm minimum.  A margin that is to thin will cause the valve head to overheat and the edge will burn.
The Valve-stem should be inspected for a bent condition and the end of the stem inspected for groves or scores.  The clearance of the stem in the valve guide must also be checked.

Testing valve springs
Fig 3.20
Valves springs should be tested for compression and for squareness.  Special testers are used.  These measure the force required to compress the spring to the specified length (figure 3.20).
To check for squareness, stand the spring on end on a flat surface against a square (figure 3.21).  The spring should be rotated slowly to see if the top coil moves away from the blade of the square. Turn the spring Upside down and check again. If the spring is more than 2 mm out of square, or if it does not have correct compression at the specified length, then it should be discarded.
Weak springs that do not close the valve quickly enough will allow gas leaks and cause excessive valve head temperatures.  They could also cause excessive wear to the lifters or cams.
    Fig 3.21
  • Springs that are not square impart a sideways thrust to the valve-stem, causing excessive wear to the stems and guides.
Replacing valve-seals or springs
On some engines, the Valve-stems seals and springs can be removed and installed without removing the cylinder heads from the engine.
An adapter is fitted to the spark-plug hole and an air line connected.  Compressed air is then applied to the cylinder to hold the valve against its seat, and the special tool is used to depress the valve-spring (figure 3.22).  The method can be used with different types of valve- mechanisms, providing a suitable tool is available and the top of the valve-spring is accessible. With some OHC engines, camshaft would have to be removed.
Fig 3.22
To remove the valve-spring, the piston of the particular cylinder is brought to TDC on the compression stroke and air pressure is applied to the adapter and the spark-plug hole. The engine should be prevented from turning so that valve will rest on top of the system should air pressure be lost. 
  • If air pressure will not hold the valve on its seat, then a faulty valve or seat is indicated.
Servicing rocker arm assemblies
Where the rocker arms are mounted on a shaft, the rocker arms are moved as an assembly and then removed from the shaft. The arrangement of arms, springs and washers should be carefully checked and laid out in order.  All rocker arms are not the same shape.
On engines with independently mounted rocker arms are dismantled separated by removing the mounting stud that is threaded into the Cylinder-head.
After the rocker arms are removed, they should be examined for the wear or damage. Rocker arms with bushings can be re-bushed if they are worn. On some rocker arms, and that operates against the cam has a roller. Other rocker arms can be ground on the valve re-facer, but excessively worn rocker arms have to be replaced.

Servicing hydraulic lifters and lash adjusters
The valve train length can be checked on OHV engines that have been hydraulic valve-lifters.  The purpose of this is to see that the hydraulic lifter is able to operate properly.  With a good lifter, the plunger of the hydraulic lifter normally operates around the center position of the body.
Check is made as follows:
1.     The crankshaft is turned so that the lifter is on the low part of that cam.
Fig 3.23
2.     Special tool is used on the rocker arm to force the oil from the lifter so that the plunger bottoms in the body (figure 3.23).
3.     Feeler gauges are then used to check the clearance between that tip of the valve-stem and the rocker arm.
4.     If clearance is too small, the valve might not close properly.  If the clearance is too large, then the lifter could be noisy in operation.
If the valve-train has no adjustment and if the valve is noisy, the hydraulic valve-lifter will have to be dismantled and cleaned, or replaced.

Lifter problems
The most common problems with the valve-lifters are caused by dirt, gum, and varnish and air bubbles in the oil. Dirt and gum can cause the plunger to stick, or prevent the check valve from functioning.  The lifter will have to be removed from the engine, dismantled and cleaned.
Air in the lifter will compress and prevent the lifter from operating properly, but this can usually be corrected by bleeding.

Dismantling hydraulic lifters
Fig 3.24
A dismantled hydraulic lifter is shown in figure 3.24.  To dismantle, push-rod cup and plunger are depressed and the circlip removed.  This allows the push-rod cup, the plunger and the valve to be removed from the body of the lifter.
Observe the sequence of dismantling, as the assembly must be done in the reverse order.  Work on one lifter at a time.  The parts are not interchangeable and must not be mixed with other lifters.
Check the condition of the parts, including the roller, or if there is no roller, check the bottom of the lifter for wear.
If the parts are serviceable, then the parts can be cleaned distillate, reassembled and the lifter tested.
  • If the parts are scored or defective in any way, then the complete lifter should be renewed.
See Testing Hydraulic Lifters

Fig 3.19

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