Sunday, May 29, 2011

Camshaft service

Fig. 3.43 (a)
The procedure for camshaft removal varies considerably with different engines. In OHV engines, the camshaft is mounted in the crankcase and is accessible from the front of the engine after removing the timing cover and timing chain.
The rocker arm assembly and push-rods also have to be removed, and the valve lifters must be removed or raised to clear the cams before the camshaft can be withdrawn through the front of the engine.
With OHC engines, the camshaft is more readily accessible on top of the cylinder head. Before the camshaft can be removed, the timing pulley or sprocket and the timing belt or chain is removed from the front of the camshaft. There is also a valve-operating mechanism that might have to be removed from the cylinder head.

The camshaft bearing caps have to be removed before the camshaft can be lifted from the lower half of its bearings. The bearing cap bolts should be released progressively and in a recommended sequence.
Fig. 3.43 (b)

Checking a camshaft
A camshaft can be checked for alignment by mounting it in V-blocks and using a dial gauge to cheek for eccentricity. Journal diameters should be checked with a micrometer, and the bearings with a telescopic gauge. The two dimensions can then be compared to determine whether bearings are worn (Figure 3.43).
With the camshaft removed, cam wear can be checked visually and also by measurement with a micrometer.

Figure 3.44 shows normal and abnormal wear patterns. If wear shows across the full width of the cam, and excessive wear of the cam lobe has taken place, a new camshaft should be installed. The valve lifter or other mechanism should also be checked.
Fig. 3.44

Checking cam Lift
Camshafts can also be checked while installed in the engine. Figure 3.45 shows how this is done on an OHV engine.
With the rocker assembly removed and the dial gauge positioned as shown, the crankshaft is turned slowly until the valve lifter is on the base of its cam. The dial gauge is set to zero and then the crankshaft is rotated until the lifter is on the top of the lobe. The reading is the cam lift. This can be compared with the other cams and with the engine’s specifications.
On OHC engines, the dial gauge can be mounted on the cylinder head so that its plunger is in direct contact with the cam, although rocker arms will have to be removed on some engines.

Camshaft bearings
Camshaft bearings of OHV engines can be removed from the engine block with a special bearing remover, but this is a fairly major operation.
The remover is a threaded puller bar that is inserted through the bearings. Puller sleeves that are slightly smaller than the bearing outside diameter arc installed on the bar behind the bearings. When the nut on the end of the puller bar is turned, the bearings are pulled from their bores in the crankcase. New bearings are installed with a similar method.
Camshaft bearings of OHC engines can be checked for clearance with plastigage, but they are not usually replaceable.

Timing gears, chains and sprockets
Gear and sprocket run out can be checked by mounting a dial gauge on the cylinder block or head, with the plunger resting on the side of the gear or sprocket.
Backlash between gears is measured by a dial gauge, or by inserting narrow feeler gauges between the meshing teeth. Excessive run out or backlash will require replacement of one or more gears.
Timing chains can be checked for tension and wear. Excessive slack, which cannot be corrected by adjustment, indicates a worn chain and possibly worn sprockets. Worn parts should be renewed.
Fig 3.46

Timing belt and pulleys
Timing belts should be checked for tension and adjustment. They should also be checked for condition as shown in Figure 3.46. If a timing belt is being removed and is to be reused, then an arrow should be chalked on the back of the belt. This is to show the direction in which it runs. When reinstalled, the belt must run in the same direction.
See Cylinder blocks, assembly and construction 

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