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11th August 1972, Page 49
11th August 1972
Page 49
Page 49, 11th August 1972 — road and workshop
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by Handyman

Hydraulic tappet troubles (2)

WHAT are the factors that can upset hydraulic, apart from the possibility of hydraulic jack-up, from excessively high revs? As this design is intended to provide valve train silence together with correct adjustment at all times, when noise occurs something must be wrong and the damping effect of the oil will be absent.

Apart from material worn away at the valve lever, stem or push-rod the main reason for valve noise must point to loss of pressure in the plunger assembly and there are a number of reasons for this. Excessive wear or clearance between plunger and body can permit the oil to escape without any pressure build-up at all; in other words, the oil pump is not setting up pressure to carry the load.

However, the fault may not be in the plunger assembly at all, as should the engine be seriously short of oil in the sump, the oil pump may begin to draw air into the system, and, of course, air is compressible and will not lift the valve, other than noisily as the slack is taken up completely.

Check sump oil The first steps then should be to check the sump oil level and ensure that this is correct; it is also possible to have the sump too full and con-rods can be touching the surface of the oil, thrashing it into foam and this will not help to operate the hydraulic lifter.

When all the tappets have suddenly become noisy this usually indicates that the fault is elsewhere in the system but is affecting the oil-pressure side. Therefore check the engine oil pressure at a point apart from the gauge or pressure switch connection and establish if the gallery pressure is as it should be or is partly obstructed. If there is a failure of a tappet to operate owing to high leakage, the engine valve is not going to operate and there will be a certain misfire.

Dirt and carbon has been known to jam the plunger in the lower position, or cause the check valve to leak heavily, the result being that there is again noise from the tappet and misfire. To locate a particular tappet that has become noisy but is not quickly found, it is best to idle the engine slowly and use a feeler gauge blade between valve stem and lever — it may need 0.025in. or more to quieten down the offending tappet, although feeling along the springs by hand will usually draw attention to the offender by the-roughness felt in the spring. Should you need to remove the hydraulic tappet, it is usual to find a snap ring fitted in the top of the main tappet shell. This ring retains the plunger, and if you find yourself with much of this kind of work, a special tool will pull the lifter up through the push-rod bore hole.

The important points to bear in mind with this particular work is that the parts are usually a selective fit and should not be mixed about; cleanliness is most important on the bench, as the most minute particle of grit can either score the barrel or plunger, or cause it to jam solid. Where trouble and some stress has been exerted on the tappet assembly, it is as well to check the push-rods and make certain that they are straight and true.

When the parts are on the bench and well washed, it pays to search the inside of the tappet barrel and outside of the plunger for any scores or scratches and if these do exist it is best to replace the complete tappet. A good test for a possible bent push-rod is to roll it flat on a surface plate; it will wobble if it is bent, and if a bend is there then the rod can bend further under load and may be completely unreliable under full-power conditions.

After overhaul and assembly, some adjustment may be called for, and on the "Chevy" and some of the Fords the rocker holding-down bolt nut takes care of this, so ease back the nut and screw down the bolt until all movement is gone.

Change the oil

With the hydraulically operated tappets it pays to change the oil at the scheduled intervals and not stretch this duty out too far, as cleanliness inside ensures long life for these parts and reduces the risk of lost engine tune from lazy operation. As the engine builds up mileage and more carbon forms it is wise to suitably shorten the oil change period.

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