Engine Valve Adjustment

Most internal combustion engines use poppet valves and rocker arms to control the flow of gases to and from the cylinders. To reduce the effects of thermal expansion, a small gap is left between the end of each valve stem and the rocker arm that pushes it. This gap will change slightly with engine usage and needs to be adjusted regularly to minimize noise and wear. Vehicles that require an engine valve adjustment should have it done every year or so. It is not a complicated task and it can be done in one hour with a spanner set, screwdriver and feeler gauge.

While most vehicles need an engine valve adjustment done every year, some do not need it done at all. Vehicles that have hydraulic lifters do not require any adjustment because each lifter maintain a zero clearance between its valve stem and rocker arm. The lifters are actually small cylinders filled with engine oil, with a piston at one end and a small hole at the other end. When the valve is closed, the hole is open and oil flows into the lifter. As the valve begins to open, the hole is shut and the incompressible oil is trapped, causing the lifter to behave like a solid rod.

Before performing an engine valve adjustment, leave the vehicle standing overnight so that the engine will be cold. Chock the tires and put the transmission in neutral so that the pistons can be moved. Open the engine hood and move any cables that are covering the valve covers. If a cable needs to be disconnected, make a note of the plug it was joined to before removing it. Next, remove the spark plugs to allow the crankshaft to be turned. It is difficult to overcome the compression in the cylinder with the spark plugs in place. Remove the bolts holding down each valve cover and slowly remove the covers, so as not to damage the gaskets sitting under them.

Find the top dead center (TDC) mark on the flywheel or crankshaft pulley, as well as its corresponding mark on the engine casing. Turn the engine over until these two marks line up so that the piston in the first cylinder will be at the top of its compression stroke. The engine can either be turned over by rotating the radiator fan or by turning the crankshaft with a long wrench. Refer to the service manual to determine the cold temperature gap and select a feeler gauge with the same thickness.

Insert the feeler gauge into the inlet valves of the first cylinder and check if there is a tight fit. If the gap needs adjusting, use a spanner to loosen the locknut on the rocker arm screw above it. Adjust the screw until there is a tight fit and tighten the locknut. Then check the gaps for any other cylinders that are also at the top of their compression stroke. Turn the engine over until the other pistons are in the correct position and adjust their valves accordingly.

Once all the intake and exhaust valve gaps have been checked, the valve covers and wires can be put back in place. Inspect the gasket on each valve cover and replace it if there is the slightest sign of damage. Clean the gasket surface and put the covers back put on. Tighten the cover bolts to their correct torque, as outlined in the service manual. Replace the spark plugs and any wires that were removed.

This is only a general description of how to perform an engine valve adjustment. It is not what is commonly referred to as a valve job, which involves grinding the valve discs. The service manual should always be consulted before attempting an engine valve adjustment, as there many be several minor but important differences that have not been covered. If there are any engine problems or unusual noises after performing an engine valve adjustment, take the vehicle to a professional mechanic.

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