Valve clearance adjustment

Valve clearance refers to the small gap between valve lifter and valve stem (or rocker arm and valve stem) that ensures that the valve completely closes, an expansion joint in the valve train.

Most engines have the valve clearance set by grinding the end of the valve stem during engine assembly, overhead cams not needing subsequent adjustment.

In earlier engines, mostly with push rods and rocker arms used adjustable tappets or hydraulic tappets to adjust for valve and cam wear.

Lack of valve clearance will prevent valve closure causing leakage and valve damage.

Valve clearance adjustment

Valve clearance measurement

Valve clearance is measured with the valve closed, typically at Top dead centre of the compression stroke. The tappet will be resting on the heel of the cam lobe. A feeler gauge must pass through the clearance space. The feeler gauge should fit in and out with a slight drag. If the feeler gauge will not fit in, then the clearance is too small. If the blade of the feeler gauge fits in too loose then the clearance is too big.

Valve clearance measurement

Valve clearance too wide

A too wide valve clearance will cause excessive wear to the camshaft and valve lifter contact areas, the pushrods can also bend and the engine will be noisy. Should the clearance become wide enough, valve timings will change resulting in poor performance.

Valve clearance too narrow

A narrow valve clearance will not allow for heat expansion and will result in the failure of the valve to close on its seat. The combustion chamber will not seal properly, producing poor compression and power. It is also possible that the valve can become hot enough to melt.

Adjusting valve clearance

Valve clearance adjustment must be performed to manufacturer's specifications. It is normal that the exhaust valve will have a larger clearance. Adjustment is performed by either adjusting the rocker arm or by placing shims between cam follower and valve stem. Most modern engines have Hydraulic lifters and require no adjustment.


Adjusting valve clearance

4strock - 4stroke atkinson cycle - 4stock valve clearance - 4strock otto cycle - 4strock valve train

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