Although most engines have hydraulic valve lifters requiring no periodic maintenance or adjustment, there are lots of exceptions, including many late-model Japanese cars. There are two adjustments in use on overhead-cam engines: replaceable shims or rockers with adjusting screws. Extracting the old shim and installing one that's thicker or thinner requires special tools and assorted shims.

An annual check of valve clearance is good practice and is typically recommended maintenance. With an overhead-camshaft engine, remove the valve cover, turn the engine with a wrench on the crankshaft pulley bolt until you see that the base circle of the camshaft (opposite the lobe that opens the valve) is against the rocker arm of valve lifter. At that point, the valve is fully closed, and you can check clearance between valve stem and rocker tip with a feeler gauge.

The specified feeler should slide in and out with light-to-moderate drag. If clearance is too great, the valve opens late and closes early, reducing engine performance. If it's too little, the valve opens early and closes late, spending too little time on its seat, which is when it transfers heat to the cooling system. Results: The valve runs hot and burns prematurely.

To reset valve clearance, loosen the locknut, and turn the adjusting screw until the feeler drags. Then hold the screw while you tighten the locknut. Invariably, the clearance changes as the locknut tightens, so you'll have to try it until the gauge just glides out once the nut is snug.