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What is a "valve lash adjustment"?

Dear Tom and Ray:

In my several years of listening to your radio show and reading your column, I have never seen you address a mysterious item that keeps appearing on Chrysler's "Scheduled Maintenance Service Bulletins" for my 1986 Dodge Colt. It says "check valve lash adjustment and correct." I once got an estimate for what it would cost and was shocked! What is a valve lash adjustment? What will happen if I don't do it? And how much (ball park figure) should it cost?

RAY: A "valve lash adjustment" is just another term for a valve adjustment, Cynthia. And it's required on all cars that have adjustable valves. There are fewer and fewer cars that fall into this category, but yours happens to be one of them.

TOM: The "valve lash" is the space between the valve stem and the rocker arm. The rocker arm is what strikes the valve stem and pushes the valve open. And for the engine to run properly, there has to be just the right amount of space between the two.

RAY: If there's too much space there, the rocker arms will slap against the valve stems, and you'll get the classic "click and clack" tappet noise--which, by the way, is the annoying noise from which our nicknames (Click and Clack the Tappet Brothers) were aptly derived.

TOM: On the other hand, if there's too little space between the rocker arm and the valve stem, the two will touch, and won't allow the valves to close completely. Eventually, that will burn out the valves.

RAY: So a valve adjustment is important, Cynthia, and it should be done every 30,000 miles. I don't know why you were shocked at the price. It shouldn't cost you more than $50 or $100 to have the valves adjusted and the gasket replaced. So my guess is that either your mechanic had a big boat payment due when you asked for your estimate, or it doesn't take much to shock you, Cynthia.
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