A torque wrench is an indispensable item in a professional mechanic's toolbox. And if you do much work on your car, it should be part of your tool collection, too.

A torque wrench lets you determine exactly how tight a nut or bolt is. That's important. If you overtighten a bolt, it may shear in two. If you undertighten it, it may loosen from vibration.

This is not to say you must use a torque wrench every time you tighten a nut or bolt on your buggy. Good old common sense will tell you how tight most should be. But if cylinder head bolts, for example, are not tightened toe the specifications recommended in the service manual for your car, you can end up with a warped head or water leaking into the combution chambers.

How do you know which nuts and bolts require accurate torqueing? Your service manual will tell you. For example, the tightening specs on a popular 440-cubic-inch Chrysler engine call for: cylinder head bolt - 70 foot-pounds; oil drain plug - 20 foot-pounds; spark plugs - 30 foot-pounds.

For the Saturday mechanic the beamtype torque wrench is not only the most economical but easiest to use. As you tighten a nut or bolt, a pointer moves along a scale indicating how many foot-pounds of twisting force you are applying. When doing more than one bolt, thighten the bolts in three increments. For example, if the maker of your particular car says the cylinder head bolts should be torqued at 90 foot-pounds, thighten each bolt to 30 pounds in the sequence recommended in your service manual. Then, in the same sequence, tighten each bolt to 60 pounds and, on the third pass, tighten each to the full 90 foot-pounds. This divides the stress evenly across the cylinder head, preventing warpage and leakage.

When using the torque wrench, apply pressure smoothly. Don't jerk it. The hand applying the pressure should be on the handle. Your other hand should rest lightly on the drive end of the wrench, just to keep the socket from slipping off the nut or bolt you're tightening.

Although the beam-type torque wrench is a precise measuring instrument, it can be kept in the toolbox with your regular tools. Just don't drop it or use it to tighten a bolt beyond its ability to measure.

The price tags on some torque wrenches are steep, but a suitable one can be found one for around $15. And by doing a little shopping, checking out sales at local stores with tool departments, you may find one hovering around the $9 or $10 mark.