Under The Bonnet: Torque Specifications

While not all of us were born with a wrench in our hand, we all can learn to work on our cars and be able to have confidence in the quality of the work. But such skill does not come easily nor quickly; it comes only through study and experience. Factory workshop manuals are written for experienced and knowledgeable mechanics, and therefore explain what to do, but not always how to do it. Even many after-market do-it-yourself manuals do not explain all the details, the tricks-of-the-trade. In great enough detail to give beginners the confidence to tackle what is, to them, major work. It is this information gap that prompted this column. While much of what will be written here is old hat to many, the topics addressed are those which have been the cause of recurrent questions or problems, not only with our customers, but also within Moss Motors. Topics being considered include dealing with pressed-in bushes, measuring and instruments used for measuring, rebuilding hydraulic cylinders, stock versus competition parts, changing brake pads, and the whys and wherefores of ballast resistors. If there is a topic you would like to see discussed here, please send your suggestions to:

Under the Bonnet, c/o Moss Motoring. 400 Rutherford St.. Goleta. CA 93117.

We are breaking the ice with some thoughts on torquing fasteners. Tightening by feel almost always results in over-tightening; while specified torque values often feel wimpy, believe in them! On the other hand, too little torque can be as bad as too much. If you are dealing with an older British workshop manual that gives torque specifications in “lb. in.”, otherwise known as inch-pounds, simply divide by 12 to arrive at the figure in foot-pounds. (Inch-pounds are normally now used only for light requirements for which foot-pounds is too coarse a measure.)

Ok, so what is a “foot-pound”anyway? It is a physical measurement of torsion, or the force required to cause or resist rotation. As far as we are concerned here, a foot-pound may be thought of as simply the torque of a one pound weight at a distance of one foot. It can also be twelve pounds at a distance of one inch, one-half pound at a distance of two feet, or any similar ratio of weight versus distance.

You may check the calibration of your torque wrench in your shop or garage if you have a well-secured bench vice. Clamp the square drive end of the torque wrench in the jaws of a vice so that the handle of the wrench is horizontal. Use a tape measure to find a spot on the handle 12” from the center of the square drive. Suspend a weight from this point—20 to 50 pounds will do nicely. The reading of the wrench should correspond to the weight suspended from the handle.

Torque specifications are established by the engineers who designed the particular component, based on material strength, clamping force requirements, and other factors. It is not at all uncommon to find bolts of the same size on a car which have quite different torque specifications. In the first place, it is essential to understand how a bolt and nut work to hold something together. When you tighten a cylinder head nut, you’re not just drawing it up tight against the surface of the cylinder head. As the nut is tightened, the stud stretches. The natural tendency of the stud is to try to return to its original length, and this is the pull that produces the clamping action that holds the cylinder head tightly to the block.

Torque based on specifications are the material and the size of the stud or bolt. Normally, a bolt will be torqued to approximately 75%of the calculated yield strength of the bolt. This gives you the most “clamping action” for a given bolt, with a safety margin. For example: a bolt 3/8th” In diameter (24 threads per inch) SAE grade 5 has a material tensile strength of 120,000 lbs per square inch. The cross sectional area of this bolt is 0.0878 square inches, so the tensile strength of this bolt is 0.0878 x 120,000 lbs. per square inch=10,563 lbs. Normal torque specs for this bolt would be 34 foot-pounds, which put a load of about 7900 pounds on the bolt, which is about 75% of the load the bolt should be able to carry without breaking.

Thanks to Eric Wilhelm of our Research Department, and Sales Manager, Michael Grant, for information on torque specs.