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How to Determine Torque Specs

by Anne Davis

Engine bolts must be torqued or bolted down to a certain weight specification under manufacturer guidelines to ensure engine stability. But not all manufacturers provide consumers with torque specifications, especially for after-market additions. Measuring torque for nuts and bolts is crucial to the longevity of your engine. Once you've calculated torque, use a torque wrench to fasten the bolts.

Determining Preload

Begin with the formula T = K x U x D x P, where T is the torque, K is a standard representing 1.33, U is the coefficient of friction, D is the diameter of the fasteners, and P is the necessary preload.

Find the diameter of your fastener. For the sake of calculations, we'll say that your diameter is 0.5 inches.

Find your friction coefficient. This value can vary, but you'd be safe to use about 0.2 for dry, or non-lubed fasteners and about 0.09 for wet, or lubed fasteners.

Determine the ultimate strength of your fastener by consulting your local hardware store. As a basic rule, preload, the value we need, should be about two-thirds, or 67 percent of the fastener's yield strength.

For the sake of calculations, we're going to use a Grade 8 fastener with a yield strength of 130 ksi, or 130,000 lb.-per-square-inch. The thread area of this bolt is one-half of an inch, giving it a size of 0.1599 square inches. The full yield strength is 130,000 lb-per-square inches multiplied by 0.1599 square inches, which totals 20,787 lb. Now, take about 67 percent of that, and its yield strength, or preload, is 13,927 lb.

Calculate Torque

Fill your values into the equation T = K x U x D x P.

Solve the equation T = 1.33 times 0.20 times 0.50 inches times 13,927 lb. The total is 1,852 inches lbs.RE

Reduce the value into ft-lb. You can do this by dividing your value by 12, the number of inches in a foot. The resulting value is 154 ft-lb., which is the amount of torque required to tighten the fastener.

Warning

  • Do not attempt to fasten bolts using your gut instincts for how tight they should be. If you are incorrect, the result could be thousands of dollars in necessary repairs.

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About the Author

Anne Davis writes pieces on domestic and international travel, automotive maintenance, education and health. She graduated with a Bachelor of Arts in English and history, and is pursuing graduate study in a related field.

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