How to Determine Torque Specs

by Anne Davis
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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

Step 1

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.

Step 2

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

Step 3

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

Step 1

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

Step 2

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.

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