How to Calculate FT Lbs. of Torque

For certain applications, such as bolting on a cylinder head or putting wheels on, you will need to make sure that the fastener you are tightening is installed using the proper FT pounds (foot pounds) of torque, which can be found in the shop manual pertaining to the specific model of car you are working on. There is really only one sure way to be certain that the torque you are applying falls within specifications, and that is through the proper use of a torque wrench.

Things You'll Need

• Torque wrench

Instructions

Using a Torque Wrench

1. Ensure that the fastener threads are clean and familiarize yourself with the manufacturer’s torque patterns (e.g., in what order you should tighten the fasteners).

2. Set the torque wrench so that the indicator on the wrench reads half the desired FT pounds of torque. When doing this, always look straight down at the scale because, when viewed from an angle, the scale is easy to read incorrectly.

3. Turn the torque wrench, with the appropriately-sized socket for the job, to tighten the fastener with a slow and steady even pull on the handle until the torque wrench “clicks.” Never use short jerky motions. Repeat this step using a setting of 3/4s of the desired FT pounds of torque and then twice more with the torque wrench set to the full amount of FT pounds desired.

Tips & Warnings

• Swivel joints can affect the accuracy of the torque wrench so only use them if there is no other way to reach the fastener you are tightening.

• If the manufacturer’s recommended torque values for your application are not available, consult a general torque value chart.

• If you don’t have information regarding the manufacturer’s torque patterns, tighten the fasteners in a criss-cross pattern.

• When tightening fasteners with a torque wrench, use the handle only.

• For some applications, the manufacturer may recommend using new bolts to accommodate a “torque-to-yield” process, in which the bolt is tightened to specifications and then is turned an additional specified number of degrees.

• Torque stretch can be used to discover weaknesses in a bolt. Generally, only specialized mechanics, such as those building race cars, ever need to use this technique, which involves measuring the length of the bolt with a micrometer. Bolt stretch indicates weakness, and a surplus of stretch may indicate a torque problem due to problems with the threads.

References

• "Modern Automotive Technology;" James E. Duffy; 1998