How to Calibrate a Torque Wrench

Any mechanic with a few years in the field will tell you how important it is to have a good working torque wrench in the tool box. They will also tell you how important it is to have the torque wrench calibrated by a shop with the proper equipment, and technical expertise. While there is no substitute for proper calibration, there is a way to determine the amount of error in the torque wrench, and make adjustments for the error.

Tools and Materials

• A torque wrench
• A bench vise
• A 20 pound weight
• Bailing wire or twine
• A tape measure
• A permanent marker

The weight must be verified to be exactly 20 lbs. This part is important.

Measure and Mark
On the back of the torque wrench, make a mark to correspond to the center of the square drive. Measure an even distance down the handle near where your hand would normally be during use, and make another mark. Record the measurement. For the purpose of demonstration, the measurement will be 24 inches.

Set the Wrench in the Vice
Clamp the head (square drive) in the bench vice, making sure that no other parts of the wrench contact the vice. Move the handle to a horizontal position. Set the torque value to the appropriate setting, for this demonstration ours will be 24 inches x 20 pounds, this equals 480 inch pounds, or 40 foot pounds.

Find the Point of Transition
Hang the weight along the line made in step 1. If you hear a click, lift the weight and move it in towards the square drive a little at a time until it stops clicking. Mark this point. If no click occurs, move the weight away from the square drive until it is heard, and mark here. When searching for the point of transition, move the weight in and out to insure that the exact point is found, making several attempts to check. Always lift the weight off the handle to move it, to insure accuracy.

Measure the Transition Point
Measure the distance from the square drive to the point of the click transition. This measurement is the second number of the ratio you will need. For demonstration it will be 26 inches. If the torque applied at 24 inches was 480 inch pounds, or 40 foot pounds, then the torque actually applied in the demonstration would be 26 inches x 20 pounds, or 520 inch pounds (50 foot pounds, 520 divided by 12).

Correct for the Difference
To make the proper correction, divide the ratio numbers. 24 divided by 26 equals .923. Multiply the amount of torque desired by this number. So, if you want 50 foot pounds applied, you would set the wrench at 46.15 foot pounds.

The actual mathematical equation looks like this: Ta=Ts x(D1/D2.) Ta is torque applied, Ts is torque setting, D1 is first distance, D2 second distance.

Following these simple calibration checking procedures will insure that no more lug nut, or head bolt tightening nightmares are ever a safety issue.

How to Use a Torque Wrench

A must-have tool for any serious home mechanic, a torque wrench can help you save a lot of time and money when tightening nuts and bolts on your vehicle. A torque wrench is very similar to a normal wrench, except it allows you to determine how much pressure is applied when tightening a particular nut or bolt.

Find Recommended the Torque Setting
Before using your torque wrench, look in a car repair manual for your vehicle to determine how much torque pressure should be applied to the nut or bolt you wish to tighten. For instance, a fly wheel gland nut is usually tightened to around 220 foot-pounds. Locate the torque foot-pounds setting for the nut or bolt you want to tighten before continuing.

Set the Torque Setting on the Wrench
Once you've found the appropriate torque foot-pounds setting for the nut or bolt, adjust the dial on the bottom of the torque wrench to the selected setting.

Tighten the Nut or Bolt
Now that you've set your torque wrench, you can begin tightening the bolt. Tighten the nut or bolt just as you would with any other normal wrench, until it "breaks," or makes a cracking noise. This is a signal that you should stop tightening the bolt, as it has been tightened to the adjusted setting.

Related Questions and Answers

How Many Different Types of Hand Torque Wrenches are there?

There are five basic types of hand torque wrenches available to the consumer. The first type was developed by Chrysler Corporation, and is called (single) beam. A second type is called deflecting beam, and differs by the torque being applied to a deflecting beam rather than a coil spring. The click type is very precise, and uses a ball-detent and spring as a pre-set slippage clutch. The next type is used by plumbers, and is called a no-hub wrench. It is factory set to slip, at a pre-determined torque, in order to tighten soil-pipe fittings. The last type is electronic, and incorporates a strain gauge to measure torque.

What are Insulated Torque Wrenches and What are they Used for?

The insulated torque wrench is used in situations where the possibility of shorting and/or shock exists. Torque wrenches are sometimes used to tighten bolts on electrical connectors, such as batteries. In addition to bolts on electrical panels, and other locations providing electricity, when precision bolt tightening is required. The design of the wrench segregates the user from the bolt carrying electricity. It incorporates a PVC insulation that is fitted by heat-shrink means, and non-conductive handle grips that are made from plastic. Moreover, insulated torque wrenches can provide insulation against voltages ranging from 50 volts up to 1,000 volts.

What are the Most Popular Torque Wrench Sizes for Auto Repairs?

Torque wrench sizes differ in actual wrench dimensions and the amount of torque they can measure. Some varieties of torque wrenches are calibrated, and measure torque in inch-pounds. While others are calibrated, and measure torque in foot-pounds. The drive, or the size of the anvil, determines the size of the sockets that can be used with that wrench. The common sizes for automotive use are 3/8 inch drive, and 1/2 inch drive. The dimensions of the wrench can range from 14 to 18 inches long. Of course, longer wrench lengths allow for greater amount of torque to be exerted.

What is a Torque Wrench?

People doing repairs on vehicles will often ask, "What is a torque wrench?". The complete answer is that it is a specialized wrench, which allows the user to be alerted visually and/or auditorially that the bolt has reached the desired torque. Simply put, it is a tool used by a mechanic to precisely tighten a bolt or nut. When vehicles are designed, the engineers have determined that certain nuts and bolts need to be tightened a specific amount in order to correctly secure parts together. Torque specifications are listed in most repair manuals, and should always be followed.

What is the Best Torque Wrench to Use for Auto Repairs?

The best torque wrench to use for auto repairs is a torque wrench that is calibrated in foot-pounds, can attain torque values of 150 foot-pounds, and clicks. This will be sufficient for most general automotive repairs the do-it-yourselfer would tackle. Moreover, the click type torque wrench is the most precise, and does not require you to read a gauge or scale while tightening. Additionally, unless dropped, this type of wrench rarely needs calibration. For very low torque requirements, a wrench calibrated in inch-pounds would be needed. However, unless it is vital these nuts/bolts be tightened with precision, hand tightening will suffice.