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How-To Calibrate A Torque Wrench
By Sleepy Gomez, stockcarracing.com

A Simple Way To Measure Torque Wrench Accuracy

The use of a torque wrench in the assembly of an engine, as well as other parts of a race car, is necessary for the correct tightening of nuts and bolts. All threaded fasteners have a torque value. Materials and designs are different, but the manufacturer has designated a tightening torque for each.

Under-tightening does not provide proper thread loading. Over-tightening can stretch a fastener beyond its ability to recover. This weakening can cause the fastener to fail prematurely.

You should use a torque wrench to achieve the correct tightening torque on any given fastener. But what if your torque wrench is inaccurate? Do you have any idea if it is? If a torque wrench doesn't tell you the truth, how do you properly assemble an engine?

The answer is that a torque wrench must be checked periodically.

Torque Lab

One business that does exactly that is Calibration Specialities of Irving, Texas. The decision-maker there is Rick Heck. During a recent tour of the company, Heck showed me the lab where they test, repair, and adjust torque wrenches, among other things. There were several workstations and devices for testing from the largest to the smallest torque wrench. There were hundreds of pigeon holes filled with small parts for any brand of torque wrench. I watched as a wrench was disassembled. A few small parts were carefully changed and it went back to the test bench where it then passed.

I asked Heck about the tolerance to which a wrench should be held. "A tolerance of less than +/- 4 percent for the older wrenches is considered acceptable," he said. "Newer-style wrenches run to about +/- 3 percent. These are the accepted norms."

What about the frequency at which wrenches are checked? "The airlines send theirs in about once a year," Heck said. "Some racing engine builders check theirs as often as every 30 days. They'll do it more often if a wrench has been dropped or mishandled. We check it and adjust its accuracy if necessary. The turnaround time is usually a week or so."

Heck says a torque wrench should be handled like the precision tool it really is. "Tossing them in the toolbox and digging them out occasionally is not the way to keep them accurate," he says.

"In order to properly test, we have to keep our equipment checked to a level of about one half of one percent of dead true," Heck says. "The professionals at the airlines and professional race engine builders all have their wrenches checked and adjusted as necessary."

MATERIALS LIST

  • 1 piece of 1-in pipe, 8-ft long
  • 2 pieces of 1-in square tubing,7-in long
  • 1 piece of 1-in x 1-in flat bar or other material for a spacer
  • 1 piece of 2-in angle, 5-in long
  • 1 piece of 13/4-in tubing, 2-in long
  • 1 5/8-in x 31/2-in bolt
  • 3 5/8-in nuts
  • 2 6202 ball bearings
  • 1 package of J-B Weld
  • 1 1/4-in or 1/2-in wide steel tape
  • 1 gallon jug of water
  • 25 pounds of balance weight

Check It Yourself

The 13/4-inch diameter tube has already been welded to the angle. A 7-inch long square tube is then welded on as shown. The individual racer working in his home shop may not have $125 or so to spend for periodic calibration by a professional testing lab. But now there is a way to do it yourself. It requires about $30 worth of materials and it can be built in a few hours.

The 5/8-inch bolt is shown with the two bearings and the 1-inch-long spacer. The spacer can be of any material. It only holds the bearings apart until the J-B Weld is set. I'll walk you through the process. You can build your torque wrench checker from whatever materials you have available. My materials list, which follows, contains things easily obtainable or plentiful in my shop.

Building the torque wrench checker is not difficult. In addition to the materials, you will need a welder. You will need a way to cut steel--so either a torch, band saw, cut-off wheel, or diamond-coated beaver teeth will do.

I purchased the 6202 ball bearings from a local auto parts store. They were selected because a 5/8-inch bolt is a close fit in the bore. There is no machine work required other than sawing parts to length. Follow the photos for the building sequence.


The materials needed to build your own torque wrench tester. Only a welder and a metal cutting saw are needed for construction. Don't laugh at the water bottle; it is a known weight.

The 13/4-inch diameter tube has already been welded to the angle. A 7-inch long square tube is then welded on as shown.

The 5/8-inch bolt is shown with the two bearings and the 1-inch-long spacer. The spacer can be of any material. It only holds the bearings apart until the J-B Weld is set.

The bearings are coated on the outside with J-B Weld and slipped into the tube. Don't let the J-B Weld run down on the bolt. Leave the bolt in place to keep the bearings located. Clean up wet J-B Weld or other epoxies with alcohol.

The last 7-inch piece of square tubing is welded across the end of the first. It should be about 3/8-inch above the surface of the angle so the beam will lay flat on it.

This is the heart of the accuracy of this unit. Purchase a gallon water jug at the grocery store. Take it to the meat department. Ask them to weigh it and print out a label with the exact weight. Multiply that number times the number of feet from the pivot bolt. The answer will be the foot-pounds of torque required to lift the beam.

This is the finished working section of the checker. The angle is clamped in a vise. The bolt is ready to be screwed into the nut on the beam. A steel tape is tie wrapped to the tubing. This shows the way it should be extended when ready for use. When the beam rises, the tape will audibly click back against the tube.

The 8-foot-long beam has a 5/8-inch nut welded to it 18-inch from one end. This will attach to the pivot bolt. A lock nut must be used against the beam nut. The photo shows the layout of the beam marks. For maximum accuracy, start with the 12-inch mark at the center of the bolt. Then mark off each foot toward the long end.

Operation

Once complete, attach the balance weight to the short end of the beam. You will need about 25 pounds. I used a set of clamp-on race car weights from A&A Manufacturing. Move the ballast until the beam comes in contact with the attached steel tape. This balance is critical to the accuracy of the tester.

Now suspend the water bottle on one of the marks on the beam. I usually test at 3 feet and 6 feet to get a low and high check. With the end of the steel tape extended under the beam, attach the appropriate socket and place the wrench on the end of the pivot bolt. With a slow, even pull, the same way you would when tightening a bolt, apply pressure to the wrench. Watch or feel when the wrench reaches a specific torque. If you are testing at 3 feet and the water weighs 8 pounds, then the beam should lift when the wrench reads 24 pounds. You will hear the steel tape snap back when the beam lifts off. Higher or lower readings indicate the amount of error.

The bearings are coated on the outside with J-B Weld and slipped into the tube. Don't let the J-B Weld run down on the bolt. Leave the bolt in place to keep the bearings located. Clean up wet J-B Weld or other epoxies with alcohol.

The last 7-inch piece of square tubing is welded across the end of the first. It should be about 3/8-inch above the surface of the angle so the beam will lay flat on it.

This is the heart of the accuracy of this unit. Purchase a gallon water jug at the grocery store. Take it to the meat department. Ask them to weigh it and print out a label with the exact weight. Multiply that number times the number of feet from the pivot bolt. The answer will be the foot-pounds of torque required to lift the beam.

This is the finished working section of the checker. The angle is clamped in a vise. The bolt is ready to be screwed into the nut on the beam. A steel tape is tie wrapped to the tubing. This shows the way it should be extended when ready for use. When the beam rises, the tape will audibly click back against the tube.

The 8-foot-long beam has a 5/8-inch nut welded to it 18-inch from one end. This will attach to the pivot bolt. A lock nut must be used against the beam nut. The photo shows the layout of the beam marks. For maximum accuracy, start with the 12-inch mark at the center of the bolt. Then mark off each foot toward the long end.

The Results

Only at a professional testing facility such as Calibration Specialties can a torque wrench be adjusted to correct inaccuracy. However, for the individual racer who knows a wrench is off by say 9 percent will be able to compensate for that error by adding or subtracting that amount.

I tested three of my own torque wrenches: a 1/2-inch drive beam type, a 3/8-inch drive beam type, and a 1/2-inch drive "clicker" type. The results are in the chart below. Remember +/- 4 percent is the outer limit of acceptable error. I suggest checking your wrench at the values it will be used, for instance at the recommended torque for rod bolts, main bolts, and head bolts. Use a calculator to predetermine the amount of error to add or subtract. Then use the corrected number when tightening fasteners.

Loose race cars may be fast but tight bolts keep them that way!

Results Of Sample Torque Wrenches Checked

Wrench TypeCORRECT TORQUECHECKED TORQUEERROR
1/2" Drive Beam TypeLow 26 ft.-lb./High 61 ft.-lb28 ft.-lb./64 ft.-lb.7.7%/4.9%
1/2" Driv Clicker TypeLow 26 ft.-lb./High 61 ft.-lb29 ft.-lb./66 ft.-lb.11.5%/8.2%
3/8" Drive Beam Type313 in.-lb.328 in.-lb.4.8%

*Acceptable error +/- 4%

You can contact Sleepy at: sleepy.gomez@primedia.com, or 9036 Brittany Way, Tampa, FL, 33619.

Article originally located Here.

In memory of Speedshop  LP Privateer  LP Racing  Aftershocks  Pirelli

March 27, 2007