The torque wrench is probably the most often-used specialty tool. If you’re like us, you may still have your first torque wrench; we got our Craftsman ½-inch drive clicker-style as a Christmas present in 1972. It is the oldest tool in our toolbox. But veteran status might not necessarily mean it’s accurate. We contacted Cornerstone Metrology in Van Nuys, California, a company that does calibration work for both industrial and individual mechanics. We needed a highly accurate test procedure to measure our torque wrenches. Director of Quality Keith Chauvie showed us the digital scale that was accurate to within 0.10 of a pound, which suited our test just fine since we weren’t going to be working on the Mars rover.?>
We originally gathered a small collection of standard ½-inch drive clicker torque wrenches, including tools from Snap-on, Craftsman, and Harbor Freight. Then we discovered a digital torque adapter sold by Harbor Freight. This small unit fits in between a standard ½-inch drive breaker bar and socket. Using a digital strain gauge, it converts the torque applied through the adapter into a digital readout. At a typical ridiculous Harbor Freight price of $39.99, we decided to include it in our test. Frankly, we didn’t expect this little unit to be very accurate. But testing proved otherwise.
We ran the torque adapter through five consecutive applications of 70 lb-ft of torque to watch it hit within 0.10 of a lb-ft twice and actually hit exactly 70 lb-ft on one occasion. The average of the five tests was an amazing 70.1 lb-ft. The unit offers a digital readout and warning lights, and it even emits a progressive electronic beep when nearing the pre-selected torque.?>
Next, Chauvie suggested an even more interesting test. He offered to keep our Harbor Freight ½-inch clicker wrench and test it every 30 days for two months while keeping the wrench loaded at 70 lb-ft. He said the problem with clicker torque wrenches is, when the mechanic fails to return the preload to its lowest setting after each use, the tool begins to lose accuracy. Chauvie says the longer the wrench is loaded, the less accurate it will become. He has since reported on the torque wrench’s performance at 30 days and 60 days, and we’ve included a graph to show the results. This inaccuracy occurs because the internal spring loses its tension, allowing the wrench to break away (click) at a lower torque value. Chauvie says this is a common problem for all types of clicker torque wrenches. The point here is that you should always return your clicker torque wrench to the base setting before storing the wrench. This simple step will maintain the wrench’s accuracy for years instead of mere months.
Harbor Freight 1⁄2-inch clicker...239 $24.99
Harbor Freight 3⁄8-inch clicker...807 $14.99
Harbor Freight 1⁄4-inch clicker...2696 $19.99
Harbor Freight Digital Torque Adapter...68283 $39.99
Craftsman digital 1⁄2-inch...47712 $239.99
Craftsman 1⁄2-inch clicker...31423 $79.99
Snap-on click 1⁄2-inch...QD3R150 $323.00