Torque Wrench Myths - Matters Of Precision

Digital Wrenches Aren’t as Accurate as Old-School Mechanical Wrenches

But that doesn’t make them any more accurate than the most modern evolution of digital torque wrenches. Snap-on’s TechAngle line of torque wrenches, for example, are all electronic and incredibly precise. They are capable of reading to a tenth of a pound of force, while most mechanical wrenches only read in one-pound increments. The TechAngle wrenches also ship with a Certificate of Calibration printed specifically for your wrench. For the wrench that we tested, no measurement was off by more than one percent (on a range between 50 and 250 ft-lbs) and most were less than one-half of one percent in variance.

Some mechanical torque wrenches use a beam scale or a dial indicator to indicate the torque. While accurate, these can also lead to user error because of a phenomenon known as “parallax distortion.” That’s a fancy way of saying “you ain’t looking at it straight.”

Parallax distortion happens when the viewer is observing the gauge at an angle. For example, imagine a needle on a round dial gauge pointed directly over the five on a scale of one to 10. Looking at the gauge directly head-on, the needle is obviously pointing at the five, but if you move your head to the side in one direction it may look like it’s over the four (like in the picture to the left), and if you move your head in the other direction far enough the needle will look like it’s instead pointing at the six. This can be a real problem with torque wrenches that use an analog gauge, especially if you’re working in tight confines in your race car and can’t position your head directly over the gauge.

This isn’t an issue with click-type wrenches or those with digital gauges. But some wrenches with a digital readout have another advantage. The Snap-on TechAngle we tested will actually read back the maximum torque setting after the pull so that you can tell exactly how much torque has been applied.

A Torque Wrench Should Never be Used to Loosen Fasteners

This one is the truth. You might argue that if a torque wrench can handle tightening bolts to 250 ft-lbs, it should be able to reliably handle loosening that same bolt. And this is true, but when it comes to loosening a fastener you don’t always know what’s going to be required to get it loose.

After several heat cycles and the accumulation of dirt and grime that can coat everything in a race car, a bolt can gall against the threads and require a lot more torque to remove than was required to originally install. Most of us don’t watch the torque when loosening bolts, so it’s quite possible to exceed the maximum torque loading of a wrench when breaking bolts loose. The wrench will still function in terms of tightening or loosening bolts, but by exceeding its maximum torque limit loosening that tight bolt may have thrown off the calibration.

Adaptors Can Affect Your Readings

Yes and no. Whether you use a standard or deep socket isn’t an issue. Neither are adaptors to fit, for example, a 3⁄8-inch drive socket on ¼-inch drive ratchet or even short extensions. But anything that moves the head, or pivoting point, of the torque wrench away from directly vertical to the fastener being torqued can affect your reading.

This includes wobble extensions, or crow’s feet. Crow’s feet extensions are necessary on some Chevy SB2 heads and other race-only cylinder heads that put a priority on port location and performance over ease of installation. If you find yourself in a situation where you must use some type of horizontal extension on your torque wrench, consult the manufacturer to see if it offers any means of compensating for the change in leverage.

Wrenches Can’t Measure in 5-Pound Increments or Less

For example, cylinder head bolts are normally torqued in three stages to help improve a consistent clamping load across the cylinder head. Conventional thinking says that these stages should be 10 to 15 ft-lbs apart. So, if you’re installing heads on a Chevy small-block and the final torque value is 70 ft-lbs, then the bolts should be torqued to 40, then 55, and finally 70 ft-lbs.

Smaller increments can often lead to incorrect readings. For example, it’s almost always a bad idea to take a bolt that has already been tightened to 65 ft-lbs and take it to 70. This is true, and if you find yourself in this situation, you should loosen the bolt and then retighten it to 70 ft-lbs.

Many people think the reason is because a torque wrench isn’t able to measure the 5-pound difference in an already tightened bolt, but this is untrue. The truth is that a bolt that has already been tightened will usually have a breaking point (the amount of pressure that’s required to get it moving again) that’s higher than the amount that it has originally been torqued. So, for a bolt that has already been tightened to 65 ft-lbs, the force required to get it moving again could be 5 ft-lbs or more on top of the original 65.

The lesson here is that although your torque wrench is perfectly capable of reading the difference between 65 and 70 ft-lbs, if you ever need to change the torque on an already tightened bolt by less than 15 ft-lbs, first loosen the bolt so that it’s in a relaxed state and start over.

With these myths now busted you’re ready for years of reliable use from your torque wrench.