Digital Wrenches Aren’t as Accurate as Old-School Mechanical Wrenches
The newest generation of torque...
The newest generation of torque wrenches are more accurate than ever before. Most use a strain gauge inside the wrench (sort of like an engine dyno), and wrenches like this one from Snap-on can measure down to one tenth of a ft-lb.
This one isn’t true, and in fact, the truth may be quite the opposite depending on conditions. Mechanical wrenches normally depend on either the ball detent mechanism mentioned earlier or a beam that flexes as more torque is applied to the bolt. They are relatively simple mechanisms and dependable.
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.
If you also happen to work...
If you also happen to work on street engines that employ torque-to-yield bolts, Snap-on’s TechAngle wrench also incorporates an angle meter in the wrench. Torque-to-yield bolts require you to reach a certain torque setting and then twist the bolt an additional certain number of degrees. This used to require a cumbersome dial contraption, but a gyroscope inside the TechAngle wrench does all the work for you.
Crow’s feet extensions or...
Crow’s feet extensions or wobble-head sockets can change the torque value at the bolt. Some manufacturers will provide you with the necessary information (like you see here from Snap-on) to correct for this so that you’ll be able to properly torque any bolts even under these conditions.
This Dodge cylinder head with...
This Dodge cylinder head with its downward-facing head bolt is an example of where a crow’s foot would be necessary.
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
One more advantage with the...
One more advantage with the new breed of digital torque wrenches is that some can read back the actual torque after you’ve completed your pull. Here, you can see I’ve overshot my 70 ft-lb mark by 1.3 pounds. Knowing this can help the user adjust his style so that he is able to make a slow consistent pull and stop as soon as he feels the wrench buzz in his hand.
The answer is this statement is incorrect, but it still isn’t a good idea to try to increase the torque on an already tightened bolt by 5 pounds 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.