A good torque wrench is critical...
A good torque wrench is critical to properly installing many important fasteners in your race engine as well as your chassis. Make sure you’re using yours properly to get the best results.
There’s more to the nuts and bolts fastening the most critical components on your race car than simply getting them “tight enough.” Bolts don’t work by simply clamping down on something thanks to the pressure you apply twisting the nut. The steel can actually stretch a few thousandths of an inch. It’s the elasticity of the steel trying to pull itself back to its original length that allows a bolt to provide a consistent clamping load. Plus, this constant pulling force that remains long after the wrench has been removed helps keep the nut from coming loose under the vibrations that come from racing.
So if you don’t stretch a bolt enough when tightening the fastener, you won’t get that “pulling” or retracting force on the nut. But if you apply too much force, the bolt will fail. The solution to making sure your most critical fasteners are tightened properly is to use a torque wrench.
In its most common form, a torque wrench simply measures the amount of twisting force, or “torque,” that is required to twist a nut or bolt as it’s tightened. It’sn’t a perfect system because changes in friction can change your torque reading, while actual bolt stretch isn’t affected by friction, but measuring torque is usually the only way to measure the amount of force being placed on a bolt or stud.
That’s why a reliable torque wrench is one of the most valuable tools in any mechanic’s toolbox. It does a job that other handtools can’t, and because of that, mechanics and engine builders place a lot of trust in their torque wrench of choice. Maybe that’s why there are so many myths surrounding these wrenches. Everything from how to store them to how they are best used.
For this article we discussed torque wrench technology and the best use practices for these specialized wrenches with Dan Eggert, Hand Tools Engineer with Snap-on Tools, one of the most trusted tool makers in racing, and Chris Raschke with ARP, which manufactures high-end fasteners and has done as much research and development on how torque affects fasteners as any company in the racing industry. We ran several of the most widely held myths past them to get not only their answers, but the reasons why.
A Torque Wrench That’s Been Dropped Can’t be Trusted
In order to be most accurate,...
In order to be most accurate, a torque wrench needs a slow, steady pull up to the desired torque. Some bolts, like those securing the main caps, require a lot of torque. Make sure you have the block secured and you’re in a position to apply the final torque in a steady pull without jerking or tugging the wrench.
It’s a fact of life that stuff gets dropped, knocked off workbenches, and generally treated to many different forms of abuse that they may or may not be able to handle. Knocking your ½-inch box-end wrench off the workbench is no big deal, but a torque wrench is a precision piece of equipment and should be treated as such.
One accidental drop to the shop floor probably isn’t a big deal, but repeated drops can affect the wrench’s calibration. This is true even if it doesn’t show signs of damage. The easiest way to check your torque wrench is to check it against another wrench. Torque a bolt with one wrench and then try to torque the same bolt with the second wrench on the same torque setting. If the second wrench pulls the bolt down farther before marking the same amount of torque, you know one is off. Then repeat the same process with the order of the wrenches reversed.
Of course, this isn’t exactly the most scientific method. Even if you have never dropped or otherwise abused your torque wrench, it’s always a good idea to have it professional checked regularly. Professional shops normally have several wrenches and send them in for professional calibration every six months or so. If you use your wrench regularly, you should consider sending it in probably about once a year—or sooner if you suspect yours isn’t working properly.
One of the advantages of purchasing a wrench from a professional tools company like Snap-on is that it has services like this in house. Eggert says Snap-on actually has facilities across the country that can check and recalibrate its torque wrenches. All you have to do is hand your wrench over to the Snap-on truck driver whenever he visits your shop (or you can flag him down) and he’ll take care of it for you. Simple as that.
The Wrench Should Always Be “Unloaded” Before Storage
A click-type wrench is popular...
A click-type wrench is popular and dependable. You set the amount of torque you want by twisting the handle of the wrench. This compresses a spring in the handle which increases the amount of torque required before a steel ball will pop out of its detent inside the wrench, giving you the familiar “click” to let you know you’ve reached your desired torque setting.
If you’re using a mechanical click-type wrench, this one turns out to be absolutely true. A click-type torque wrench presses a ball into a detent that’s held in place by a spring. The wrench is normally adjusted by twisting the handle on the wrench. Twisting it in compresses the spring and requires more torque to pop the ball out of the detent. This is how it measures torque.
But if the spring is stored with the wrench “loaded” or set for a high torque rating, the pressure on the spring can cause it to weaken over time. To protect the wrench it should always be returned to the lowest setting before storing it back into your toolbox. For big wrenches (measuring in ft-lbs increments) the lowest setting is usually 20 ft-lbs. If, for some reason, you have a click-type torque wrench that goes all the way down to zero, leave the wrench set to 10 or 20 pounds. You always want to keep a minimum amount of pressure on the spring so that the ball can’t fall all the way out of the detent.
For other styles of torque wrenches, this isn’t an issue. The newest digital torque wrenches use an electronic strain gauge to measure torque, so when the wrench is not in use the only thing you may need to do is remove the batteries so that there is no chance of corrosion ruining your expensive digital wrench.
A torque wrench can’t measure...
A torque wrench can’t measure friction. For most fasteners the final torque will depend on the type of lubricant used. Motor oil, for example, has a different friction coefficient than moly lube or ARP’s new Ultra Torque lubricant. Make sure your torque number takes into account the lubricant being used. You can get this type of information from fastener manufacturers like ARP.
A beam-type torque wrench...
A beam-type torque wrench measures the flex in the beam of the wrench as force is applied to a bolt to measure the torque. With a gauge like this it’s important you keep your head over the markings and not at an angle in order to get an accurate reading. It’s rare to see this type of torque wrench in a professional shop any more.