Torque wrenches are as important to the diy auto mechanic as a stethoscope is to a doctor. If you’ll open your Hayne Repair Manual to the Engines Section you will find a subsection titled “Torque Specification.” This section gives you the torques to apply to a bolt in either inch-pounds or in foot-pounds for everything from an oil pan bolt to a main bearing cap bolt.
I’m certain that most of you have heard the word “torque” and know what it means, but, just so we are all starting out on the same page, in this case it refers to the twisting force that we apply to a bolt. Why is being able to tighten any given bolt to a specified torque important? Well, obviously it’s possible to snap a small bolt, like an oil pan bolt, off if you apply too much twisting force to it. Then you have to try and remove it by drilling and the use of an Easy-Out™ . It’s also possible, if you apply too much twisting force, to strip out the treads, especially with today’s use of aluminum engine components. Stripping the threads out in a bolt hole is even more of a headache then breaking a bolt off because you have to bore the hole out oversize and then rethread the hole for a bigger bolt or insert a thread insert for the original bolt size. Although these are the more obvious reasons why we should watch the amount of twisting force that we apply to a bolt there are some far less obvious reasons and far more important reasons why torquing to the proper number of inch-pounds or foot-pounds is important. Let’s use the connecting rod cap nuts as an example. For a particular 1.8L GM engine the rod cap nuts are supposed to be torqued to 35 to 37 ft. lbs. (foot pounds). For proper lubrication the connecting rod to crankshaft journal should be between 0.0035” and 0.0125”. Proper clearance depends on the rod end caps being torqued down properly. Over tightening reduces the clearance and prevents proper lubrication of the bearing and crankshaft journal increasing bearing and journal wear and, if over tightened sufficiently, possible bearing to journal seizure. Under tightening reduces oil pressure which in turn prevents the proper lubrication of all the engines moving parts. Uneven tightening of the two rod cap nuts can also cause the rod cap to become distorted which can lead to damaged crankshaft journals and/or broken connection rods.. Aluminum engine head are another good example, if the bolts securing the heads to the block aren’t torqued down using the right sequence and torque they can become distorted. Distorted heads can cause the engine to loose compression, burn and/or leak oil, loose coolant, and/or get coolant in the oil. All of these things spell disaster and major engine overhauls. Considering what a major engine rebuild will cost you, even if you do all the work yourself, what a top quality torque wrench will cost you is minuscule in contrast.
There are basically two types of torque wrenches, “Beam” and “Click” torque wrenches. A beam torque wrench is the least expensive of the two but the hardest of the two to use. With a beam torque wrench, accuracy depends on your eyesight because you have to read the torque on a scale attached to the beam. The click torque wrench is more expensive and by far the most accurate. With a click torque wrench you dial in the number of inch-pounds or the number of foot-pounds torque you want to tighten to and when that torque is reached the wrench clicks. The clicking is caused by the wrenches internal clutch mechanism which slips when the desired torque is reached. With a click type torque wrench you can tighten a fastener to the same exact torque over and over again with a good micro-adjustable torque wrench will have an accuracy of ± 4%. A click torque wrench will cost you four times as much as a beam torque wrench but they are well worth the additional investment.
Torque wrenches are delicate instruments and need to be recalibrated from time to time. Store them away from other tools so they don’t get knocked about. Above all don’t use them as a substitute for a breaker bar, many people are tempted to do that because they are longer then most ratchets and provide better leverage. Buy quality and with the proper care a torque wrench will last a life time.
Sources
Sears Craftsman Tools http://www.sears.com/shc/s/s_10153_12605_Tools_Mechanics+Tools_Torque+Wrenches
Mac Tools http://www.mactools.com/productdetails/tabid/119/c-23287-dial.aspx
Snap On Tools http://buy1.snapon.com/catalog/tools.asp?tool=hand&Group_ID=954&store=snapon-store
