Automotive Machining. Mike Mavrigian
scale may offer both ft-lbs and Nm).
Torque wrenches are precision instruments and should be handled as such. Care and storage of a torque wrench is critical in terms of maintaining calibration. Any adjustable torque wrench (the commonly used micrometer-handled click type, for example) should be set at its lowest torque reading when not in use. This is something that many technicians commonly forget. If left stored at a high-torque setting, the calibration may be affected over a long term. When you’re done with the wrench, readjust it to the minimum setting before storing it in the toolbox.
By entering the angle mode, you set the desired angle. When the set angle is achieved, the wrench alerts you via a beep and a vibration in the grip. This is a handy feature for applications where you need to meet OEM torque-plus-angle tightening.
When using an adjustable torque wrench, be careful not to overtighten by applying torque past the release or signal point. With a ratcheting “click” type, the “click” may not be heard at low torque settings, especially in a noisy shop. It’s best to become familiar with the “feel” of the release, rather than relying only on the sound of a click.
When using an indicating type torque wrench (such as a flex bar or dial indicator type), try to read the indicator while viewing it at 90 degrees to its surface. Reading the indicator at an off-angle provides errors due to improper line of sight.
Most torque wrenches operate accurately only when held by the center of their handle grips. Don’t use cheater bars to extend your grip farther away from the wrench head, and don’t grab the handle closer to the wrench head. Only grip the wrench by its designated grip area.
Torque Values and Fastener Clamping
The torque applied to a bolt or stud creates clamping force by stretch-loading, which can be loosely compared to the stretch of a rubber band. When the underside of the bolt head (or nut) makes contact with the parent surface, the additional rotation of the bolt head or nut causes the bolt shank or stud to begin to stretch. The objective is to reach the ideal point where this stretch provides the needed clamping force to properly secure the component being installed. When tightened properly (to specification), the fastener has stretched within its designed elastic range.
When the fastener is loosened, the elasticity allows the shank to return to its normal, uninstalled length. If stretched to its yield point, it can permanently weaken. If the bolt or stud retains no elastic ability, it can’t do its job in terms of providing clamping force. If severely overtightened, a bolt can shear.
Bolt or stud diameters are based on the load required for component clamping performance. That’s why 1/4-inch bolts may be used in one location and 3/8-inch bolts in another. A smaller-diameter bolt requires less torque value to achieve ideal clamping load, and a larger-diameter bolt requires more torque value to achieve ideal clamping load. Although not a perfect analogy, you can sort of view threaded fasteners as “fuses.” The diameter is based on the requirement for the specific job, just as the amp rating of a fuse is based on the requirement for a particular circuit.
Taking advantage of a threaded fastener’s clamping load potential isn’t a matter of guesswork. Especially for critical fasteners, such as any involved in the brake system, steering system, suspension, engine, transmission, differential, and wheels, all threaded fasteners must be tightened to their specific-application torque value. If you don’t pay attention to torque values, it’s like buying a set of pistons and sticking them into cylinder bores without measuring oil clearance.
In addition to adjusting the setting and/or monitoring the preset level via a click, listening for a beep, or watching a dial, consider the variables. First of all, is the torque wrench accurate? If it’s a cheap one, or if it’s been lying around the shop for years, it may be out of calibration. Second, is the fastener being tightened lubricated properly? Third, is the clamping force being created suitable for the diameter and type of metal?
About 90 percent of the torque applied during tightening is used to overcome friction. Friction occurs between mating threads, as well as between the underside of the bolt head (or nut) and the parent material of the object being installed.
Excess friction can occur if galling or “thread seizing” takes place. This is especially common with threaded fasteners made of alloys such as aluminum, stainless steel, and titanium. If galling occurs (at any level of severity), this makes your torque readings inaccurate, since the galling effect will add significant friction at the thread mating area, which results in a severely undertightened fastener.
Several factors can affect fastener tension, including type of material, material hardness, lubrication (or lack thereof), fastener hardness, surface finish/plating, thread fit, and tightening speed.
• Make sure the threads (both male and female) are clean.
• Make sure the threads are in good condition, and free of deformation or burrs.
• When necessary, apply the required lubricant to the threads before assembly (this may involve engine oil, molybdenum disulphide, an anti-seize compound, or an anaerobic thread-locking compound, depending on the situation).
• Keep your torque wrenches clean and calibrated. Depending on their amount of use, consider sending your torque wrenches out for recalibration once per year. Also, store your torque wrenches in a safe place. Don’t toss them around the shop. They’re delicate instruments.
• When tightening, whether using a common hand wrench or a torque wrench, slow down! The action of tightening quickly can increase friction and heat at the thread area, which can lead to thread galling. Speedy tightening can also lead to inaccurate tightening, as the torque wrench must overcome the increased friction.
• When you reach the torque limit (your desired torque value), approach this slowly and watch the needle or feel for the click or vibration (depending on the style of torque wrench). If you tighten too fast, you may pull the wrench past the preset limit (unknowingly adding a few more foot-pounds of torque).
The use of a torque wrench or other torque value or fastener stretch monitoring device is absolutely necessary for anything engine related (not only cylinder heads and connecting rods, but intake manifolds, carburetors, water pumps, oil pumps, rear seal housings, oil pans, timing covers, valvecovers, exhaust manifolds or headers, power steering pump brackets, etc.). If you own a torque wrench, take the time to look up the torque value for the carburetor fasteners, and adhere to the specs. Tightening anything on the engine by “feel” is more often than not the root cause for annoying fluid or vacuum leaks. The skilled engine builder/assembler never guesses about anything. Undertightening or overtightening can and does lead to problems, ranging from the mildly annoying to the most severe. Don’t guess!
Adapters and Extensions
As long as the adapter (socket extension, etc.) is in-line with the torque wrench drive, no compensation is required. However, if an adapter that effectively lengthens the wrench is used (such as a crow’s-foot wrench), a calculation must be made to achieve the desired torque value.
For those occasions when a straight socket can’t be used, a special attachment might be needed (such as a crow’s foot). The use of an offset adapter changes the calibration of the torque wrench, which makes it necessary to calculate the correct torque settings. Following are two formulas for calculating this change:
TW (where the adapter makes the wrench longer) = L ÷ L + E × Desired TE
TW (where the adapter makes the wrench shorter = L ÷ L - E × Desired TE)
Where:
E = Effective length of extension, measured along the centerline of the torque wrench
L = Lever length of the wrench (from center of the wrench drive to the center of the adapter’s grip area)
TW = Torque setting on the torque wrench
TE = Torque applied by the extension to the fastener
If