torque wrench
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torque wrench .

How to Use a Torque Wrench

Copyright AA1Car

A torque wrench is a "must have" tool for do-it-yourself auto repair. You need a torque wrench to accurately tighten nuts and bolts, especially those that must be tightened to a specific load so they do not stretch, break or come loose.

Automotive applications where a torque wrench is essential include tightening engine cylinder head bolts, connecting rod bolts, main bearing cap bolts, flywheel bolts, camshaft timing gear bolts, crankshaft pulley bolts, intake and exhaust manifold bolts, lug nuts, axle nuts, ball joint studs and many other critical fasteners on your vehicle.

How Does a Torque Wrench Work?

A torque wrench is a calibrated tool for applying a known amount of load to a bolt or nut. The amount of torque applied depends on the force you apply on the tool handle and the length of the wrench (torque = force x length).

For example, if your torque wrench is one foot long and you apply 30 pounds of force on the handle, you are applying 30 ft. lbs. of torque on the fastener you are tightening. If your torque wrench that is two feet long, and you apply the same 30 pounds of force on the handle, the longer tool will increase the leverage effect and multiply torque, applying 60 ft. lbs. of torque to the fastener.

A torque wrench indicates how much force you are applying to a fastener with a deflection beam or a calibrated spring mechanism. A scale or display on the tool shows you the load that is being applied. The scale may be calibrated in foot-pounds (ft. lbs.), inch-pounds (in. lbs.) or Newton-meters (Nm). A typical automotive torque wrench will have a scale that reads up to about 200 to 250 ft. lbs. (or 150 to 200 Nm).

Torque values for fasteners with wrench head sizes of 1/2 inch (12 mm) or larger are usually specified in foot-pounds (English) or Newton meters (metric). Torque values for smaller fasteners (wrench size less than 1/2 inch or 12 mm) are usually specified in inch-pounds.

Typically, torque wrenches with a 1/2 inch drive will have a dual scale that shows the reading in both ft.lbs and Nm. Torque wrenches with a 3/8 or 1/4 inch drive will be for light loads and calibrated in in-.lbs.

Torque Wrench Conversion Factors:

One foot pound equals 12 inch pounds.

One foot pound equals 1.3558 Nm

One Newton Meter equals 0.7375 ft.-lbs.

One inch-pound equals 0.1129 Nm

Torque Corrections

To achieve an accurate torque reading, you must use a torque wrench correctly. You have to hold the tool on the hand grip, and either push or pull on this part of the handle only. If the hand grip has a pivot point, you have to hold the handle so the load is balanced over the pivot point.

Torque wrenches are designed to be used with socket wrenches. Using an extension won't alter the torque reading, but you must use care to keep the extension straight (parallel) to the fastener you are tightening.

If you are using a crow foot open end wrench on a torque wrench, position the crow foot so it is 90 degrees to the tool handle. This will minimize any change in leverage that could affect the accuracy of your torque wrench reading. If you position the crow foot so it is straight with the tool handle, it will extend the length of the tool slightly and increase leverage slightly. This, in turn, will increase the actual amount of torque applied to the faster over what your tool indicates. The difference can be 4 to 5 percent depending on the length of your tool. To compensate, you need to reduce the amount of torque applied by a corresponding amount (4 to 5 percent).

types of torque wrenches

Types of Torque Wrenches

The four basic types of torque wrenches are beam style, click adjustable, dial gauge and digital electronic. The beam style are the least expensive and simple to use. You can usually buy a beam style torque wrench for less than $30 in most auto parts stores or online. Most professional technicians prefer to use an adjustable click style torque wrench because they are faster and more accurate (assuming they have been properly calibrated) than a simple beam style torque wrench. The dial gauge and digital electronic torque wrenches are easiest of all to read, but also are the most expensive.

Beam Style Torque Wrench

There's not much that can go wrong with a beam style torque wrench unless you severely overload it to the point where the shaft is bent, or accidentally bend the beam indicator. If the shaft is bent, you need to replace the wrench. If the indicator is bent and does not read zero with no load on the wrench, just bend the indicator until the pointer at the end lines up with the zero on the calibration scale.

TIP: When using a beam style torque wrench to tighten a fastener, you must look at the pointer on the scale straight on, otherwise you might not read it accurately. Looking at the pointer from an angle may make it appear to read slightly higher or lower than what it is actually reading.

Click Adjustable Torque Wrench

Click style torque wrenches have an internal spring and adjustable handle. Twisting the handle adjusts the torque value of the wrench. When you reach the preset torque value while tightening a fastener, the wrench "clicks" and slips slightly, preventing you from applying any more torque (unless you keep on twisting the wrench handle). The nice thing about this is that you can check the torque by feel and don't have to look at the scale or a gauge.

Tip: When you have finished using an adjustable torque wrench, always return the torque setting back down to near zero. This will relieve the pressure on the internal spring so it doesn't take a set and produce inaccurate readings later. If you forget to relieve spring pressure, and have not used your torque wrench in some time (say more than a month), readjust the torque to a mid-range value, then tighten and loosen a fastener four to five times to work the spring. Then adjust to the desired load to work on your vehicle.

Note: Adjustable torque wrenches can get out of calibration over time. The wrench should be accurately calibrated from the factory (unless it is a really cheap one, in which case it's hard to say how accurate it might be!). But with repeated or heavy use, the spring inside may take a set and not apply as much actual torque as you have preset. There are companies that specialize in checking and adjusting the calibration of torque wrenches. Many professional tool distributors such as Snap-On, Mac and Matco also have equipment that can check the accuracy of a torque wrench. So if you've never checked your wrench, or have been using it more than a couple of years, it might be a good idea to have it recalibrated. Accuracy should be within plus or minus 3 percent.

Dial Gauge & Digital Electronic Torque Wrenches

Dial gauge and digital electronic torque wrenches have easy to read gauges, and are typically the most accurate (accuracy within 0.5 percent is claimed by many). They are also the most expensive, ranging in price from several hundred dollars up to several thousand dollars! These are precision instruments and should be handles carefully. And like adjustable click torque wrenches, they need to be accurately calibrated from the factory, and recalibrated periodically to assure continued accuracy.

Unless you are building NASCAR engines or other high end professional racing engines, a high end torque wrench would be more of a luxury than a necessity. A click style or beam style torque wrench should be adequate for most auto repair needs.

How to Use a Torque Wrench the Right Way

First, look up the torque specifications for the fasteners you will be tightening. If you don't know, don't guess. Look it up online or in a service manual. Also make sure the torque specifications and tightening procedure is the most recent version, as it may have been updated or revised since it was originally published. Using incorrect specifications or out-of-date specifications may cause problems later on.

NOTE: The torque values specified for most fasteners are for CLEAN, DRY, UNDAMAGED threads.

The loading on the bolt will depend on the amount of friction generated by the threads as the fastener is tightened down. If you oil or lubricate the threads, it will reduce friction and INCREASE the applied load on the bolt. This may overload the boat, increasing the risk of stretching or breaking the bolt, or crushing a gasket. The only exception are cylinder head bolts, which are typically lightly oiled with 10W-30 motor oil. Everything else goes on dry.

When tightening a bolt or nut , use an ordinary wrench or socket to tighten the fastener down to where it is snug but not overly tight. Then use your torque wrench to tighten the fastener to final specifications.

On many beam style torque wrenches, the plastic handle on the end has a pivot point inside that allows the handle to move or wobble slightly. The idea here is to hold the handle so that it is centered on the pivot and neither end of the handle is touching the bar. The reason for doing this is so your torque reading will be accurate. If either end of the plastic handle is touching the bar, it changes the effective length of the arm and the amount of torque applied to the fastener. This, in turn, throws the accuracy of the gauge reading off slightly (maybe a couple of foot pounds, or inch pounds in the case of an inch-pound wrench). So if you want to be as accurate as possible, try to keep the handle centered on its pivot while tightening the fastener.

Follow the Specified Tightening Sequence

With head bolts and intake manifold bolts, the bolts must not only be tightened to a specific value, but also tightened down in a specific order to equalize the loading on the castings and gaskets. The pattern usually alternates from one side to the other, starting in the middle and working your way towards both ends of the head or manifold.

TTY Head Bolts Usually Require Special Procedures

Many late model engines have "Torque to Yield" (TTY) head bolts that are designed to stretch slightly when tightened. The torque specifications for these bolts often involve extra procedures, such as gradually tightening down the bolts in steps until a certain torque value is reached, then giving the bolt and extra twist (specified in number of degrees) to achieve its final load.

For these kinds of applications, you will need an extra tool called an Angle Gauge (see photo below) or an electronic torque wrench that can also measure the degrees of rotation as a fastener is being tightened. If you are using an angle gauge, mount it on the end of your torque wrench. Zero the pointer, then note how many degrees you have rotated the fastener as you are turning it.

torque angle gauge

For example, a specification might say to tighten down the head bolts in three incremental steps, going from 15 to 30 to 45 ft.-lbs., then giving each bolt a 1/4 turn (90 degrees) to achieve the final load.

Use a Torque Wrench to Tighten Lug Nuts and Axle Nuts

There are several reasons why you should always use a torque wrench to final tighten wheel lug nuts:

* Over-tightening a lug nut may break off the stud, weakening the wheel mounting (always replace broken or damaged wheel studs)

* Under-tightening a lug nut may allow it to work loose from road vibrations and bumps. This could result in the loss of a wheel!

* Uneven tightening of the lug nuts can distort the brake rotor, causing it to wear unevenly. This can eventually create a brake pedal pulsation or shudder when braking.

* Never use an impact wrench to final tighten the lug nuts - unless you are using "torque stick" extensions with the impact sockets that automatically limit the amount of torque applied by the impact wrench. An impact wrench can apply a LOT of torque, making a wheel very difficult to remove if you have a flat tire later on. The high force applied by an impact wrench may also stretch, damage or break the lug stud. The force applied can also vary considerably from one lug to the next, causing the rotor distortion and uneven wear problem mentioned above.

* Axle nuts should also be tightened with a torque wrench and NOT an impact wrench. The ratta-tat-tat jolts of an impact wrench can shock load and damage the balls inside a constant velocity joint. The axle nut must also be accurately tightened to specifications so it won't overload bearings or come loose. Many axle bearings must also staked or locked in place with a cotter pin after final tightening.

NOTE: Axle nuts may require loads as high as 240 ft.-lbs. or more depending on the application. This may require using a handle extension on the torque wrench or a device called a "torque multiplier" on the socket end to achieve the desired load.





Related Articles:

Torque-to-Yield Head Bolt Installation Tips
Common Causes of Head Gasket Failure
Preventing Repeat Head Gasket Failures
Engine Rebuilding Tips

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