Torque WrenchA torque wrench is a specialized socket wrench used to apply a specific torque to a nut or bolt. Its purpose is to tighten without over-tightening. Torquing a bolt stretches it with a "preload" that holds the joint together. The danger is in "pulling a stud", i.e., stripping a bolt hole.
When tightening a bolt, either with or without a torque wrench, if the torque suddenly drops while tightening then you are in trouble. The bolt has stripped its hole or the bolt has exceeded its elastic limit. In the former case there is no easy fix other than Helicoiling® or TimeSerting® the hole. In the later case, replace the bolt. With steel bolts removed from a stripped aluminum hole, the threads often are filled with aluminum bore material.
What we would really like to measure is bolt tension (bolt stretch), which a torque wrench only indirectly measures. Due to variations in friction between the fastener and mating surface, it is not an accurate measurement. A torque wrench applies preload tension with about +/- 25% accuracy (this is not the accuracy of the torque wrench, which is usually about +/- 3%).
While the majority of nuts and bolts on Airheads have a published torque spec, BMW Airheads are pretty tolerant of ham-fisted over-tightening except for a few bolts and nuts for which a torque wrench should be used. These include cylinder head stud nuts, nose cam nuts, pan, flywheel, rod and driveshaft bolts, thermostat banjo nuts, valve cover center nuts and disc carrier fasteners. Even professional mechanics use them in some applications, such as on cylinder head bolts. Also, unless you are experienced, it is safest to use a torque wrench on any steel bolts which thread into aluminum such as spark plugs. For a good reference on Airhead torque specifications, see Snowbum's web page on torque values. If you have a R100/7, Mike Valenti's torque chart is great.
There are three commonly used units of measure for torque wrenches: foot-lbs, inch-lbs and Newton-meters. One foot-lb = 12 inch-pounds (surprise, surprise) and one foot-lb = 1.356 Newton-meters. BMW manuals specify torques in kiloponds (mkp) or Newton-meters (N·m) and foot-lbs.
Some of the Newton-meter torque specifications were incorrectly converted to foot-lbs in BMW factory manuals. The Clymer and Haynes manuals duplicated some of these erroneous values. Generally, the published kilopond and Newton-meter values are correct; the foot-lb values in parentheses have the errors. To be safe, recalculate the published ft-lb values.
The parameters for specifying a torque wrench are:
BMW hobbyist mechanics (the target audience for this web page) should not open their transmission or rear ends. This eliminates the need for the noted above high torque values, and everything else on an Airhead can be torqued with just two torque wrenches, one for 16 ft-lbs and below, and the other for 16-ft lbs and above:
Low Torque (200 inch-lb) Clicker
A good example is the Harbor Freight 2696-2VGA clicker. This wrench would be used for low-torque Airhead fasteners such as:
Medium Torque (150 ft-lb) Clicker
A good example is the Harbor Freight 239-1VGA Clicker. This wrench would be used for the higher torque fasteners such as:
Use a smooth, even pull to the stopping point. Movement needs to be slow, without jerking, but also without stopping. This is especially important with bolts with crush washers like drain plugs and spark plugs (when either using a torque wrench or tightening by feel). Jerking the wrench will result in inaccurate settings. Apply your hand force at the center of the handle.
For multiple bolts like the cylinder heads, oil pan, clutch, rear drive cover, etc., the tightening should be done in stages, in a cross pattern. By the same token, the tension on these bolts should be released gradually, in a cross pattern, when being removed. However, not all torque wrenches can be used in reverse without damaging the tool. All beam types can, and a few clickers, but most clickers cannot. The owner's manual must be read to determine if a given torque wrench is safe to use for loosening fasteners in staged torque values.
It is not possible to determine the torque a bolt has been set at. The "breakaway" torque to start a tightened fastener moving, because of initial friction, is always higher than the torque the bolt was originally set at. For this reason, a bolt can only be re-torqued by loosening it first and tightening it again. Most stripped oil pan gaskets are caused by improper re-tightening by owners who think they are "snugging everything up". Generally, fasteners using thin or no gaskets do not need re-torquing, but fasteners using thick gaskets such as head bolts and oil pan bolts do.
Calibration and Repair
Like many tools, there are American brands which cost a couple of hundred bucks and Chinese brands which cost less than $50. When the Chinese brands fail they are thrown out. When the American brands (CDI, KD, Sears, Snap-On, Central Tool, Mac, SK, Sunex) fail you have them repaired by a Snap-On tool dealer or sending it to Griot's Garage in Tacoma, Washington 800-345-5789 or Angle Repair in West Virginia. 304-253-5729.
A torque wrench can be checked by placing the square drive in a vice, handle horizontal, and hanging weights from the mid-point of the handle.
Torque Wrench Extensions
An adapter can be added to a torque wrench to increase the tool's range or help reach a bolt in an awkward location. The new effective torque value = value on gauge multiplied by (X+Y/X). For example, adding Snap-On's 10mm socket torque adapter (FRDHM10, $25) to to their 3002LDINSS 0-300 in-lb torque wrench converts it to a 0-350 in-lb wrench.
In another example, a 30" extension added to the Precision Instrument D2F200HM 40-200 in-lb dial torque wrench converts it to a 160-800 in-lb tool. The overall length (over three feet) is a little unwieldy, but isn't really a problem when working on motorcycles. Note: never place an extension on the handle end of a torque wrench.
A 90° adapter of any length can be used without changing the indicated torque value.
It is sometimes necessary to use a 6" rod extension and/or adapters (3/8" to 1/2"; 1/2" to 3/8") with a torque wrench; they don't affect the torque value. Rod extensions may be needed if deep sockets are not used. It is not all right to use a universal joint with a torque wrench.
Torque Wrench ManufacturersAIT (Armstrong), ATD, Balkamp (NAPA), CDI Torque, Central Tools (Storm), K-D Tools (GearWrench), K-Tool International, JC Whitney, Mac Tools, Pittsburg (through Harbor Freight), Precision Instruments, Sears, SK, Snap On, Proto (Stanley), and Sunex.
Online Stores with discounted Torque Wrenches:
Amazon , Northern Tool , Sears , SJ Discount Tools , and The Tool Warehouse .
Inch-lb (Low Range) Torque Wrenches
Foot-lb (Medium Range) Torque Wrenches
Torque Wrench Types
Beam TypeAlso called "needle" wrenches, these are the simplest torque wrenches. These have a pointer that indicates the torque as the fastener is tightened. They are easy to calibrate (just bend the pointer to point to 0) and work both tightening and loosening.
Beam torque wrenches are very rugged. However, a direct, head-on view is necessary to read the actual torque value. This is sometimes difficult on an Airhead unless a motorcycle lift is used.
Clicker TypeThese typically have a handle that is turned to set an internal spring and clutch for the desired torque. They usually have a rated accuracy of ±3% (i.e., about ±1 ft. lb. at a 30 ft. lb. setting). There are two types: one where the value is set on a circular vernier on the handle, and the other with the torque value displayed in a digital window. The advantage of clicker wrenches is a positive unambiguous signal at the set point.
The disadvantages of clicker torque wrenches are:
Split Beam TypeThey are easy to set to the desired torque and there is no need to release it back to zero after the fastener is tightened.
Dial TypeThese are usually very accurate torque wrenches. The amount of torque that is applied is not affected by where they are held. A disadvantage is that the user of the dial type must be in a position to read the dial. For cylinder head work, this is much easier if a motorcycle lift is used.
Strain Gauge TypeAlso known as "electronic" or "digital" torque wrenches, these are usually the most accurate (and most expensive) torque wrenches. Measurement is by means of a strain gauge attached to a torsion rod. The signal generated is converted by the transducer to the required unit of force (N-m, lb-ft etc.) and shown on a digital display.
Last updated December 5, 2009