Tools

Torque Wrench

A 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.
  • mkp is the symbol for the meter kilopond equal to 9.80665 newton meters (N·m) or 7.23301 pound feet.
  • N·m is the symbol for Newton-meter equal to .7376 foot-lbs or 8.85 inch-lbs


The parameters for specifying a torque wrench are:
  • range (in foot-lbs, in-lbs, or N-m)
  • drive size (1/4", 3/8" or 1/2")
  • accuracy (1%, 2%, 3%, or 4%) over a stated range, usually from 20% to 100% of full scale
  • type (beam, split beam, clicker, dial or strain)
There is a wide range of torque values used on Airheads. Some of the lowest ones are:
  • lower fiberglass fender brace (1.5 ft-lbs)
  • steering pre-load (2.5 ft-lbs)
  • cam nose nut (4.5 ft-lbs)
  • timing chain cover bolts (5 ft-lbs)
  • front engine cover (5.5 ft-lbs)
  • oil pan bolts (6 ft-lbs)
  • swing arm pivot pins (6 ft-lbs)
  • oil sump pickup bolts (6.5 ft-lbs)
  • oil pump cover bolts (7.3 ft-lbs)
  • oil cover plate (7.5 ft-lbs)
Some of the highest torque values are:
  • exhaust spider nuts (110 ft-lbs)
  • transmission input pinion gear (115 ft-lbs)
  • transmission output flange center large nut (162 ft-lbs)
  • driveshaft coupling nut (180 ft-lbs)
One torque wrench cannot work at both the low and high torque values accurately. However, just because every nut and bolt has a torque specification doesn't mean every nut and bolt needs to be tightened with a torque wrench. At the low end, no one uses a torque wrench for the 1.5 ft-lb torque on bottom fender braces, and at the high end, no one uses a torque wrench on the exhaust spider nuts. Feeling carefully with fingers, a torque wrench isn't needed on drain/filler bolts or valve adjustment nuts.

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:
  • banjo nuts on oil thermostat
  • cam nose nuts on ATUs
  • fill and drain plugs
  • oil pan bolts
  • swing arm Allen bolts
  • valve cover center nut
Dial or beam types aren't recommended for this application because often the dial or needle can be difficult to see (as with tightening oil pan bolts).


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:
  • 27 mm rear swing arm locking nuts
  • engine mounting bolts
  • flywheel, clutch, and u-joint bolts
  • fork crown nuts
  • shock absorber bolts
  • spark plugs
A favorite torque wrench of Airhead mechanics has been the Sears 5-80 ft-lb Digitork. This is a clicker with a digital window instead of a vernier dial. Unfortunately, Sears has been having a reliability problem with these. If you want a professional quality torque wrench, select a new generation strain gauge model from GearWrench, Sears, or KD Tools.

Use
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.

Re-Torquing
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-adapter
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. extension 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.

another torque adapter


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.



warnings! Warnings!

  • Do not use a torque wrench as a regular wrench, i.e., to just tighten or loosen a bolt without setting its torque.
  • Do not use a torque wrench to loosen a nut or bolt unless you are gradually reducing torque AND the wrench is designed to allow loosening. Do not use a torque wrench as a hammer or pry bar. Do not drop the wrench or allow it to bang around in the tool box.
  • Do not let a torque wrench travel past its set point. Many torque wrenches can travel past the torque setting without a definite release. They will continue to apply torque beyond the set value. This can be dangerous and is often the cause of stripped fasteners, especially at lower torque values. For beam type wrenches, going beyond the maximum value can permanently distort the beam, destroying the wrench.
  • Do not "re-torque" a bolt without first loosening it. You can check a bolt to see that it is not loose with a torque wrench, but that is not the same as re-torquing it.
  • For a clicker type wrench, store it with the tension released (which is not the same as the tension set to zero). This is needed to keep the tool in calibration.
  • For a clicker type wrench, the bottom 10% of the scale is not accurate and the "click" is not pronounced. Do not use a clicker at this end of the scale.
  • The correct torque to apply is different when the threads are clean and dry (which is assumed unless specified otherwise) than when a lubricant such as Loctite or oil is used. Torques should be 10% less when using Loctite Blue or Red, and 20% less if the threads are oiled. For spark plugs with anti-seize, torque values should be reduced by 30%. (It is an Airhead controversy whether anti-seize compound should be used on spark plugs!)

    For example, the four cylinder head nuts on the rocker arms are assumed to be oily when assembled and torqued.

    Keep these lubricant reductions in mind however, when a procedure assumes dry components and Loctite or oil is actually used.
  • If using Loctite, only use one or two drops covering three revolutions of threads. Whenever a Loctited bolt is removed, clean off the old Loctite with acetone or BraKleen and a wire brush. Most parts using Loctite on an Airhead use Loctite Blue (Loctite 242).

    An exception is where the BMW manual specifies Loctite 2701 or Loctite 271. This is now Loctite "Red". Loctite Green, Pink, or Purple shouldn't be used on an Airhead.

    Before using any Loctite, shake the bottle to re-mix the active chemicals.
  • Torque values assume the threads are clean. If they or the bolt undersides are dirty, or rough, the torque value may fluctuate up to 25% from the actual strain required. Insure all the parts are clean!
  • Several torque specifications on this website differ from that specified in the BMW manuals. This may be due to later service bulletin updates, practical experience, or modified service procedures. Such Airhead components include:
    • alternator rotor Allen center bolt
    • cylinder heads nuts (25  +/- 2 ft-lbs!)
    • drive shaft U-joints
    • exhaust pipe finned nuts
    • oil pan bolts
    • rocker arm adjustment nuts
    • spark plugs
    • steering head bearings
  • Some Airhead bolts cannot be re-torqued (i.e., reused). These include flywheel and clutch bolts. These instances are mentioned in the service procedures on this website.
  • Do not use generic torque tables for Airhead bolts and nuts, including BMW's own generic table (BMW N 600 02.0). Most Airhead nut and bolts have a published tightening specification and these often are different from what the generic tables specify. Many tables assumed the bolts are phosphate treated, with no after treatment. However, our original Airhead BMW bolts were cadmium plated which take approximately 25% less torque. Because of European environmental laws, BMW no longer ships cadmium plated hardware, but what they do ship can be used with the standard torque values.
  • Never put an extension on the handle end of a torque wrench.
  • When purchasing a torque wrench, if doesn't come with a case, purchase one.
  • While most bolts on an Airhead can be re-used unless they have been stressed beyond their elastic limit, there are three exceptions: the flywheel, clutch and driveshaft bolts should be replaced whenever they are removed.

References

  • Snowbum's Torque Specifications and Problems web page. (http://www.bmwmotorcycletech.info/torquespecs&problems.htm).
  • Snowbum's Torque Values web page. (http://www.bmwmotorcycletech.info/torquevalues.htm).
  • Snowbum's Increasing the Range of a Torque Wrench web page. (http://www.bmwmotorcycletech.info/IncTorqWr.htm).
  • Oak Okleshen, AirMail May 1997 – "What kind of torque wrench to use - and why etc."
  • Oak Okleshen, AirMail May 1998 – "Torque Wrench recommendations."
  • Oak Okleshen, AirMail May 2002 – "Torque Wrench protocol, Tools..."
  • Oak Okleshen, AirMail Dec 2004 – "What kind of torque wrench to use - and why etc."
  • Mike Valenti's excellent R100/7 Torque Chart.


Torque Wrench Manufacturers

AIT (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

low range torche wrenches


Foot-lb (Medium Range) Torque Wrenches

medium range torche wrenches



Torque Wrench Types

Beam Type
beam wrench Also 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 Type
clicker wrench These 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:
  • At the low end of their scale, many do not offer a good "feel" for when the desired torque is reached. The release point is not always obvious. This can lead to over-tightened fasteners.
  • They must be set back to zero or to the "stop" setting after each use to release the tension on the internal spring.
  • They are not accurate below 10% of scale.


Split Beam Type
split beam wrench They are easy to set to the desired torque and there is no need to release it back to zero after the fastener is tightened.


torque wrench
Dial Type
These 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.




KD torque wrench
Strain Gauge Type
Also 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


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