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Changing the range of a torque wrench.
Adjustment correction when using extensions.


Copyright, 2014, R. Fleischer

There are only a few places on an BMW Airhead motorcycle that require torque settings over 72 foot-pounds.   Common ones that are over 72 foot-pounds are the transmission output flange center nut; the ring nut & input nut on the rear drive, & the exhaust finned nuts.   The swing arm locking nuts can be 72 to 75 ftlbs, not critical. The finned exhaust nuts should be done by feel (IMO); and, I do NOT torque them as tight as the factory says.   Cheap beam wrenches are adequate for most higher torque areas.  NEVER use a cheap beam wrench for the cylinder stud nuts or other critical places.  Quality beam wrenches and quality tubular clicker wrenches are what should always be used for more critical fasteners.

It is possible to increase the torque from a tubular clicker type wrench or a beam wrench.

Extending the length of any torque wrench, if used straight out, will multiply the torque shown on the torque wrench by a factor.   You CAN set the extension at 90 and avoid any mathematical corrections; but that does not change the applied torque.  BUT, if the wrench is used straight-out, then you must compensate for the added leverage, by setting the torque wrench to a lower value than the specification; or, calculate the new torque.   Thus, you can increase the maximum torque your wrench could apply, to a higher value, depending on the amount of extension used. Under no circumstances should you ever use an extension at anything but either straight-out, or, 90, as the calculations are much more complicated.

This article explains how to use an extension for the purpose of INcreasing the range of any torque wrench.

This article ALSO explains how to use an extension for such as the U-joint flange bolts, where you simply cannot get a torque wrench withOUT an extension to fit properly, due to the limited room between the bolt and the U-joint body.

Perhaps you have just one torque wrench, say one that reads to 75 or 100 foot-pounds maximum, and you desire to increase its maximum.  That is easy to do with an extension.  In order to increase the output of a torque wrench beyond its maximum setting, you must increase the leverage.  This is done by EXTENDING the working end of the torque wrench,  NEVER by extending the handle in ANY way.   In most instances the EXTENSION is used straight out, that is, parallel, in-line, with the wrench.  

A calculation you will need is how much the torque is INcreased.  TYPICALLY, however, you will just want to know how to set the torque wrench for a specific actual torque value.  

In the instance of such as the Airhead U-joint bolts, you are LIKELY to not be interested in extending the torque wrench beyond its maximum setting; rather, you are interested in calculating what setting to use on the torque wrench, for a specified actual torque on the bolt. This is because the torque wrench already has the required range of torque settings, but you have to use an adapter to fit the bolt area.

Nearly the very same process and calculations are used for all of these ideas.

Torque wrench design assumes that the point of pressure applied is centered under your hand.   In some instances the owners manual tells you there is a mark of some sort on the handle, used for measuring purposes, such as you will now learn in this article. Torque wrench design is different for SOME types, but the assumption remains for that same pivot point, whether it an actual pivot, or effective pivot point....or not.

NOTE!  Consult your torque wrench owner's booklet to be sure how to do the following things for YOUR wrench!  The information in this article is usually applicable to all torque wrenches, but BE SURE, for yours!

NOTE!  This article uses inch type format.  To help avoid confusion, measuring your torque wrench distances in metric is not shown.   For Nm values, simply take the ftlbs value and apply a standard conversion, or start with Nm, and apply a standard conversion.  The conversions are:
Nm x 0.738 = foot-pounds.
Foot-pounds x 1.356 = Nm.

1.  Measure the distance in INCH MEASUREMENT from the middle of the handle (handle pivot point) to the center of the 1/4" or 3/8" or 1/2" square male drive.   Write down the measurement, call that measurement "L".

2.  Measure the distance of the extension, from the center of the drive to the center of the output. The extension output could be a square drive or could be a hex, such as the box end of a box end wrench, etc.    As in the previous measurement, do it in inch measurement.  Write that down. Call that measurement "E".

THINK!!     With the torque wrench added extension extended straight-out, whatever READING YOU SET on the torque wrench MUST BE SET LOWER than the ACTUAL applied torque that you want!   RESTATING: This means that in setting the wrench for a specific torque you want to apply to a bolt or nut, the torque wrench must be set to LESS than the value the books or literature ask for, since the straight-out extension is a multiplying factor, it gives more leverage.

4.  Most commonly you want to know what setting to use on a torque wrench to get some specific torque, when using an extension.

The torque you wish to apply is found by dividing L into the sum of L + E and then applying the result as a correction factor.

L + E
You want to torque the Airhead U-joint 10 mm bolts.  The value of torque you want is 26 ftlbs. You  measure your torque wrench from the center of the handle to the center of the 3/8" square drive & find it to be 12.75 inches, that is "L".  You are using an adapter/extension that is being used straight out from the torque wrench, just as in the example sketch.  The center-to-center of that extension is 1.5 inches.   "E" is then 1.5".

L + E is 14.25.   Dividing that by L gives you 1.118, rounded.      THAT is the correction factor!

You WANT 26 foot-pounds to be applied to the bolt, so, divide 26 by the 1.118 correction factor.  The answer is 23 foot-pounds to be set on the wrench.

If you use the torque wrench with E (the extension adaptor) at 90 to the torque wrench, you do not need calculations.  This usage can be physically awkward however.

07/16/2006:  Slightly revised for clarity.
06/13/2011:  Because of three math-challenged folks who have contacted me; I have revised this article extensively.
10/06/2012:  Add QR code; add language button; update Google Ad-Sense code; simplify article once more, clarify a few details.
10/19/2012:  Revise some for clarity.  Remove one multiplying formula, condense and clarify methods, add sketch.
2013:  language button removed due to javascript problems.
08/16/2015:  Clarify a few details, and make the example one for a very common TW and common aftermarket adapter.
03/05/2016:  Update meta-codes and layout.
07/31/2016:  Update metacodes, scripts, H.L., add conversion factors, clarifications.

Copyright, 2014, R. Fleischer

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Last check/edit: Wednesday, August 10, 2016