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HOW TO use a torque wrench in odd situations...
HOW TO use a torque wrench in odd situations...
Published by J!m
09-21-2007
Default HOW TO use a torque wrench in odd situations...

OK I read this, and obviously I submitted this late at night, drunk, or both. It is not very clear at all exactly what I am saying here, so I thought some clarification was in order. And a better title.

Ideally, you install an appropriate socket onto your torque wrench, set it to the factory spec for the fastener in question, hold the handle right in the center and slowly apply force until it clicks (in a click-type) or the arrow aligns with the desired torque (in a flex-beam). This article was intended to help when you do not have direct access, as is sometimes the case when installing a head with overhead cams.

In this case, to gain access, a typical box wrench can be used to extend into hard to reach areas. The wrench is placed on the fastener, and the torque wrench acts on the other end of the wrench. Be cause you are not directly in line with the axis of rotation of the bolt being tightened, you need to correct for the change in length, as explained below.

The box wrench must ALWAYS be parallel to the torque wrench handle, but it can be extended forward of the torque wrench as well as coming back under the torque wrench if this is easier.

You also need the (fairly accurate) length of the wrench from center to center (in inches to make the calculation easier for US foot pounds).

Now you can calculate the change in torque, due to the change in length.

If it goes out in front, your torque increases (because your effective lever is longer), if it comes under the handle, it decreases (in all cases, your hand must remain on the torque wrench handle for accuracy). If it is at an angle, you must calculate the change as if the wrench was straight out- use trig to determine effective lever length change. It is more complicated that way, so just try to have it straight out in front, or straight under the handle if you can. A slight angle can be fudged a bit, if you have to. You can also use an extension on the torque wrench to clear the top of the cams for example.

If you do need an extension, try to use the largest diameter extension you can (even when not using a wrench for offset) to limit the torque robbing of winding up the extension. If you have ever used a long extension, you know how different it feels as some of your torque is used to wind up the extension.

How much to add/subtract? This is tricky, but not impossible. There are 12 inch pounds in a foot pound (believe it or not), and a foot pound represents one pound of force exactly one foot from the center of the nut/bolt. With this in mind, you convert foot pounds to inch pounds, so you have common units. We'll use 30 ft/lb for this first example. So, we multiply 30 by 12 and get 360 inch pounds. The wrench we will use in our contraption has a center to center measurement of 10 inches. In that case, if we have the wrench out in front, we must add 10 inches to the figure, or make it 370 inch pounds. If the wrench is under the handle, we subtract that amount and we get 350 inch pounds. Now we just convert back to foot pounds, and we get 30.8 foot pounds (if we go longer) or 29.2 (if we go shorter).

Second example: if you set 30 ft/lb, and the wrench is 11 inches long, you have 30.9 ft/lb. Based on that, you would back off one foot pound to make the correction (and you see how the wrench is 0.9 foot, and the torque increased the same amount). Since there is usually a tolerance on torque figures, the best thing to do is shoot for the exact center, to account for any errors that may occur. Obviously, errors are more common with a "contraption" than with the torque wrench alone…

Oh, also, the wrench should be flat- meaning parallel to the torque wrench handle. Many combination wrenches have a knuckle clearance off set on the box end, but not on the open end, so do be aware of that. If the torque wrench is not perpendicular to the centerline of the bolt being torqued, there will be error (slight, but an error).

The easiest way to do this is with a tool designed for the job. This is a torque adapter wrench. It is placed on the nut or bolt in question, and the torque wrench (or extension) is placed in the drive hole:


Since you are applying the torque set on the tool at the drive hole and not at the bolt or nut, the adjustment is required.
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