Make a Torque Wrench Extension

Background

I have a decent "clicker" type torque wrench with a range of 15 – 150 ft-lbs. I recently had to replace the pitman on A friend's Jeep. When installing it back on, the nut needed to be torqued to 185 ft-lbs. What to do? My first inclination was to buy another torque wrench, at a cost of about $79.99 from Harbor Freight (for a decent one, ha!). But how often am I going to use it? Practically never.

An alternative was to make a special extension which will multiply the torque. I DO NOT mean an extension like this:

A regular straight extension like the one above does not change the effective length of your torque wrench, and will therefore not have any affect on the torque setting.

Now look at this:

The big difference is that the extension shown above does increase the effective length of the torque wrench. As a result, the actual torque applied to the bolt will be higher than the setting on the torque wrench.

The extension shown in the above picture is called a crowfoot extension, which can be handy when trying to torque hard-to-reach bolts. You must, however, use a formula to account for the length of the extension:

wrench setting = desired torque x wrench length / (wrench length + extension length)

For example lets say that the length of the torque wrench is 15 inches, and the length of the extension is 8 inches. The desired torque is 185 ft-lbs.

Wrench setting = 185 x 15 / (15 + 8)

= 185 x 15 / 23

= 120.7 ft-lbs.

If the torque wrench is set to 120.7 ft-lbs, the actual torque applied to the bolt will be 185 ft-lbs.

This tool I'm making allows my 150 ft lb torque wrench to place 230 measured ft lbs on a fastener.

Making The Extension

The extension must have a female fitting on one end, for attaching to the torque wrench. On the other end it needs a male fitting to attach sockets. Since my torque wrench takes 1/2" sockets, all the examples below are for a 1/2" extension. If your torque wrench is 3/8" or 1/4", then resize as needed.

Parts List:

1/2" x 3/8" socket adapter (available at Sears for $5.99)

Serpentine belt tool ( available at Harbor Freight for $17.99 )

Access to a Flux wire welder (available at Harbor Freight for $89.99)

(the wire feed welder is an awesome tool capable of doing many hobby type projects. I use mine frequently for making tools, brackets, and even did some exhaust work with it. Because of this I won't include it in the total cost of this project)

Total cost- $23.98 + tax

I chose the serpentine belt tool that looks like this:

Notice the small bar. It has a 1/2" drive male end and a 3/8" drive female end:

We're going to use this piece along with the 1/2" female to 3/8" male socket adaptor:

1. Prepare the extension by cleaning the paint off of the 3/8" female end. I did this with some emery cloth and a file to clean any burrs off the metal. I clamped it in the vice to do this.

2. Next clean the chrome off of the socket adaptor. Chrome makes toxic fumes when it gets hot (as in welding).

3. Insert the 3/8" male end in the 3/8" female end of the extension to test the fit. Both the male and female end of the extension are square with each other meaning the squares are oriented the same.

4. Clamp the 2 pieces together. I used a large pair of vice grips to do this. This is only until the first weld is complete. Double check to make sure that the two pieces are sitting flat against each other.

5. Clamp the ground cable from the welder on to the bar extension and suit up (gloves, visor, long sleeve shirt). Turn on the welder and do your tack weld somewhere clear of the vice grips. Hit it a welding hammer to remove any slag and clean it with a wire brush. I used wire speed 9 for my tack weld and it looked good, so I removed the vice grips and completely welded the fat side of the adaptor to the bar.

6. Flip the whole thin over and then weld the 3/8" square side to the bar. Clean it up and then quench it with water. This allows the molecules of steel to align properly for hardness and durability. Dry it off and grind the welds smooth, but don't take off too much of the weld.

7. Paint the entire piece with primer and then paint. I chose black. The finished piece should look like this:

Sources of Errors

Worried that using an extension might cause inaccurate torques? I don't blame you. Here are the various errors that can be introduced when using an extension:

A. If you do not accurately measure the length of the torque wrench.

B. If you do not accurately measure the length of the extension.

C. If the extension is not perfectly lined up with the torque wrench.

The above errors are in addition to the calibration accuracy of your torque wrench. If you are reasonably careful, each of the above errors will be negligible. But just how careful do you need to be? See the following:

A. You measured 15" from the mark on the handle to the center of the head of the wrench:

But actually should use the center of the handle to the head which is 14"

So you do the math with an 8" extension and come up with 120.7 ft lbs to get 185 ft lbs of actual torque. Because you mis-measured the wrench you have actually put 189.7 ft lbs of torque on the fastener which is a 2.43% error.

B. you measured the extension at 8" but it is actually 7.75" center to center. Here a 15" wrench set for 120.7 ft lbs should give 185 ft lbs of torque but because of the error in the extension length it actually yields 183.1 ft lbs at the fastener which is a - 1.09% error.

C. The wrench is not aligned perfectly with the extension. In this case this would be due to the ratchet mechanism inherent to the wrench. As I place the wrench on the extension it is perfectly aligned, but as I apply force to the handle of the wrench the angle changes by as much as 10 degrees. This is what 10 degrees looks like:

So, here 120.7 ft lbs on a 15" wrench turned 10 degrees relative to the 8" extension does not yield 185 ft lbs. instead it yields 184 ft lbs. At 20 degrees the 185 becomes 181.2 ft lbs. at 30 degrees it becomes 176.4 ft lbs.

Obviously degree errors do not yield linear errors in ft lbs so be careful.

10 degrees is a .53% error

20 degrees is a 2.10% error

30 degrees is a 4.66% error

40 degrees is an 8.14% error

An interesting thing happens at 90 degrees. The effective length of the extension becomes zero and it is exactly like using the torque wrench without the extension. So if you set 135 ft lbs it will yield 135 ft lbs.

CONCLUSION

Making a torque wrench extension is relatively easy and inexpensive, and is a viable alternative to purchasing a separate torque wrench that will get very little use. With a little care, potential errors can easily be kept to negligible levels.