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Wrench Tip: Torquing Unreachable Fasteners

by Nikita Rushmanov

 

Have you ever looked at your service manual for a torque spec, only to curse, “What the hell were those engineers thinking?” We certainly have, but there is always a way to get things done by the book. The perfect example of an impossible torque spec is the driveshaft to differential flange on a 240sx. In this case, it’s impossible to get a socket on the nut because there is not enough room between the flange and the differential housing. Additionally, the hex head of the bolt on the other side is jammed against the flange, so you can’t torque that either. As a result of this, a lot of 240 owners we know never actually use a torque wrench on their driveshafts, instead opting to torque to “2 uga ugas” with an open ended wrench. To compliment this, we’ve also seen a few 240 driveshafts rattle loose, or in some cases, come off completely, punching holes in the floor pan.

So what’s the solution? You use a crowfoot wrench and some math. A crowfoot wrench is basically an open ended wrench that has a ⅜” or ½” square drive instead of a handle. This means that if you attach it to your torque wrench, you get an open ended torque wrench! You can pick up a set of crowfoots from Harbor Freight for as little as $10. However, there is one little caveat. Attaching that crowfoot changes the lever arm of your torque wrench, since the nut or bolt is no longer spinning on the same axis as the square drive of the torque wrench. We can compensate for this lever arm offset using math.

 

We usually use this Snap-on torque wrench, which has a menu where you can enter your crowfoot offset, and it will automatically compensate. However, for this demo, we will use my “missile” Harbor Freight torque wrench.

So once you have the crowfoot, you need to know 4 things to calculate the new torque setting:

  1. The target torque setting.

  2. The length of the lever on your torque wrench.

  3. The axis offset distance of your crowfoot.

  4. The angle of the crowfoot relative to the torque wrench.

 

The length of the lever is the distance between the axis of rotation of the square drive and the center of the handle. For this torque wrench, the distance is 11 inches.

 

To calculate the crowfoot offset, we usually take off the nut or bolt I’m torquing down and put it inside the crowfoot. This allows you to see where the axis of rotation is. For this 14mm crowfoot, the offset is 1 1/16 or 1.0625 inches.

The service manual calls for a torque spec of 29-33 ft-lbs, so our target torque setting is 30 ft-lbs. For this particular application, we find it easiest if the crowfoot points straight, which means our crowfoot angle is zero. Now we have everything we need to calculate our new torque setting. For the calculations, we like to use this online calculator to make things easier.

 

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Comments
nissannx
nissannxlink
Friday, June 16, 2017 7:12 AM
What's this! More proof that paying attention in class at school can have positive benefits! Ha Ha - great article. Love to see some more.
Entropy
Entropylink
Friday, June 16, 2017 1:34 PM
Only bolts I've ever torqued are internal engine parts and lug nuts. Never had an issue. Just have to be careful and resist the urge to keep tightening.
Crousti
Croustilink
Friday, June 16, 2017 3:59 PM
are these crowfoot really straight ? Spanners are usually angled 5° , i wonder if it is the same here.
StrangeLiform
StrangeLiformlink
Friday, June 16, 2017 4:57 PM
Naw, they're dead flat. I used crowfoot wrenches all the time in aircraft maintenance school. This trick is frequently useful on smallish turbine engines like the Alison 250 and other tightly-packaged aviation hardware. You torque (and safety wire) EVERYTHING when it comes to aircraft.
CrickiKaze
CrickiKazelink
Saturday, June 17, 2017 12:43 PM
If you have the space a "dogbone" or a line fitting crows foot does a better job but if you lack space around the nut the crows foot works great, I just worry sometimes that rounding the hardware is easier pending the torque on a regular crows foot.
Cam wow
Cam wowlink
Saturday, June 17, 2017 2:48 PM
If you'd actually paid attention in school you could skip the math, rotate the crows foot to a 90° angle, torque to the specified value, profit. Check with the calculator - story checks out.
SM_Clay72
SM_Clay72link
Thursday, June 22, 2017 8:34 AM
^ They mentioned that.
Rockwood
Rockwoodlink
Thursday, June 22, 2017 2:20 PM
For the sake of simplicity and possible lack of Interweb access: the additional length is determined by the percentage of 90* it's offset by. So, a 1" crow's foot at a 45* angle adds roughly 1/2" to the lever. From there, it's simple math to determine the percentage change: a 10" torque wrench lever with an additional inch of crow's foot needs 10% less setting to reach the same target (30ft-lbs would be 27 now).

Realistically, since the torque range for most fasteners varies by more than 10%, you could just go with the low spec and be fine. :)

Also: F you for using the driveshaft as an example. :) My damned Jeep's 1310 yoke has threaded holes with 5/16" fasteners holding that driveshaft in place (comical, I know). These are hardened and break ludicrously easily. I know this. I also know that since they're loc-tited into place and are hardened, extraction is not easy and the small fastener diameter means EZ out options suck (and break). Since I just dropped a bunch of cash on a locker and re-gear, I'm now dealing with the pain of removing the yoke, making sure backlash didn't get AFU'd, and reassembling. Woe. Woe is me. :)
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