Torque wrenches and caliper bolts
 

So yesterday I was installing my restored calipers. I had cleaned up the caliper bolts (wire wheel) and they looked OK in general. A little antisieze and I started tightening.

The shop manual states 120 ft lbs, so I set my torque wrench (a halfway decent Craftsman click type) and had at it. It seemed like as I approached what "felt" like it should be right, the wrench wouldn't click. The bolt just kept "yielding" a little more and a little more and then... bang. Broke the head right off.

I thought I was doomed (easy out for that kind of torque? how to drill in that location?) but managed to get some needle-nose vice grips in through the caliper and back the bolt out (dodged a bullet there!).

So, two questions:

- Are the caliper bolts so standard that I can get one at OReilys or is back to payroll deduction at LMC for me?

- Anyone experienced this with a torque wrench? It normally seems just fine, but there have been three times over the years when it seemed never to click and I wound up breaking something.

I don't mean its miscalibrated (though it could be), or it would always be off. It's just one out of 50 times it seems to NEVER agree to click.

Re: Torque wrenches and caliper bolts
 

A thought, was there anti seize on the threads? One must remember, that unless the threads were lubricated from the factory or the torque spec is noted as "oiled" you must reduce the torque by approximately 25% to get the same clamping force as a "dry" fastener. By applying the same torque to a lubricated fastener (such as anti seize), you are creating a much higher clamping force, and thus you are more apt to stretch the bolt to yield.

As far as the torque wrench not clicking, I have a newer craftsman torque wrench that seems fine above 100ft lbs or so, but below that it seems to do what you are speaking of. I don't trust it very much, and tend to use my older wrench instead.

Re: Torque wrenches and caliper bolts
 

Good point. You're right that the listed torque values are almost always for "clean and dry", and that alone would have led me to overtorque it.

Whether that's the explanation or there's still something amiss with the wrench, I don't know. I'm not even sure where to take it to be checked or calibrated without mailing it off somewhere.

Re: Torque wrenches and caliper bolts
 

Not doubting you, or your manual, and maybe I'm thinking of different caliper bolts than you, but 120ft-lb. seems WAY excessive. The caliper bolts are 7/16" if we're on the same page, and a 7/16" bolt (depending on grade and type) shouldn't ever be over 80ft-lbs. (60ft-lbs. lubricated) unless they are some special alloy (which I doubt). I would bet they are Grade 5 or so, and would torque to even less.

I just installed new Wilwood calipers on my truck, and I believe the instructions say 35ft-lbs. or so (I don't ever actually torque mine). The instructions also specifically stated to NOT lubricate the threads.

You should be able to pick them up on the Help rack at you local parts store, and I know CarQuest (others too I'm sure) usually stocks them boxed up in the back somewhere too.

Re: Torque wrenches and caliper bolts
 

There is now way that 120 is correct. A grade 8, 7/16 bolt doesn't even take that much torque before breaking , let alone a grade 5 caliper bolt. Lucky you didn't break the thing off. As stated above maybe 35-40. Just snug it down

Re: Torque wrenches and caliper bolts
 

My shop manual says 35 ft lbs. I use anti-seize and I use a torque wrench only so I don't over tighten them. You can get new bolts at any DYI store.

Re: Torque wrenches and caliper bolts
 

120 is definitely not correct, re check that spec!

Re: Torque wrenches and caliper bolts
 

I agree with all of the above.

Don't have my manual available, but this sounds much more like it, IIRC. I just did this very rebuild on my truck as well. And I don't own a torque wrench. I think for engine rebuilding and the like, they are essential. But most other stuff, not so much IMO.

Re: Torque wrenches and caliper bolts
 

I've never torqued a caliper bolt. Just a good "gorrila grunt" and it's tight.

Gary

Re: Torque wrenches and caliper bolts
 

Well, I prefer to torque everything to spec rather than just strong-arm it. I figure the factory wouldn't have gone to the expense if it wasn't worth it.

That said, those that noticed my torque value was too high were spot on: I checked and the 120 ftlbs is for a "Caliper Housing Bolt", on the Corvette only. The caliper bolt on front calipers is only supposed to be 30 ft lbs (for passenger cars, anyway... I don't have the '72 truck manual, just a '70 before discs were available, so 35 sounds right).

As you can imagine, there's a big difference between 120 and 30, certainly enough to snap a bolt head off!

Thanks for all the input!

Re: Torque wrenches and caliper bolts
 

Dave, when all else fails, you can usually go by a chart according to the size of the bolt and the bolt material. The exception being when threading into something softer like aluminum, or TTY bolts etc.

Here is one you can go by, note that the max torque is 54 ftlbs for a 7/16 bolt like that in grade 5, and that is a dry torque. Any anti seize or oil used would reduce that to roughly 40.

http://www.engineershandbook.com/Tables/torque.htm


Of course if you can first get proper information like that 30ftlbs for the bolt as spec'd by GM, go by that.

Re: Torque wrenches and caliper bolts
 

It does mention lubing the bolt. not the threads specific.

Re: Torque wrenches and caliper bolts
 

Yes, lubricate the sliding portion on the bolt, not the threads though.

Re: Torque wrenches and caliper bolts
 

That's a very handy site. I actually have their chart saved on my computer, printed out & posted in my home garage.

It should also be noted also that torque specs can deviate a tad according to the bolts fit class: 1A is loose, 2A is normal to most of our trucks fasteners and 3A is for where very tight tolerances are needed.

As GCncsuHD mentions, he material the bolt is made of has a huge difference too, as seen in the second screen shot from Engineers Edge.

Very good thread you have started for us Dave! Thanks! :metal:

Re: Torque wrenches and caliper bolts
 

120 ft lbs is way to much 35-40 but i been doing brakes forever i i get them tight but i'm using a ft long wrench never had one back out or come off