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Hey folks. When re-torquing a bolt that is not tight enough, can one just tighten it straight away to spec or is it best to unscrew it a bit and then torque it to spec?

cyccommute 

If the bolt isn’t tight enough, just torque it to spec. That’s the way you would torque a freshly installed bolt anyway.

sweeks

How did you know it wasn't properly torqued inthe first place?

Iride01 

It's sort of a decision you have to make for the unique circumstances that you are having and seeing for that particular bolt or nut that you want to tighten. Stiction is a thing sometimes. And if the bolt or nut is almost there but not quite if may not break free of the stiction for that last little bit of torque. Also, if the threads are not in good condition, corroded or contaminated with certain substances, then it' might be a good idea to loosen first. (better maybe undo it completely to clean it up and put a little grease on it)

Conversely when trying to undo bolts or nuts, I've sometimes found that if they won't turn to loosen, I can sometimes tighten them a tad more and get them to break free of whatever is keeping them from turning and then they loosen easily. Though seldom on a bicycle have I had to do this.

13ollocks

Personally I'd back it off and re-torque "fresh"

Andrew R Stewart 

I'm with the back off and retighten to torque spec crowd. That's what I was taught. What I violate is the dry bolt only when needing a torque spec rule. I add some sort of lube or locker to most all my threaded fasteners.

Funny story about bolt torque and experience and ego. As the whole small bolts replacing big ones and AL or carbon fiber constructs became the thing many/most pro mechanics were reluctant to use a torque wrench for tightening stems, bars, posts and such. They, as I was/am, very experienced with how strongly to tighten the various bicycle fittings that were the norm in the 1970s/1980s. "I know how tight to make it" was a common claim. For the LBS level of wrenching this was good enough most of the time, between the experience with both big and small bolts and that we tend to only touch a repair bike more than once or twice in a season a "good enough" tightness is fine. But at the pro ranks the bikes see nearly daily attention from the mechanics. At a Tour De France a Park Tool guy (I think he was a Park guy?) went around with his own torque wrench and was checking various teams bikes on his own. What he found was that at the start of the tour the mechanics had set the various torques about right, as though they did use a torque wrench before coming to the race. But as each day's stage wear and grime was washed off and checked for the hardware would get a small amount more tightening "just to make sure". By mid tour this guy found many fasteners were far tighter than they should have been.

This just reinforced my belief that the bolt/nut should be slightly loosened before retightening when checking for torque spec. And that even pros should learn new ways. Andy

Koyote

This.

If you're using a beam-type torque wrench, you can see how much a bolt is over-torqued; if you're using a click-type torque wrench, the only way to know that a bolt is at the correct torque (vs over-torqued) is to back off a bit and then tighten it.

Of course, if a bolt is visibly loose, then just have at it.

Calsun

I use a torque wrench on cars and on boat engines but seldom on bikes. Most of the bolts are so small that I tighten by hand with a small wrench. No vibration with a bike unlike motorized vehicles and if there is a worry then there are usually special washers designed to keep a fastener from loosening. Common with bike in the 1970's were broken bolts used to fasten the front derailleur to the bike frame. Being 8mm and hardened steel (and more brittle) if someone use a socket wrench they were going to apply enough torque to snap the head off.

Andrew R Stewart 

"If you're using a beam-type torque wrench, you can see how much a bolt is over-torqued" Koyote

Unsure about this as I believe you're also sensing the bolt's stiction or corrosion. I was taught that the torque wrench readout (and we could talk about the difference between a scale and a gage) is only valid while the bolt was turning. Andy

sweeks

This is what I learned as well. The applied torque is supposed to be stretching the threaded part of the bolt (using the threads as an inclined plane). When the bolt isn't turning, there's static friction added to the torque wrench reading, which renders the wrench reading inaccurate. Also, the bolt's threads are generally lubricated to get the proper amount of stretch for a given applied torque.

maddog34

remove, clean threads, inside and out, lube, re-torque to spec.

sweeks

This doesn't actually answer the original question, but nicely shows what happens when a bolt is tightened. ( )
The consensus ^^ supports at least backing off, then re-torquing to spec.

grumpus

Agreed on all but the very last point. Many fastener torque ratings are quoted dry - greasing will potentially let you stretch or strip the fastener before you reach the stated torque. This is possibly more applicable to diesel engines than bike parts, but is something to bear in mind.

redshift1

Great question. To do it properly, you should back it off first ( or if the bolt is holding the position of something that you don't want to move, then back it off most of the way ) before torqueing. And the reason is because static friction in the only partially tight fastener may exceed the applied torque if you don't first loosen the bolt. So the bolt will not move / get tighter. If you noticed that the bolt did in fact move as you tried tightening it, you might be o.k. but it's really hard sometimes to know whether a fastener moved a bit or not, so it's best to back it off and start from scratch. In the older days before torque to yield bolts came along, and earlier than that, when "no-retorque" gaskets were used, good auto. mechanics would always back off all cylinder head bolts before re-torquing a cylinder head.

Yes on this. Lubricants have an effect on the so-called " nut factor" which is an experimentally determined value that attempts to include friction effects when calculating the correct torque for a fastener. For many greases, you reduce the dry thread torque value around 10 % and for many oils, around 20 %. Many years ago, I went to a machine shop and the proprietor there told me of a car race team in the U.S. that decided to use "STP" on the connecting rod bolts without lowering the tightening torque setting. As this stuff was "super slippery", it lead to rod bolt breakages through over-tensioning the bolt. The moral of the story is, if you use a book torque wrench figure, verify what conditions that figure is given for e.g. dry threads, oiled threads, etc. But it's not always easy to find this information on given torque figures ! That's very remiss of the people who provide torque values in publications without including this information.

SurferRosa

Depends on the bolt. If we're talking about a crank bolt in a square taper BB, you don't just remove it and reinstall, willy-nilly.

Andrew R Stewart 

Don't confuse the arm's interface on the axle with the retaining fastener's. Crank arm bolts and nuts can be removed and replaced willy nilly with no other effect and as long as they are torqued to the correct spec the arm will keep on the axle.

Now if we're talking about a cotter pin... Andy

SurferRosa

A good way to damage a crank arm's square taper is to retorque its bolt. Do it once during installation and forget it.

Sy Reene

Couple comments/questions.
How is it known that the bolt isn't tight enough if not using a torque wrench?
If you don't loosen a bolt first, you wouldn't know (unless using a non-click wrench) that your bolt could already be over-tightened (past the torque rating).

frdfandc

As a former lead bicycle mechanic, if I find it better to just re-torque it to the proper specs. However, if there is more than one bolt not at the correct torque specification, loosen then torque them all to spec. Stem bolts should have a little bit of blue Loctite on them to help prevent loosening. Some people use grease, but with grease, I find it easier to over-torque the bolts, than either dry or have blue Loctite.

KerryIrons

A perfect summary with a great anecdote to back it up. The OCD nature of some people to "check their bolts" can result in chronic over-tightening.