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Old 05-18-09, 07:41 PM   #1
xenologer
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Aluminum Thread Stripping Help?

The other week I was replacing the brake cable on my cantilever brakes, and ended up stripping the threads for the anchor bolt. -the threads on the brake arm itself (not the bolt)
Ended up buying new brakes... ohwell

Just now today, I was working on my brother's bike adjusting his brakes, and once again stripped the threads out of the brake arm's anchor bolt hole.

I don't *think* I'm being unreasonable with torque, I hand tighten the anchor bolts with a 6inch crescent wrench.
Maybe the fact that the brakes are made of aluminum and I'm using steel bolts doesn't help?

Any advice or tips on how I can avoid doing the same thing in the future?
I don't have a torque wrench...


thanks
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Old 05-18-09, 08:58 PM   #2
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A 6" adjustable wrench will easily strip a small bolt. A cable binder bolt only has to be tightened enough to clamp the cable in place - any more is totally unnecessary and will either damage the cable or strip threads.
If the threads that were torn out were 5mm, use a 6mm tap to cut some new ones and replace the binder bolt with a 6mm one too.
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Old 05-19-09, 12:33 AM   #3
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How much is enough?
I've been doing it as-hard-as-possible-by-hand and of course I worry about undertightening considering this is a brake cable and I don't want to discover it wasn't tight enough next emergency stop.
Having real problems discovering this magic margin between tight enough, and whoops stripped threads again...

Almost makes me want to switch to steel-only brake lever parts...
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Old 05-19-09, 12:46 AM   #4
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Well, I would suggest you are indeed being unreasonable with torque given you're stripping things. Been there, done that. But thankfully on my own bikes as I learned to wrench on them. I tighten and test the brake by clamping the lever with both hands, if the cable slips I reposition it and tighen more, when it dosen't slip then I figure it's good enough. This being said, an easy way to recover stripped brakes is by drilling through them and putting a longer bolt with a nut at the other side.
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Old 05-19-09, 02:27 AM   #5
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With a 6 inch leverage from the crescent or regular wrench you will easily strip out the binder bolt when you tighten with that much leverage as hard as possible. In fact I'm surprised they withstand even the FIRST time doing it that tight rather than only stripping out later on.

The Park Tools website with a PDF listing all common bicycle part torques lists the cable pinch bolt torque at 53 to 69 inch pounds. That translates on your 6 inch crescent wrench to roughly a 4 to 5 lb bag of sugar hanging from the hole on the end of your 6 inch crescent wrench. Now if you can't hold up a 5 lb bag of sugar with only one finger held horizontal then there's something wrong with your strength. And we know this isn't the case since you stripped out those bolts. So if you need more than one finger working as hard as holding that 5 lb bag of sugar to apply the correct torque then you're overdoing it. Clearly you overdid it by your own description.

The Park Tools torque spec page is.....
http://www.parktool.com/repair/howtos/torque.pdf

To equate inch-pounds to foot lbs you just divide the spec they give by 12. If you find you can't judge that sort of torque then a torque wrench should be high on your priority list.
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Old 05-19-09, 03:42 AM   #6
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Cool table
thanks


Huh, this would also explain why when I tighten down derailer clamps, I tend to slice through cables... steel clamp just bites em in half.

guess I need to tone down the caffiene or something...

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Old 05-19-09, 03:51 AM   #7
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How much is enough?
I've been doing it as-hard-as-possible-by-hand and of course I worry about undertightening considering this is a brake cable and I don't want to discover it wasn't tight enough next emergency stop.
Overtightening is actually worse than under. You end up pinching and cutting the cable. On the next emergency stop, the cable snaps, leaving you with zero brakes. At least if it was loose, you'd still have some braking ability. Use a torque-wrench. A lot of shops require sign-off sheets on critical parts like handlebar stems, crankarms and brake binder bolts; unless a torque is written down in those spots, the bike doesn't go out the door.
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Old 05-19-09, 05:53 AM   #8
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when I first started to wrench on things I was told to use the "proper tool for the proper job". A crescent wrench is a toy NOT a toll. Used by someone who is to lazy or cheep to get what they really need. You have learned valuable lesson.
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Old 05-19-09, 06:44 AM   #9
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Well, the concensus, and the physical evidence, is that you badly over tightened the anchor bolts. I agree if you used a 6" wrench and tightened the bolts "as-hard-as-possible-by-hand", you certainly did over torque them.

So now what? You have two ways to fix the damage.

The first is to drill and tap the damaged hole for the next size up bolt. That will be difficult as the blind threaded hole is in a relatively thin brake arm and getting the hole drilled and re threaded will require a bottoming tap and these are difficult to get started and a regular tap won't have much to work with.

The second alternative is to drill the damaged hole to clearance diameter all the way through the brake arm and buy a longer, same diameter bolt, a nut and a lock washer. Fasten the cable using the nut and bolt arrangement.
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Old 05-19-09, 08:17 AM   #10
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Thats way too much torque. Thiose are teeny tiny bolts that need very little torque.

First, be sure all the threads are perfectly clean and oiled. Next, just use a hex or nut bit on a screwdriver style 1/4 inch driver or use one of those 3 in one bike specific tools and just hand tighten.

Now, pull on the brake lever as hard as you can, pull it all the way to the bar and hold it there for a few seconds. If the cable doesn't slip, you're fine.
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Old 05-19-09, 08:47 AM   #11
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One third of people who start fixing bikes overtighten. One third undertighten. One third don't do either.

You're an overtightener.

I was an undertightener.

I learned.

You'll learn. Or you just did.
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Old 05-19-09, 10:21 AM   #12
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One third of people who start fixing bikes overtighten. One third undertighten. One third don't do either.
I've also heard the general rule that new mechanics tend to over torque small fasteners and under torque large ones. The numerous posts here about stripped small bolts and loose crank arms and bottom brackets seems to substantiate this observation.
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Old 05-19-09, 10:25 AM   #13
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Probably more accurate than my assertion. My point is that we learn from our mistakes. Well, I hope so. I'm sure xenologer will.
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Old 05-19-09, 10:48 AM   #14
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I think people who learned on steel bolts going into steel parts learn a different feel. You can feel the bolt start to just yield a tiny bit. Steel can withstand that kind of treatment even if it is a bit heavy handed. Aluminum can't and you'll fatigue the threads in just a few tightenings.

You don't want to go to aluminum bolts. It won't be just as strong just because the hole is made from the same stuff. The root of the threads is a lot smaller on the bolt than the hole.

Just go on Ebay and get a torque wrench. Find a nice small one, then do a cost-benefit analysis while riding and looking at your aluminum stem!
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Old 05-19-09, 11:10 AM   #15
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Better to go easy/undertighten and bench test with a simulated hard stop... you can always readjust.
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Old 05-19-09, 12:31 PM   #16
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Thing is, you can fatigue aluminum threads pretty easy even if you don't strip them the first time. I'd go torque wrench.
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Old 05-19-09, 12:40 PM   #17
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Imagine me saying, in a scratchy old man's voice, "Why in my day, we didn't have torque wrenches. And we LIKED it that way."

:rofl:
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