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Old 08-12-10, 12:50 PM   #1
andy.c
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What happens if you undertighten pedals?

I installed my pedals myself... I think I may have undertightened them. I don't have a way to measure the torque.

So then today my right pedal feels funny, and I'm wondering what's going on. Then it falls right off.

I am confused because the pedal threads were totally stripped. There were many metal coils coming out of the hole. I got special short crank arms that were tapped by hand, so I guess that was the insert to make threads in alumnium. I'm just wondering if it's more likely that undertightening somehow caused this problem, or the threads weren't tapped well.

My understanding is that if you undertighten the pedals, they should naturally tighten just by pedaling? This thread seems to say that: Installing pedals - how much torque?

I rode the bike probably ~10 miles with this setup over a couple days. So it failed pretty fast.
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Old 08-12-10, 01:00 PM   #2
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"special short crank arms that were tapped by hand"?
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Old 08-12-10, 01:07 PM   #3
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Is that how a helicoil would self-destruct?
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Old 08-12-10, 01:10 PM   #4
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Originally Posted by Torchy McFlux View Post
"special short crank arms that were tapped by hand"?
I have no idea what that means either, but it sounds like you've got some old and/or junky cranks/pedals. I never tighten my pedals, after they're finger "tight" I just barely snug them with a pedal wrench. And I do mean barely. Never had a problem at all. The problems come when you over-tighten, especially if you don't clean and/or grease the threads before installing.
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Old 08-12-10, 01:17 PM   #5
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OK thanks for the replies. Yes I bought very short (100mm) cranks since that were shortened from from stock cranks since I have a knee flexion problem. It sounds like the undertightening is probably not a problem, although perhaps there is some weird interaction with the helicoil.

Do normal aluminum cranks have some kind of steel threaded insert? I think that would be normal since aluminum is perhaps too soft to have a steel pedal threaded into it.

It basically seems like the helicoil did self-destruct. Half of the hole is smooth and the coils came out in pieces.

Also I used white lithium grease on the pedal threads which I believe should be fine.

Last edited by andy.c; 08-12-10 at 01:22 PM.
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Old 08-12-10, 01:22 PM   #6
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Are you certain that you didn't cross-thread the pedal when installing it?
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Old 08-12-10, 01:25 PM   #7
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While it is true that pedals will tend to self-tighten during use there's still a window of opportunity where a too loose assembly will allow the pedal to wiggle back & forth and chew up the threads that way. What do the crank arms look like? Such a radical shortening might mean that there isn't enough metal to cut threads in at that position.

And no, normal aluminium arms do not use any inserts. Fofr the overwhelming majority they're still strong enough.
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Old 08-12-10, 01:28 PM   #8
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Are you sure that the cranks were helicoiled? I've never seen a properly installed helicoil back out the way you described. Heck, I've never seen a badly installed one do that. Could you flex or break the coils that came out? If so, they weren't helicoiled, and those are the shards of the aluminum threads.
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Old 08-12-10, 01:33 PM   #9
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100mm crank arms? I would like to see a picture of those. I have never heard of anything close to that short, except for unicycles.

My guess is that half the threads came out, and it is not a helicoil. Most crankarms are pretty soft aluminum, and you can peel the threads out if you are not careful.
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Old 08-12-10, 01:40 PM   #10
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Yeah i sounds like the cranks were shortened, drilled and tapped. OP, by not tightening the pedals properly the pedal wiggled in the threads breaking them from the arm and stripping the hole. Take the crank and get a helicoil put in for the pedals at a good bike shop. Then when screwing the pedal back on after reassembly, grease the threads before hand and the torque the pedals properly. NEVER ride loose pedals or expect them to "tighten" themselves.
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Old 08-12-10, 01:41 PM   #11
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Yes, undertightening was the most likely cause of the problem - the Helicoil does not "self destruct." Once pedals come loose and continue to be ridden, even a relatively short distance, havoc results.
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Old 08-12-10, 02:04 PM   #12
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In my experience, helicoils fail - and fail often - when they're used in a crankarm/pedal capacity. They're a "last resort" type of repair thing that isn't reliable.
The person who customized your cranks shouldn't have used them. All forged aluminum cranks have the pedal threads cut directly into them. The fact that yours didn't makes me think that the person who did the work either didn't know what they were doing, didn't have the proper tooling to do the job well, or they just screwed up.
The problem isn't you or your pedals - it's your poorly modified crank.
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Old 08-12-10, 02:18 PM   #13
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Hmm... so yeah I'm not 100% sure it is helicoil. Here is a picture:

http://chubot.org/share/stripped.jpg

I thought those coils were from a helicoil, but it could just be aluminum threads stripped out? I have actually never seen a helicoil so I don't know how to tell for sure. I e-mailed the guy to ask. Sorry I didn't want to call attention to the mechanic who did it if it was my fault.

Is that not how helicoils break?

It would be a little surprising to me if aluminum threads would come off in "clean" pieces like that. I could see them getting damaged easily but not coming off in coil. But I am a noob when it comes to these things.

Has anyone heard of this happening before? Undertightening, and then the aluminum threads come out like this?

Last edited by andy.c; 08-12-10 at 02:24 PM.
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Old 08-12-10, 02:29 PM   #14
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That's not a helicoil. Those are aluminum threads that have torn out.
You can't drill and tap a crankarm by eye and hand and expect any kind of precision or quality. A drill press should have been used to ensure the squareness of the bore and threading, at least.

Last edited by Torchy McFlux; 08-12-10 at 02:36 PM.
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Old 08-12-10, 02:39 PM   #15
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Quote:
Originally Posted by Torchy McFlux View Post
In my experience, helicoils fail - and fail often - when they're used in a crankarm/pedal capacity. They're a "last resort" type of repair thing that isn't reliable.
I'm sorry you have had problems with. I never have. They are usually stronger, esp. metal helicoils in aluminum.

Quote:
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The person who customized your cranks shouldn't have used them. All forged aluminum cranks have the pedal threads cut directly into them. The fact that yours didn't makes me think that the person who did the work either didn't know what they were doing, didn't have the proper tooling to do the job well, or they just screwed up.
The problem isn't you or your pedals - it's your poorly modified crank.
We don't know that he used them. It doesn't sound like they were used. Where do you get that from?
And riding with loose pedals and expecting them to tighten on their own is a problem. Stripped pedals are a certainty in that situation, not a possibility, modified crank or no.

Quote:
Originally Posted by andy.c View Post
Has anyone heard of this happening before? Undertightening, and then the aluminum threads come out like this?
Yes, loose pedals will cause this . And yes, aluminum strips in coils from crank arms like this.
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Last edited by canopus; 08-12-10 at 02:43 PM. Reason: Sorry Late post.
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Old 08-12-10, 02:48 PM   #16
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10 cm crank arm sounds like a kiddy crank conversion may be where to get replacements. now that the others have been damaged by leaving the pedals under tightened.
http://www.sjscycles.co.uk/product-T...+-110-3079.htm
ttp://www.sjscycles.co.uk/product-Th...-110-11106.htm
A top notch bike shop may be able to fit pedal inserts, a bit different from heli-coils ,
as its a sleeve , a tube threaded inside and out, whereas Helicoil is a coil of square wire,
wound sortof like a spring..

Last edited by fietsbob; 08-12-10 at 03:02 PM.
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Old 08-12-10, 02:52 PM   #17
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Quote:
Originally Posted by canopus View Post
I'm sorry you have had problems with. I never have. They are usually stronger, esp. metal helicoils in aluminum.
And that opinion runs counter to my own experience. Deal with it.

Quote:
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We don't know that he used them. It doesn't sound like they were used. Where do you get that from?
And riding with loose pedals and expecting them to tighten on their own is a problem. Stripped pedals are a certainty in that situation, not a possibility, modified crank or no.
His stating that there were threaded steel inserts might have had something to do with it. Try reading the thread. He never stated that the pedals were loose - just that he had concerns about them not being torqued down enough.
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Old 08-12-10, 02:56 PM   #18
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I dunno, but I always tighten pedals on the crank pretty good. Just don't like the idea of riding on anything just "finger tight". I've seen enough instances where parts get broken from undertightening because of the play that develops between the two parts.
Anyway, it will take a huge amount of force to strip the threads on a crnkarm hole by over tightening the pedals on it. And if you have enough mechanical experience, you tend to develop a feel for things that you don't end up stripping parts from overtightening.
Never had any problems taking off my pedals from my bike's cranks either, despite having them on tight.

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Old 08-12-10, 03:19 PM   #19
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I also set mine pretty tight. I don't want any movement in the threads and this is why.

Since these pedals were custom modified, I believe they should be returned to the mechanic who did the work for a repair or replacement.
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Old 08-12-10, 03:39 PM   #20
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It would help to post pics of the damaged crank arm, and if possible, remove the pedal from the other arm and post a pic of that threaded hole as well, so we know what sort of custom job was done to create these special arms. And it might inspire some folks to suggest how to proceed either to fix the damaged arm, or specify something else to meet your unique short-arm challenge.

To your one question in the original post, no, there are no inserts, typically. The pedal shaft threads directly into the arm. AFAIK, the BEST thing to use on the threads is the thing you generally use with dissimilar metal threading situations like this (similar to a steel spark plug in an aluminum cylinder head) -- anti-seize thread compound. This allows you to torque the shaft down fairly tight, and still remove it later even after some heavy pedal pressure's been applied over time. I just pulled the semi-cheapo Wellgo pedals off my 105 crankset after five years and, Idunno, 5000-6000 miles (yeah, I was off the roadie for too long), and although they required a cheater pipe on the combo wrench to start loose, they came off just fine with no damage to the cranks, and totally clean except for some of the anti-seize still visible on the threads.
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Old 08-12-10, 03:40 PM   #21
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Threads are supposed to deform slightly when tightened. Finger tight with a little extra isn't enough in an interface like pedal to crank. You don't need to "Conan" it, as in the barbarian, but in 30 years I've never seen pedal strip out a crank that wasn't either cross-threaded or not tight enough to start.
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Old 08-12-10, 05:32 PM   #22
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I have had an undertightened pedal back out and strip the crank arm.
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Old 08-12-10, 05:38 PM   #23
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Quote:
Originally Posted by Torchy McFlux View Post
That's not a helicoil. Those are aluminum threads that have torn out.
+1 Those are threads stripped out.
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Old 08-12-10, 07:11 PM   #24
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Torchy, did someone piss in your cornflakes this morning? You seem to have a lot of angst going on today. All this sunshine got you down or something...

I'm far more inclined to suspect that there are no heli or other coils in these arms and what he's seeing is just the peeled away aluminium. But unless he only used one finger on the wrench I can't see the pedals being so under torqued as to cause this to happen in only 10 miles. After all our OP has a stated knee issue that lead to the modifying of the crank arms to this odd length.

It's important to note that pedal threads are NOT standard. They are up in the extra fine category. Then there's the left hand threading for the left hand crank. All of this makes me wonder if the shop guy that did this mod used an expensive custom order pair of taps or if this was done by grinding some chip cutting channels into a set of old pedal axles. While doing this to a set of pedal axles produces a set of sweet occasional use threading chasers it's hardly the optimum solution for cutting new threads of the proper depth. And then if a drill press and the optimum drill size were not used then it's very hard to say if the original threading was up to snuff or not.

To our OP, you're the only one that can accurately comment on this part. Was the guy you got to do this job for you standing in a shop full of fancy machines or was it the local bike dude that may be able to fix your bike but is short on machine tools for doing things such as this?
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Old 08-12-10, 07:19 PM   #25
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The good news is that a good shop can fix it, but probably will better for you to get a new set (or single crank) because the guys will ask you a few bucks for that. And might be better go new, costes.

Besides the links posted with shorter cranks, there was another guy who was doing it also, the good about him was that he was saying that not all the cranks were good for shortening because of the aluminum or the way the cranks were forged. Either way is werid what happened to you, once the pedal is in the threads shouldnt "move" at all in my opinion, just wonder if somebody put the pedal wrong or the pedal is so cheappy that the threads of it are pretty rough, and in your case all needs to be perfect.

Good luck
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