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agh! stripped my second crank in a few miles...

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agh! stripped my second crank in a few miles...

Old 06-27-09, 12:53 PM
  #1  
ianarch
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agh! stripped my second crank in a few miles...

I bought an old Peugot road bike a few months back. It has a stronglight bottom bracket and crankset, and I'm not sure the make on the pedals. They say made in Germany. Anyways, I've never put a bike together, but managed to put this one together pretty easy with some help from a cycling friend. I've worked with cars and metal threads before, so I made sure everything was greased and threading correctly. After taking the bike for a spin around the block a few times, maybe two miles total, the pedal stripped all the aluminum out of the crankshaft. I figured it's an old crank, and maybe I bought it on it's last leg. No problem, I'll buy a nice, brand new Campagnolo Veloce crank. I asked a couple of people if the pedals were okay, and if I needed new ones as well. Everyone said the ones I have should be fine. While I'm rebuilding the BB, I might as well take the whole apart, strip it, and repaint it. I put the whole thing back together this yesterday and this morning, and took it for a spin around the block this afternoon. Right away I noticed my pedal felt a little wobbly. Before I had even gone a hundred yards, the pedal stripped the inside of the crank threads smooth...

Needless to say I'm a bit frustrated. My first thought is that aluminum is just a terrible choice for a crank, but I'm new and so many cranks are made of aluminum, I figure that's not the case. In both instances I've tightened the pedal to crank with a medium hand torque, and insured they were being threaded properly.

I figure everyone is going to say, well duh, your pedal is junk. it's ruined two cranks already, but I want some more advice. Why would a pedal ruin two cranks, especially the original one it came with? I've attached some images and am begging for some input.
Attached Images
File Type: jpg
crank1.jpg (79.6 KB, 17 views)
File Type: jpg
crank2.jpg (52.6 KB, 24 views)
File Type: jpg
crank3.jpg (82.1 KB, 38 views)
File Type: jpg
crank4.jpg (35.5 KB, 28 views)
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Old 06-27-09, 12:58 PM
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This time replace the cranks AND the pedals.

Or have your LBS shoot a helicoil into the stripped arm. And buy some new pedals. You aren't doing yourself any favors the the ones you have.
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Old 06-27-09, 01:09 PM
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Your pedals could be French threaded, which is 14mm x 1.25mm. Threading that into a slightly larger 9/16" threaded crank could result in the disaster you experienced. Pitch is almost the same, so that would be why they threaded in and seemed the right size. Looking at your last photo though makes me think they could be 1/2" threads...which may thread in, but not sure if they'd get tight without destroying the threads while tightening. Either way, not good.

https://sheldonbrown.com/gloss_p.html#pedal

I'd do what John suggested.

Last edited by sickmtbnutcase; 06-27-09 at 01:13 PM.
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Old 06-27-09, 01:22 PM
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I've discovered the hard way that the alloy on cranks is very soft and the threaded interfaces need to be delicately approached. A person needs to be very ginger with threading the parts together, especially in the first few turns. Good luck with the next pair.
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Old 06-27-09, 01:23 PM
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As noted the threads could be wrong but looking at the close-up of the threads the second one is very flat so if it was like this when you installed it most likely it was damaging the threads on the arm as it was going on, it most likely felt a little tight going on, but only you would know that.
Bet another crank arm but change those pedals for sure.
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Old 06-27-09, 01:30 PM
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Looks to me like the pedals have 1/2" thread rather than 9/16". Most of the crappy low-end stamped steel pedals are in that smaller size for steel 1-piece cranks.
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Old 06-27-09, 02:09 PM
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Aluminum IS a terrible choice for any threaded feature. The reason it works on a crank is the threads don't get used (threaded and unthreaded) very much.

You have a problem here. You apparently have mismatched threads on the pedals and the cranks. Do as the others have advised.
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Old 06-27-09, 02:16 PM
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Originally Posted by Mike Mills View Post
Aluminum IS a terrible choice for any threaded feature.
Right, better go ground all those airplanes. Disaster waiting to happen.

Perhaps some education is needed about assembling threaded parts before one goes dismissing something as terrible. One look at those pedal threads and it's pretty clear where the problem occurred. Hint: standard threads should not have flats at the peaks.
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Old 06-27-09, 02:33 PM
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wow, thanks for all the speedy responses!

I suppose I should try to find just the crank arms for the veloce's, as the sprockets are fine. I'll check ebay. My LBS is a good mechanic... I think, but really small and only carries a few Raleigh bikes and parts. it would be a special order for him.
As for the pedals, when you say 1/2" vs 9/16" thread, is that referring to the length of the threaded rod or a dimension of the actual threads themselves. I did notice when I put them on that the threaded rod sticking out of the pedal, that the rod was shorter then the width of the crank arm. You'll notice in the pictures that only the first half of the crank is stripped. The pedal never touched the back of the crank arm. This did concern me when I was putting it together, but the threads felt right to me. I guess I'm not experienced in threading aluminum. should I just buy a matching campagnolo pedal for the cranks to ensure that nothing goes wrong next time? I'm not too picky about my parts, I just want something that works.
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Old 06-27-09, 02:37 PM
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oh, I forgot. The mismatched threads don't seem to explain why the pedals stripped out the first stronglight crank. I bought the bike from an estate sale where it had been sitting in a garage for years. I just don't think someone would have switched the original pedals, but I could be wrong. I would assume that the threads matched on the original pair.
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Old 06-27-09, 03:54 PM
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Indeed you could be very wrong. There is absolutely no reason to assume that some shmuk did not put those pedals in the original crank and that is why it got stripped out. Buy new pedals or get used ones that match the thread pitch of current model cranks...or keep f_ing up cranks, its your wallet.

-j
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Old 06-27-09, 03:58 PM
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Originally Posted by joejack951 View Post
Right, better go ground all those airplanes. Disaster waiting to happen.

Perhaps some education is needed about assembling threaded parts before one goes dismissing something as terrible. One look at those pedal threads and it's pretty clear where the problem occurred. Hint: standard threads should not have flats at the peaks.
Common use does not make something good. For threading, aluminum sucks. Aluminum is notch sensitive. What is a thread other than a tapered notch? I could go on and on, but won't bore you.

Many, many threaded aluminum features in airplanes, spacecraft, and other high reliability applications use threaded inserts (i.e., Helicoils). The only time I have ever had threads fail is when they are in aluminum. Aluminum should not be used for threads when the feature is going to be repeatedly assembled and disassembled.

Even the fix for this disaster will use a Helicoil.

I rest my case.
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Old 06-27-09, 06:05 PM
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Originally Posted by Mike Mills View Post
Common use does not make something good. For threading, aluminum sucks. Aluminum is notch sensitive. What is a thread other than a tapered notch? I could go on and on, but won't bore you.

Many, many threaded aluminum features in airplanes, spacecraft, and other high reliability applications use threaded inserts (i.e., Helicoils). The only time I have ever had threads fail is when they are in aluminum. Aluminum should not be used for threads when the feature is going to be repeatedly assembled and disassembled.

Even the fix for this disaster will use a Helicoil.

I rest my case.
Pedals generally aren't something that is often assembled and disassembled. Millions of bicycles have aluminum cranks and the vast majority of those bikes never have an issue with the aluminum threads. Most issues that do arise from insufficient torque on the pedal or like in this case, a damaged pedal thread leading to damaged threads in the crank.

That Helicoils are used to repair damaged threads proves nothing. Helicoils are stainless steel and if you ask the experts, stainless steel isn't a great material for threaded fasteners either. But, just like aluminum, stainless steel fasteners are used in many applications without issue (think marine).

If you don't know how to properly assemble something, it doesn't matter what the threaded material is, you'll risk damaging it.
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Old 06-27-09, 07:06 PM
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Best guess is that the pedals have the 1/2" thread pitch.

This failure has nothing to do with aluminum being a crappy material.

My 12 year old rockhopper has aluminum cranks, cheap ones to boot, and they've never failed me yet.
over the last five years I have changed the pedals from flats to clip-less in the early spring and back to flats in the winter. So the repeated assembly / disassembly point has been beaten to death here.

I rest my case.
 
Old 06-27-09, 07:11 PM
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Originally Posted by Mike Mills View Post
The only time I have ever had threads fail is when they are in aluminum.
You haven't worked with threaded fasteners enough. I've seen threads fail in aluminium, all sorts of steel, and titanium. You have to consider that EVERY single Honda cylinder head and block uses threaded aluminium. That alone probably accounts for a significant portion of all threaded holes in the world! Not to mention adding in Toyota, Nissan, Mazda, BMW, VW, Audi and even Porsche has threaded aluminium holes in their autos.

Failed threads isn't a problem with the material, rather a mistake in the installation procedures.

The flat peaks of the 1st couple threads on those pedals are due to beveling after the threads are rolled. This is a cheap method to make starting the threads in the crankarm hole easier. However, it's not as optimal as tapering the diameter with fully-formed threads.

Last edited by DannoXYZ; 06-27-09 at 07:23 PM.
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Old 06-27-09, 08:22 PM
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With an older Peugeot and Stronglight crank, both French, I would suspect French threaded pedals as being quite possible, depending on the age of the bike. To me more likely than 1/2" pedals.

I admit though that the pedals look like dirt cheap old rattrap pedals so hard to say what thread they are.
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Old 06-27-09, 10:24 PM
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It has been quite a while, like 35 years, but I used to race a peugeot PX-10. I still have it and am in the process of restoring it. When I trashed the original french threaded pedals (about one season), We rethreaded the cranks to 9/16". The reason I did this was becasuse there were no high quality French pedals. Leotard made the originals but they were not very good. It is also possible that the Campy crank , depending on age, could have Italian threads. There were three metric sizes in that era: French, Italian, British(Japanese). Almost all bike today use the British metric thread size. We even had a hard time at that time getting French threaded repalacement parts.
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