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Something really strange happened to me - 2 years ago I bought a bike (Carrera Gryphon Hybrid from Halfords). I had just about 100 miles on that bike the month I bought it and than I locked it to a covered bike stand. I now tried to use it again but within a mile the left pedal came off. I tried to go on with the other pedal and after another mile it came off, too.

First thing I noticed is that one of the pedals must have not been screwed in till the end (thank you, Halfords!). The other thing is that the threads seem stripped. Here are the pics:

Left crank:

Left crank

Right crank:

Right crank

Right pedal:

Right pedal

Left pedal looked similar but I just scratched with my fingers and was able in just a few minutes to remove much of the stuff that filled the threads:

Right pedal

These metal rings were what came off when cleaned the threads of the right pedal:

enter image description here

So, given that it was relatively easy to get the stripped metal off the right pedal I am thinking of cleaning the cranks and the other pedal and of trying to just screw in the pedals. However, I am a bit worried - if it was so easy for them to come off (OK, one of them was probably not screwed in till the end) what is the guarantee that putting them back in will last? I know, it doesn't cost anything to try out but I don't want to subject the threads to additional wear if there is possibly a better solution. I was even thinking of maybe trying something like a thread lock (e.g. locktite) but wanted to first ask for advice before I do something that I might regret.

  • Don't use thread locker on pedals. – John Zwinck Jan 2 '15 at 0:32
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    Odds are that whoever assembled the bike at Halfords put (ie, forced) the left pedal in the right crank and vice-versa. (Although, looking at the pictures, the left-hand pedals does have a left-hand thread, as it should.) You need to either replace the crank arms or have a machine shop install "helicoils". – Daniel R Hicks Jan 2 '15 at 2:35
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    What Daniel said. They definitely mixed up the pedals and then forced them on, ruining the threads. – Eric Gunnerson Jan 2 '15 at 4:12
  • IMHO Its more likely the pedals were left hand tightened. Its hard to imagine someone cutting a left hand thread into a right threaded hole and visa versa without noticing something was wrong. (although I might be wrong, after all some shops do give spanners to gorillas and call them mechanics.) – mattnz Jan 2 '15 at 8:52
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    They were either cross-threaded or threaded into the wrong side. It's amazingly easy to drive a steel thread (especially the "rolled" threads of a pedal) into aluminum at the wrong angle or even the wrong direction. There is resistance, but it doesn't get harder as you go, making the "mechanic" think he's "making progress". Unfortunately, after two years convincing Halfords that it's their fault will be difficult, but it's worth a try. – Daniel R Hicks Jan 2 '15 at 12:49
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The threads are completely stuffed. Do not just screw the pedals back in and do not rely on loctite or similar compounds. Pedals falling off is at best inconvenient, at worst can lead to crash and serious injury.

Easiest option is to replace the cranks. Its not a big job but parts cost might mean a repair is a better option for you. The bottom bracket will probably be a square taper, so you need a special tool, or the LBS can do it in a few minutes.

The only other realistic option is to insert a thread insert (Helicoil) thread repair into the cranks. The left hand crank is a left hand thread. (Park tool have instructions)

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The only thing you can do that is safe and reliable and sane is to get a new crankset. You'd need a helicoil or something similar to do the repair reliably and this would be less strong than a new crankset and cost more than the new crankset + installation.

Next time, make sure everything is greased and tightened properly.

  • How can I find a compatible crankset? The only thing I know about the current one is that it's FSA Tempo (2 rings, bike has 16 speeds) and that the arm length is 175mm. What other measurements or factors shall I take into account when looking for a compatible crank set? – Nick Jan 2 '15 at 21:18
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    The crankset is square taper. You want to match that and the size of the chain rings and crank arm lengths. You may as well do this through your local bike shop since you will need a crank puller most likely, and you might be able to get a cheap used crankset there too. – Batman Jan 2 '15 at 23:02
  • I had a master mechanic once tell me that that helicoil was actually stronger than the original since you are replacing aluminum threads with steel ones. If you can find a shop that helicoils for a reasonable price, it's usually a pretty good option. I've done a couple sets myself and it isn't anything too magic, just some hand tools. – Deleted User Jan 5 '15 at 17:05
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    I don't believe that the repair would be stronger than the original. On a cheap crankset like this, parts and labor would exceed the cost of just throwing on a new crankset probably. – Batman Jan 5 '15 at 17:19
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Those cranks arms are gone/dead and need to be replaced. The pedals are steel and the crank aluminum. You should grease the threads so the metals do not fuse.

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First let's think why this occured.

I believe this is what would happen if one used the left pedal for the right crank and the right pedal for the left crank arm. You see the pedals are made so they screw in with the same rotation as the pedal spins (on forward motion of course). So as you press the pedal the pedal, even if not screwed in correctly, it would go inwards rather than outwards and fall off. Assuming you did not engage into crazy back-pedaling (en route or as the bike was standing which I do not think you did) it would only make sense that these pedals were placed right-left reversed. That would have the effect of what you are seeing i.e. the more you pedal the more they come off and eventually fall off. (Note your FSA Tempo crankset is alloy as I found online so the thread would not just go bad without special force as the one described above)

Now to move on (assuming this is what happened) there are two possibilities:

1- the threads on the crank arm are damaged and cannot hold the pedal (you say pedal seems ok) OR 2- the threads on the crank arm are not so damaged and can hold the pedal

Firstly, avoid using anything out of the ordinary for your pedals. Even if nothing else you will probably end up with a permanent pedals/crankset bond.

To find out where you are at just place the pedals correctly this time (and using proper grease) tighten them but not with too much force. Examine and if it looks ok take your bike out for a careful spin on a level road. If it looks and sounds ok try a bit of uphill progressively. If everything stays in place for all the uphill you tried and no funny sounds (even ticks) can be heard when moving then you can start to carefully continue your rides. You probably saved the money for new crank arms.

If not you only need the crank arms. Make a note of your BB-to-crank arm interface (most likely square tapered), BCD measurement and the length of the crankarms and hit ebay or LBS to get them they should be quite cheap if they are not carbon and are used. Avoid no-names. If price is ok you can get the whole crankset (in that case note your bike frame BB type e.g. BSA/road).

Always be careful when riding especially a mended bike. Ride into different situations slowly and cautiously.

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