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I changed the pedals on my bike from clipless to regular pedals. But after pedalling a few kilometers the pedal fell off. I put it back on by hand but it kept falling off. I thought they tighten as you pedal. Is there a certain trick to keep them on? Or is there different/wrong size pedals now?

PS It is an old 1990's downhill racing bike and the clipless pedals worked fine and never came loose. But I didn't want to clonk around town with clipless shoes anymore.

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I assume it's the left pedal, which can indeed work itself loose as you pedal.

So, unfortunately, when a pedal comes loose, most people's immediate reaction is to thread it back on and go about their day. In fact, it really ought to be examined closely to determine why it came loose the first time, and then carefully reinstalled with the proper torque. You MUST use a wrench to tighten pedals on, and at that it should be a pretty big wrench too.

The problem is that loose pedals can damage the threads in the crank, making it a problem that eventually can't be fixed. If your pedal, which you retightened after if first came off, is still coming loose, I fear that may be what's happening to you right now.

Take the pedal off and closely examine the threads on the inside of the crank. The threads on the pedal are hard steel and will likely be fine. If there's any sign of damage, replace the crank. If it truly looks good, first apply grease to the threads and retighten it properly, with a pedal wrench. The grease is essential. Not only will it prevent it from seizing and permanently welding to the crank, but it will actually help make it tighter.

PS - there are only two thread sizes of pedals and it is extremely obvious if you have the wrong one.

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  • I've got an abandoned bike where the right pedal came loose. Either side can come undone accidently.
    – Criggie
    Sep 26 '20 at 4:21
  • indeed, it's almost always the left though @Criggie. which is convenient, since they're (often) much easier to replace! Sep 27 '20 at 19:59

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