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Repair newbie here just getting back into mountain biking after a long absence.

I recently bought a Cannondale Catalyst 4 from REI. It's a fine introductory bike for what I'm looking for, but I hated how cheap the pedals felt. So I replaced the stock pedals with these:

Unfortunately, maybe 30, 40 miles after the installation, the left pedal fell off today. When I tried to put it back it seems like the threads on the arm are completely stripped...

I can only assume I messed something up with the install. Probably didn't thread it properly or tighten it fully. Oh well, learning experience.

I'm just unsure what the best way is to replace it. I see these as options:

  1. Repair the thread (can't do myself, as I don't have a workbench or the tools to do so).
  2. Replace just the arm.
  3. Replace the entire crank.
  4. Return the bike to REI and upgrade to a better one, since I'm fairly confident I want to buy a full suspension bike with a dropper post and better everything.

I appreciate any advice.

EDIT:

Thanks for the replies everyone. Local bike store is going to replace the crank for $15 + labor. Should be good to go from there. I'm going to be more careful with my maintenance from now on!

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    I'd bring it back to the shop where you got it and ask for a refund or replacement. – Max Jun 6 at 14:42
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    It sounds like qotsa42 replaced the pedals after buying the bike and installed them himself. Not the shop's fault if that's the case. – Carbon side up Jun 6 at 14:44
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    @Max it's unlikely they will do either as from what I can work out the OP has caused the damage by changing the stock pedals. I'm sure the shop will happily repair what damage is done though for labour and parts cost – Dan K Jun 6 at 14:44
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    For future reference - cranks are usually made out of much softer metal (aluminum alloy) than the pedal spindle (hardened steel), so it's very easy to damage the threads on the crank. Always get the threads started by hand and make sure they're lined up properly before putting a wrench on it. Grease the threads before starting as well, and don't forget that left pedals are reverse threaded - LEFT (counter clockwise) to tighten, right to loosen. Right pedals are the normal "righty tighty, lefty loosey". – Andrew Jun 6 at 15:57
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If you plan on upgrading to a better bike, replace the cranks, don't repair them. You won't be able to return the bike to REI, so you'll be selling it. Cranks repaired with helicoils will not look good to potential buyers.

Square taper triple cranksets are cheap, but there are a couple of pitfalls. If you are replacing the crank yourself you'll need a special crank puller to get the old crank arms off. When installing new cranks the bolts need to be tightened to the proper torque spec. No offense, but you should probably get a bike repair shop to do that for you (and install the pedals at the same time).

You may not be able to find the exact Prowheel crank that comes with the bike. An inexpensive Shimano crank with the same chainring sizes will be fine, but be aware that different cranksets sometime require a different bottom bracket axle lengths to get the proper chainline (how far offset the rings are from the centerline of the bike), so check the specs of cranksets you consider to make sure they require the same axle length you have.

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    A HeliCoil repair will not be noticed by most inexpensive bike buyers. Done properly it's also stronger than the original crank arm threading and will last forever. A helicoiled crank is much less likely to be noticed than a non-oe crankset. – Deleted User Jun 7 at 4:43
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    @DeletedUser but an honest seller would disclose the repair – Argenti Apparatus Jun 7 at 11:29
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If you installed the pedals incorrectly, you likely damaged the other crank as well. I'd buy a new set of pedals and have a competent repair shop heli-coil the cranks (both, if necessary) for you. I believe this would be the cheapest fix (if you plan to keep the bike).

As for the return and other issues, I have no idea what REI's policies are and whether or not they would take back a damaged (and currently unusable) bike. Perhaps they will refund your purchase price minus repair costs. Again, heli-coil is likely the cheapest repair. However, you need to talk to them to find out what options they are offering you.

Based on this incident, I'd recommend you have a shop do any repairs you need until you get some experience wrenching under your belt (from a class, competent friend, internet, etc). That is, whatever you decide to do (new cranks, heli-coil, etc) I recommend you do it with some sort of supervision or instruction to save yourself additional hassle.

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    have a competent repair shop heli-coil the cranks Probably a lot cheaper to just get a whole new crankset. 24/34/42, square-taper, 6/7/8-speed cranksets can be found for well under $30. – Andrew Henle Jun 6 at 15:17
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    Yes and perhaps the same to install them. Assuming nothing else is needed. A HeliCoil repair is perhaps $20 and actually leaves the crank threads stronger than they were from the factory and is not noticable. – Deleted User Jun 7 at 4:36
  • @DeletedUser On a slightly relevent note, I've seen a crankset with arms paired with a machined, tapered metal thread insert that not only makes the crank arm's threads replaceable, but you can also configure it to have an effective length of either 170mm or 175mm. – Gregory Leo Jun 7 at 20:42
  • Yes. Additionally there is the fact that you are not throwing away a pair of cranks that are repairable with a small amount of effort and money. – Deleted User Jun 8 at 2:35

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