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Broken Pedal on the left side, stripped crankshaft arm thing

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Broken Pedal on the left side, stripped crankshaft arm thing

Old 05-14-21, 09:39 AM
  #1  
Tim_T
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Broken Pedal on the left side, stripped crankshaft arm thing

Okay, first, I'm new. So apologies up front for the failure to properly label the bits.

Second, I'm 300 pounds on a Mongoose Hitch fatbike (actually, I'm 300 pounds off it, too, really). Bike is about 6 weeks old.

I'm told I should mention that I ride it pretty hard, there are a couple of modest hills around here and the bike only has the 7 speeds (of which, only 5 actually work, but I can get the top and bottom gears). I ride in dirt, gravel, streets, a bit of sand, yards if I make a boo boo somewhere, trails, the whole bit. But I'm still fat and basically lazy, so I don't ride particularly far. 5 to 7 miles is about my limit, but I'm working on it. None of that is this, though. I digress, and I apologize. Mighty fine weather out today, though, isn't it?

Okay, anyway, while I was huffing and puffing my way up a bit of an incline yesterday, my foot kept slipping off the pedal. Figured I had a rock or something wedged in there, but no.. pedal had a groovy little angle to it. By the time I got to my destination, it was maybe 10 degrees off plane? But when I tried to take the pedal off, it was completely stripped. I did wrenchy things, and got the pedal to come off (yay!) but the hole in the crankshaft arm (is that what it's called?) is nice and smooth now. No threads at all. Which, we can all agree, is just special.

So. Here's where I petition the wisdom of the wise. I know the components on this bike are pretty crap, and that I should expect them to break. Plan is to upgrade them as I go. And I'm all for that. Should I conclude, therefore, that this is a good time to replace the ... crankshaft.. thing.. ? If so, how much hardware should I consider replacing? What gear should be on my shortlist for consideration? What are next week's winning lottery numbers?

I don't know what I don't know in terms of specs that I should mention about the existing gear. What are the relevant measurements that helps me figure out what will fit and what will work best?

Also, what other things should I put on my radar? Would it make sense to get a front derailleur at this point, or is that a whole 'nother ball of pain? Or should I just replace the arm critter (assuming that's possible, but I bet it is) and tally ho for a bit longer? Who the hell uses "tally ho" in a conversational manner?

Thanks for slogging this far into the post. You are a hero. You deserve a drink. If you have answers to any or all of this, doubly so.

Appreciate all the help, in advance!

-Tim
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Old 05-14-21, 11:26 AM
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dsbrantjr
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The crank arm can have an insert called a Helicoil® installed to create new threads which are stronger than toe original ones. Pedal threads which get stripped out are usually caused by the pedal not being torqued sufficiently.
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Old 05-14-21, 11:51 AM
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That is a beautiful bit of advice, thanks!

I'll make sure they are both torqued down properly, too. Thanks again!
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Old 05-14-21, 12:05 PM
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A helicoil would work but it's not worth the cost in this case unless sourcing parts ends up being difficult because pandemic. If you look on the back side of the left crankarm, there should be a number stamped and my guess is it's 175 for the length of the arm. You can order a new left crankarm in the same length and don't need to match exact brand or model, but do need to match the orientation of the square taper interface. Replacing it will require a basic square taper crank puller and it's likely something you can do yourself. Make sure the pedal gets reinstalled with grease and it's tightened down pretty darn tight.

I wouldn't be in a rush to replace the crank. Cheap cranks tend to shift ok. The chainrings are often not replaceable so you can upgrade if and when they wear out or of you do a full drivetrain upgrade.
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Old 05-14-21, 02:39 PM
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A heliocoil will certainly works as @dsbrantjr suggests. But depending on the crank, it may be more cost-effective to simply replace the arm.
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Old 05-14-21, 06:40 PM
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6 weeks old, Is it still under warranty?
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Old 05-15-21, 05:31 AM
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Warranty back to seller for repair.
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Old 05-15-21, 05:44 AM
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I would suggest taking it to a shop or co-op. If it's a square taper crank (Google/read) I'd let them replace it with anything available in the same length. A cosmetic match is not necessary.
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Old 05-15-21, 12:32 PM
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sch
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If the seller is not cooperative I have an NDS 170mm sq taper Shimano 1057 crank I could send you for $5 which
may or may not cover shipping. $ 7.50 may be a better cost estimate considering what USPS charges lately.
sch1943atgmail If the bike is NEW, deal with the seller first. Parts availability can be a problem however
and gratis exchange will depend on goodwill from the shop.

Last edited by sch; 05-15-21 at 12:37 PM.
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Old 05-16-21, 07:06 AM
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Generous offer, sch, and I may take you up on it!

It looks like I broke this thing on my own, though.. should have paid more attention to the pedals being properly installed. So while the warranty thing might be an option, this was my fault and I'm happy to learn from it. I'll hit the local bike store as soon as I get the extraction tool, which is supposed to arrive today.

Thanks for all the help, folks.. I'm sure I'll be back with my next muck up!
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