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I ruined the threading in my crank arm, and would like to weld or braze it back on, instead of buying a new crank set. It is a cheap old bike, so the result does not have to be perfect, but it should be cheap and durable.

I have welded on iron (an exhaust pipe and the like) in the past, and if this was just iron, I could fix it with no problems, but in this case it is not that easy.

Pedal arm: I think it is of aluminium. It seems light. Not magnetic. Looks like aluminium. I have no other guesses.

Pedal shaft: Steel, maybe even cast iron? I don't know how to tell the difference. It is definitely not aluminium. The material is stronger than the alu in the pedal arm, since the thread is intact. It looks sort of dirtyish / dark gray. Dark gray like some heavy parts in an engine which I believe are made from cast iron. Only slight rust despite a long life. Very far from the look of a stainless bolt.

Crank arm with ruined thread and pedal

Can you help me figure out what metals they are made from, so I can move on to finding the appropriate welding or brazing? Thanks.

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I couldn't say for sure but from my experience that's an aluminium crank and a steel pedal shaft. –  Matt Potts Jul 25 at 10:41
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It's not a good idea to weld it. Find a replacement crank. –  Carel Jul 25 at 12:11
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depending on the type of crank arm, it might be easier to find a used replacement. can you get a picture of the other end of the crank arm? If it's something common, it should be easy to find a part. You could probably salvage one off a bike that's no longer usable because of some other unrelated problem. –  Kibbee Jul 25 at 12:28
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The other option is to install a Helicoil in the arm. Easier if it's the right side, since left-hand Helicoils are probably hard to find (though bike shops may carry them). (I wouldn't expect epoxy to last more than a few days.) –  Daniel R Hicks Jul 25 at 15:06
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My bike shop tapped and put in a new helicoil for $20. –  RoboKaren Jul 25 at 18:22

5 Answers 5

You will spend far more $$ on welding supplies than the cost of a new crank arm.

The crank arm is an aluminum alloy and the pedal shaft is a tool grade steel alloy. You can replace both for the cost of just the gas to attempt brazing the two metals.

You'll also have to be really good at brazing not to completely destroy the aluminum crank arm in the process. Aluminum has a relatively low melting point.

If it was a really old cast steel crank, you might have a chance. But it still would cost more than replacing the crank outright.

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Welding aluminum and steel is not a DIY skill - read This.

If you must repair rather than replace, a helicoil is the correct way to address the problem. A crank would be cheaper than the coil alone, let alone the time to fix it. Chemical bonding (AKA. Glue) is probably the only DIY solution.

The issue I see is that when a pedal comes off while riding, it usually ends in tears (at best) up to a stay in hospital or worse. Why take the risk?

Second hand cranks are readily available - I have a box of bit with at least 2 sets and an old bike with another if I needed it. Your LBS will probably have something for a few dollars. Visit you local rubbish tip/recycle center - I can almost guarantee they will have an old, unride-able bike with usable cranks.

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It depends on your welding set-up and experience, but I wouldn't bother. Assuming the threads still engage a little bit, I'd get some hardcore epoxy resin (the sort that's specially designed for metal-on-metal; it often contains iron filings). Stuff the crank eye with it and screw the pedal in as far as it goes. Once it's gone off if should be good enough for rock n roll.

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There are replacement threads to repair that kind of worn threading. Your LBS might help. –  Carel Jul 25 at 12:09
    
As it is probably an aluminum crank (I'm pretty sure it's not carbon / titanium) and steel pedal, you can't wield it. So epoxy resin will be best thing. Later you can replace it whenever you want. –  Alexander Jul 25 at 12:57
    
After some googling, it seems that many people have had bad experiences with epoxy, when trying to fix my problem. There is no grip in the threading here, which makes it even less likely to succeed. So I will not try epoxy. –  Mads Skjern Jul 25 at 14:33
    
Rather than epoxy polyester resin (similar to car body filler) holds a thread better. Probably still a temprorary repair though. Sold in the UK as "plastic padding" or "chemical metal". –  Chris H Jul 25 at 21:06

Welding the pedal to the crank should NEVER be done. Pedal threads are oriented such that in the event of a pedal bearings becoming jammed the pedals will unscrew from the cranks rather than injure the rider. Severe injury is possible if the pedal jams while pedaling at a fast cadence or on a bike that does not have a freewheel or freehub body (e.g. fixed gear).

I have personally have had pedal bearings jam, while pedaling at an incredibly high rate of cadence and was thankful that the pedal simply unthreaded rather than damage my knee or ankle.

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That's something I'd never considered. Wow. Even more scary when you think how difficult it can be to unscrew a stubborn pedal. I ride fixed a lot... with clipless pedals... I think if the bearings jammed on me, it would damn near rip my foot off before I crashed... or perhaps dislocate my knee? Time to service my pedals... –  headeronly Jul 26 at 7:51
    
Very interesting. I always assumed the threading was "the other way" to prevent it from unscrewing over time. In my experience it takes so much force to unscrew a pedal, sometimes a vice is needed, but I guess that is different for new and well kept bikes. I hope fixie owners are aware of this. –  Mads Skjern Jul 26 at 9:55
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@headeronly - your legs are stronger than you think, if the pedal bearings jam, you would be surprised how quickly it will unthread a pedal, even a stubborn pedal. –  Rider_X Jul 28 at 16:27
    
@MadsSkjern - When installing a pedal you should use grease on the threads, pedal washers and only a moderate amount tension. It should be easy to remove the pedals, even a couple years later after riding under horrible conditions. –  Rider_X Jul 28 at 16:28
    
@Rider_X: I don't think I have ever seen a bike with washers on the pedals. Maybe it is only used on high end bikes? –  Mads Skjern Jul 30 at 6:23

Either a helicoil or a replacement crank arm (or set) is a better choice. You local bike shop might have some compatible used cranksets they'd sell for cheap. Or look for some on your local internet flea market. The shops are getting in old bikes as trade ins all the time and might use the parts off those bikes for this kind of job. I'd replace the pedals too, otherwise you risk damaging the crank arm threads again. I think in the long run you'll be money ahead, have a more reliable and safer bike, by not trying to do this w/a patch-welding job. Think about it. The only problem you have is a a damaged hole in the crank arm. A faulty hole. That's not such a big problem, right? So that's what you need to focus on. Don't overcomplicate it.

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