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A long time ago, when I was younger and stupider, I decided to fix my creaking crank arm by welding the bolt. Yes, I know it was so damn stupid, but I thought that was the problem. I didn't use the bike and welding the bolt didn't even fix my problem. So now I need to fix my bike, but I have no idea how to take off that pedal. I thought of cutting the whole arm off on the bottom bracket with a grinder, but I actually have no idea if that would work. I don't really care if I need to replace the bottom bracket, I just want to know if it would come off without many problems. I know that I will need to buy a new crank arm, because the one I have currently is almost broken.

TL; DR Any suggestions on how to remove a crank arm that has the bolt welded and can't be taken off with a normal tool?

EDIT I'm adding pictures to help you understand what I got myself into enter image description here

enter image description here

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    Pictures would help -- Cutting the crank is probably easier than cutting the bottom bracket spindle, but with the right hacksaw blade or grinder wheel, you should be able to do either. You may also want to see if you can drill out the bolt or something. You will need to get a new bottom bracket and a new crankset along with a bottom bracket tool (but be aware of the threading -- right side is usually counterthreaded).
    – Batman
    Commented May 19, 2016 at 19:10
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    I would use a grinder with a relatively small cutoff wheel, to allow more precise work. If you only welded through the bolt hole, I'd first attempt to simply cut the arm off by making slices parallel to the crank shaft to see if you can get the arm to come loose. If it's welded too tightly for that, first cut most of the arm off to get it out of the way, then make a vertical cut through the shaft, as close to arm as you can get.
    – Daniel R Hicks
    Commented May 19, 2016 at 22:11
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    I'm waiting for "some guy on the internet said... and now I've cut my arm off".
    – Móż
    Commented May 20, 2016 at 2:17
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    Updated the post with pictures
    – InSaneOxT
    Commented May 20, 2016 at 12:47
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    After looking at the pictures I'd say flip the bike over point the crank arm down (out of your way), and take a hacksaw to the BB spindle. With some cutting oil, you should be able to get through it relatively quickly.
    – Deleted User
    Commented May 20, 2016 at 21:42

2 Answers 2

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The core problem is that you need the thread on the bottom bracket axle to attach the new crank, and that will have probably been damaged by your welding. If you can get the bolt out after unwelding, it might be possible to keep the current BB.

bottom bracket with bolt

If your BB is this style, you might be able to cut the crank off, leaving enough of the bolt to weld on a small arm that will allow you to twist it out of the BB shaft. I would make cuts like this diagram, depending on which crank is affected - the one with the spider and chainrings is more tricky. I think you're trying for two things. First, if you can cut the weld and then cut the crank so it comes off the bike you win. If not, cutting it so you can remove the bolt then slide the crank off is also a win. Cutting into the axle means you lose, so the cuts shown try to avoid that. The middle diagram is supposed to be a sketch of a side view showing my guess as to how you'll need to cut the bolt head & weld off. Assuming you didn't just fill the whole bolt cavity with molten metal... in which case you're probably overheated the BB axle and you lose.

cutting diagram for crank

If you're unlucky you will have welded the bolt or crank to the shaft of the BB, and in that case you lose - you need a new BB as well as a new crank.

You will do much better with a 100mm/4" cutting disk (or bigger) than trying to do this with a Dremel or similar "rotary tool". I wouyld also use a thick cutting disk (3mm or 1/8") rather than a thin one, because you are likely to stress the disk a bit with the fiddly angles. But a 200mm/8" grinder is probably too big.

Also, try to mount the crank solidly - if you get the bike frame horizontal you should be able to put the non-welded crank into a a vice. That way the bit you're cutting is fixed, even if the bike can move a little around it.

Edit: seeing the new pictures, it looks as though you have an older style bottom bracket that has nuts rather than bolts. Like the one below:

enter image description here

That means you have welded the threaded part of the bottom bracket axle, so that is now junk. Since you need a new bottom bracket, like Suspended User says just cut the shaft (on the red arrow below) and bin the parts. Get a new crankset and matching BB, plus the tools to install them.

enter image description here

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Based on your photos, take the other crank arm off, pull the whole bottom axle unit out attached to the welded side. and fit a new cartridge bottom bracket. Cost is not small but its not huge either - certainly cheaper than cranks. Cup and cone bottom brackets are old tech, and cartridges are far superior.

If you're lucky, you can now work on unmounting the axle from the crank, but in the comfort of a vise rather than on the bike. If this fails, then you will have to source replacement cranks, either new or used.

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