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This is my bicycle. I mounted a new Thompson 124mm bottom bracket (the narrowest of this kind I could find) and a new single-ring sqaure-tapered crankset. When set as in the pictures, there is a thread sticking out on the left side, the whole thing is visibly asymmetrical and yet the chainline is still offset by about 5mm (chainring should be even closer to the frame). I can't push the crank any further onto the spindle. Did I choose the right bottom bracket and crankset for my bike or should I replace some of those parts? Is it possible to make something sensible out of it?

Links to the parts I bought (website is in polish): bottom bracket crankset

If you need more clarification from my side feel free to leave a comment.

skewed chainlineleft sideright sidecrank fixing screwbottom bracket

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I'm only dimly aware of Thompson bottom brackets, but:

  • The visible thread on the spindle isn't a concern or a sign that something is wrong. My understanding is that, like Ashtabula/one-piece/American bottom bracket shells, the shell widths found on different Thompson shell bikes can have some variance due to either low tolerance or lack of real standardization, and the BBs have the threaded length to accomodate that.
  • It looks like the chainstays are in the 490-500mm range or perhaps longer. The front chainline being out by 5mm on a bike with chainstays this long is likely to be inconsequential. If there's no roughness or noise when pedaling, it will be fine.
  • This appears to be a coaster brake and/or internally geared hub. If so and if the cog has offset, make sure it's flipped out. If it's not offset and if it's the normal three-notch type, and if you wanted or needed to make the chainline better, getting an offset cog can buy you a few mm.
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The crankset should have a bottom bracket spindle length specification. I didn't see one listed in your link, but I don't know Polish, only Latvian :) Can you find out the correct spindle length from the crankset manufacturer? Either that or try to find a crankset that uses a 124mm spindle length. From my limited experience, 124mm seems like a long spindle length; from Shimano cranks, I am used to e.g. 113mm or 122.5mm for a triple ring.

You may be able to replace your new spindle with a shorter one (e.g. https://www.ebay.co.uk/i/194024320681 ), as long as the spindle diameter and bearing-to-bearing distance are the same.

Something like this example might work for you -- https://www.ebay.co.uk/itm/153543351766 I would think it is available in different spindle lengths.

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