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I have a bike that I'm doing up as a project and I need to replace the bottom bracket (old-school square taper). Since the front chainring (single, 46 teeth, riveted) is pretty badly worn I was going to take the opportunity and replace that as well. But what to get as replacement?

At the back I've got a 6-speed freewheel with a chain line of 38 mm. The bike is nothing special, it's more for fun and the learning experience (my first project..), so I'd like to keep the costs down. I was looking at some budget cranksets on ebay but most of them don't specify what kind of bottom bracket length to use. How do I find a crankset/bottom bracket combination that will work and also give me a narrow enough chain line? Is it just a matter of trial and error (that could get quite expensive if I'm unlucky I guess..) or is there something I should look out for?

I found a cheap one that for a change specified BB and chain line: 131 mm BB and 50 mm chain line. https://www.rosebikes.com/article/m-wave-single-crankset/aid:485066 Can I just use a shorter BB to get a narrower chain line or will that not fit? Do you know of a combination that could work?

Thanks a lot!

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  • 50mm is a MTB chainline. You should go down to a shorter BB as 12mm difference is probably too much - I would try a 110mm. You really need to be able to source several sizes and try them out.
    – mattnz
    Jul 7, 2016 at 21:46

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Chainline is important, more so for modern 10+ speed systems. A six speed system is more forgiving of misalignment than one would guess. Since both BB are square taper all you need to do is remove it and see what the size is on the BB itself. The shaft will only allow a crank on so far (that's why its taper). By buying the size currently in the bike you should come pretty close. You'll see plenty of cross chaining with modern bikes and they survive. Although not the same exactly, I think you'd be OK as well.

(Not so cheap but) Simple Solution: Why not just replace the chainring if it is that worn? Much easier to do, and much cheaper. Also, if your chainring is in that bad of shape it is likely your freewheel is damaged too, as is the chain. You'd be better served by replacing the drivetrain entirely. Removing freewheels are not as easy as hubs, but it can be done. You'll have the ride you wanted without the compromised components causing mischief and making you think it is a chainline issue.

Good Luck!

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  • Thanks so much for the helpful comment! Unfortunately I can't just take the chainring off as it's fixed/bonded to the crank. You are right, I definitely have to replace the chain as well. The freewheel looks okay so I was going to see how it goes and only replace it later if I notice any problems.
    – user26993
    Jul 8, 2016 at 9:57
  • Don't be surprised if the rear freewheel cogs are shot. There are some things working in your favor, and that is they are essentially straight cut cogs. Wear will have less of an effect on them. Eventually you'll notice the chain jumping off a cog when you stand up to pedal. You might want to look around on eBay for a back up one They are becoming difficult to find and sometimes require their own special removal tools. Thanks for the compliment and good luck!
    – user26705
    Jul 8, 2016 at 23:07

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