Replacing crankset - chainline and bottom bracket question

[dax42]

Hi all,

I have a bike that I'm doing up as a project and I need to replace the bottom bracket (square taper). Since the front chainring (single, 46 teeth) 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!