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I am converting an old Trek 660 frame to fixie. It takes a 68mm bottom bracket and I'm putting in an SKF bottom bracket (like this) with both a BMX profile racing 110mm 46t and redline BMX cranks 165mm (like this).

Now I talked to my friend at my local shop and he says these older frames run 110mm. I've looked on the internet and I believe he is correct in that I can use a 110mm bottom bracket and use spacers if need be to get the correct chainline. But I've also read that I may wanna run a 113mm bottom bracket.

I was wondering if there were any gurus out there that could help me out? I know that it's only a matter of 1.5mm but I just wanna get it right because the bottom bracket I'm getting is mailorder only and $129, so I'd like to limit my mistakes on this build.

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  • I think you want adjustable cups. There's 3-5mm of adjustment possible in most cases.
    – Daniel R Hicks
    Commented Nov 19, 2011 at 22:59
  • @apatz87, welcome to the site. I cleaned up your question a little; please revert my edits if I've accidentally changed the meaning of your question.
    – Goodbye Stack Exchange
    Commented Nov 19, 2011 at 23:58

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There seems to be a bit of confusion in your question about how chainline is measured, and it's relationship to bottom bracket spindle length and the rest of the bike - you haven't given enough information to answer your question. The rear hub and sprocket's chainline measurement is required to decide how to adjust the chainline in the front. Even then, the published information about chainline for most cranksets (including the one you have linked) is scare at best.

The easiest thing for you to do would probably be to acquire a used/cheap sealed cartridge bottom bracket in your desired length and see how things measure up. Once you've made measurements, you can lock in the size for your SKF.

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  • The rear hub and sprocket spacing for that frame is 120mm. Im putting a standard fixed/SS flip hub wheel also spaced at 120mm. According to SBrown I want it between 40.5-42mm I was wondering if anyone knew what the preferred length of bb for running a single bmx chainring with bmx cranks would be instead of standard road cranks with a two or three stack. I dont know much about bmx parts and from what Ive read the sizing runs larger so I wasnt sure if that was cause of the frames or parts. Thanx for the suggestion I will try a junker at the shop, I suppose thats the only way to figure this out
    – apatz87
    Commented Nov 20, 2011 at 2:53
  • Yeah, the crank arms can easily make a difference of several mm. And even with an old spindle it's hard to know where to measure to, given that the taper may be longer or shorter by an mm or two.
    – Daniel R Hicks
    Commented Nov 20, 2011 at 3:26
  • @apatz87 It's unlikely that the original spacing on that frame was 120mm; the Trek 660 was introduced in 1984 after Trek had switched most frames to 126mm spacing. If it's been respaced you should verify that the dropouts are aligned and properly centered, as that will affect your chainline as well.
    – lantius
    Commented Nov 20, 2011 at 3:32
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Just did a conversion last week and ended up with a mash of Road, MTB and BMX parts. I did all the measurements but at the end of the day I had no assurance the chain line was straight so my advice is get hold of old parts or cheap parts and fit them and refine the spec of the parts visually. For example the two chain rings I used varied the chain line by 1.5mm due to material thickness. Go to bike shops and buy (or get given) old bits such as bottom brackets that are kicking around.

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