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I have a problem with finding the right bottom bracket.

In my old Puch Pace bike the bottom bracket BB-CT90 was installed (made weird noises, no longer available)

Shimano Singapore VIA 68 SK
BC1.37x 24
(it is 116mm long)

I bought a BB-UN300 in 117mm, the problem now is that the front derailleur doesn't come far enough out despite all the adjustments. The crank is now a few millimeters (roughly 5) too far to the right.

I also think that strangely the left side is on the short side (so the axle is too long on the right side and nearly too short, but okayish on the left)

The bike has a chain guard (that is fixed with the bottom bracket), maybe that's why the axle is too far to the right... but it's not thick enough to fully explain the misalignment.

Is there a different bottom bracket that fits? I've read that there are "E" and "K" versions of Shimano, but I don't get the difference.

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  • 1
    Worth a read. bicycles.stackexchange.com/questions/23938/…
    – mattnz
    Commented Oct 6 at 2:14
  • Is your crank ISO or JIS standard? I assume is JIS but what you are describing sounds like this quote from Sheldon Brown's website "ISO/J.I.S. Interchangeability If you install an ISO crank on a J.I.S. spindle, it will sit about 4.5 mm farther out than it would on an ISO spindle of the same length." Is Puch is a European bike? sheldonbrown.com/bbtaper.html
    – MindDBike
    Commented Oct 6 at 12:29
  • JIS (Shimano FCT-90), Puch is a European (Austrian) company. What I assume now ist that I bought the wrong bottom bracket, i should have bought the UN-300E instead of the UN-300. I will buy another derailleur that is mounted on the seat tube. So the UN-300 should fit (if i remove the chain guard mount thingy on which the derailleur is mounted).
    – JavaMan
    Commented Oct 6 at 14:20
  • The -K bottom brackets are similar to -E in that they allow for something to be mounted between BB shell and BB flange. -K and -E (and -EK I believe) each allow for different thicknesses. Rather than introduce potential other issues by changing derailleur styles, why not just measure the thickness of the chain guard and get an appropriate -E, -K, or -EK BB?
    – Armand
    Commented Oct 7 at 11:59
  • Shouldn't you measure the length on the inside of the cups on the original BB as this is the original clamp-width (which in most cases just is shell width, but in your case is shell+FD-thingy)?. Those cups look pretty beefy, perhaps you actually had a 68mm BB originally? Fun if you do the measurement and give us an update.
    – WornChain
    Commented Nov 6 at 21:48

1 Answer 1

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To account for your front derailleur attachment method, you must use a bottom bracket meant for a 73mm bottom bracket shell. You can use any brand 115, 116, 117 mm bottom bracket, so long as it is for the 73mm shell width.

The front derailleur attachment makes the effective width of the bottom bracket shell wider.

The Shimano E-type bottom bracket allows for styling (marketing) a bottom bracket of very similar (identical) proportions to the one I have described, for use with a 68mm bb shell. There are good reasons why this is done, but at this point it only makes confusion if you have not encountered the system before.

I hope this makes sense to you.

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  • Doesn't the E-type BB result in a spindle centered on the BB shell/frame centerline, while a 73mm BB used in a 68mm shell+attachment results in the spindle being slightly shifted to the drive side?
    – Armand
    Commented Nov 12 at 1:30
  • @Armand The e-type derailleur mounting plate is roughly 2.5mm thick, so a bottom bracket designed for a 73mm shell will mount properly when used with a 68mm shell plus e-type derailleur. The "special case" is would be an e-type derailleur on a 73mm shell. I have never seen one but if it exists, the bottom bracket would be based on a 78mm shell width.
    – Noise
    Commented Nov 12 at 12:22

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