Bottom bracket finesse

Preamble

Mildly confused how the plastic cups on a Shimano BB-UN70/71/72 operate? Read on…

Each is slightly different:

  • UN70 has one plastic thread cup, the other is metal and welded to the bottom bracket itself (although the photo below would suggest otherwise)
  • UN71
  • UN72 (both ends have metal threaded cups)

UN70

From Shimano BB-UN 70 600 dura-ace bottom bracket 70 mm 36×24 ITA 115 spindle VGC, Note the lack of a groove on the right hand side

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Note this one, Vintage Shimano Deore XT BB-UN70 1.37, appears to have a metal threaded cup on each end

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UN71

From Shimano XT Innenlager BB-UN71 Vierkant 115 mm, note the groove on the right hand side (for an O-ring?)

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This one, Shimano BB-UN71 Square taper bottom bracket 68 bb – 107 mm axle, shows a metal cup on the left hand side (more clearly than above)

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UN72

From Shimano Deore XT BB-UN72 Bottom Bracket,113mm spindle, 68mm No shell/thread cups, note the groove on the right hand side (for an O-ring?)

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From Shimano BB-UN72-E Bottom Bracket – RARE – 110mm

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This is the left hand side (I assume)

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From Shimano BB-UN72 Retro Bottom. Bracket

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My personal problem

The issues that I’m having can be seen here

There is a slight crack in the plastic threaded cup, that I’m trying not to worry about. However, the threaded cup appears to be firmly jammed on to the end at a bit of an angle (see photos). I have applied a fair bit of pressure trying to twist it off, but I haven’t applied that much, for fear of making the crack worse.

My assumptions are:

– The plastic cup is not threaded on to the BB, but rather it is just a tight fit
– There is an o-ring sealing the space between the BB and the threaded plastic cup (which is making it tight/stiff to turn)
– The metal threaded end does not come off, as it does with the BB-UN72

Should I just give the plastic cup a good old twist and it should readily come off? Or is there another technique? Would it help to have two BB wrench sockets (one for each end?), as I only have one.

I assume that gripping the body in a vice/mile grips and then applying force to the plastic cup via the BB socket spines should work – but as I stated above, I would like confirmation of that, as I am worried about the crack worsening… or am I worrying needlessly?

I have done a fair bit of searching, reading and video watching and there’s not much addressing the issue that I’m having. FWIW, The gap between the cup and the BB has been soaked in WD-40 and 3-in-1 for a day or two.

 

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