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  1. #1
    Elitist Troglodyte DMF's Avatar
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    Torque spec for plastic BB (e.g. UN53)?

    Shimano finally took down its old Australian web site that had all the tech docs back through 2003. (F**K you, Shimano ), so I can no longer find the Service Instructions for square-taper cartridge BBs like UN53 (and UN73?). These use plastic adapters (separate screw-in collars) that accept the 20-spline tool like the main body.

    The torque spec for metal cartridge BBs is 35 ft-lb, but what about the plastic side? Seems like a lot for non-metal. Does anyone have an installation sheet for these BBs? The torque specs are on them.
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  2. #2
    Senior Member Retro Grouch's Avatar
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    I just tighten it until it's just snug. It only serves to keep the bottom bracket from tipping inside the frame's BB shell.

  3. #3
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    If you want to get it a little tighter than snug, install the metal cup until snug then back it off about a half turn. Install the plastic cup and snug it down then go back and torque the metal cup. You effectively get another half turn worth of torque on the plastic cup without worrying about stripping/deforming the splines. Not sure if there's any benefit since as Retro Grouch pointed out, the plastic cup doesn't do much, but it made sense to me at the time I did it. Still going many thousands of miles later.

  4. #4
    Elitist Troglodyte DMF's Avatar
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    Well, I took it down to 25 then 30 ft-lb. It turned quite a lot between the two, 20-30°. A metal adapter won't turn nearly that much (at least, not one with clean threads), which indicates to me that something is stretching - probably the rim distorting. So I didn't continue to 35 ft-lb.

    If anyone has the instructions that came with a UN53 or similar Shimano with a plastic adapter, I'd appreciate you checking to see if it differentiates the two.

    I'd also welcome a copy of the .pdf file of the Service Instructions and/or Exploded Drawings for any Shimano square taper cartridge BB. PM me if you have one, please.
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  5. #5
    Elitist Troglodyte DMF's Avatar
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    Quote Originally Posted by joejack951
    If you want to get it a little tighter than snug, install the metal cup until snug then back it off about a half turn. Install the plastic cup and snug it down then go back and torque the metal cup. You effectively get another half turn worth of torque on the plastic cup without worrying about stripping/deforming the splines.
    That's an interesting idea. I'll try it at some point. After my experience, though, I don't think the splines are the weak point. More likely the outer lip.

    The adapter does bottom on the cartridge, so maybe the technique isn't a good one when the hanger is significantly <68mm. (Not sure any technique would be good in that case. )
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  6. #6
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    Quote Originally Posted by DMF
    If anyone has the instructions that came with a UN53 or similar Shimano with a plastic adapter, I'd appreciate you checking to see if it differentiates the two.
    They do not differentiate. They only specify one torque number (100% positive on this). I could scan my instructions tonight when I get home if I can find them.

  7. #7
    hello roadfix's Avatar
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    Quote Originally Posted by joejack951
    If you want to get it a little tighter than snug, install the metal cup until snug then back it off about a half turn. Install the plastic cup and snug it down then go back and torque the metal cup. You effectively get another half turn worth of torque on the plastic cup without worrying about stripping/deforming the splines. Not sure if there's any benefit since as Retro Grouch pointed out, the plastic cup doesn't do much, but it made sense to me at the time I did it. Still going many thousands of miles later.

    Great tip......I've got to remember that.
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