Square taper torque
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    Square taper torque

    Generic square taper bottom bracket. Torque spec says tighten to 45Nm. Using a Park Tool beam torque wrench. At about 35Nm I get a sudden lack of resistance that feels like I'm either about to strip something or sheer something. Do I trust the tool/specs and press through to 45 or leave it at 35 and hope it doesn't work loose?
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    Has the torque wrench been calibrated recently?
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    Quote Originally Posted by old_fuji View Post
    Has the torque wrench been calibrated recently?
    Yep. And I greased the bolts before the install. It's one of those things where I'm probably over-thinking it, but it's really going to suck if I end up stripping the bolt or sheering it off. I'm just not sure if 35Nm is good enough to prevent it from loosening down the road.
    Last edited by Opus51569; 1 Week Ago at 06:23 PM.
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    I would back it out and examine the threads just to play it safe. That sudden lack of resistance doesn't sound good.
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    Quote Originally Posted by Lombard View Post
    I would back it out and examine the threads just to play it safe. That sudden lack of resistance doesn't sound good.
    This is what I would do. I would also pull off the crank arm to see if something was trapped between the crankarm and spindle. There may have been some sort of obstruction binding things causing a higher than normal reading and then some how broken free. Disassemble, clean and try again.
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    Quote Originally Posted by velodog View Post
    This is what I would do. I would also pull off the crank arm to see if something was trapped between the crankarm and spindle. There may have been some sort of obstruction binding things causing a higher than normal reading and then some how broken free. Disassemble, clean and try again.
    Sounds like good advice, Lombard and velodog. I'll take a look tomorrow and see if there is anything amiss. Assuming there isn't, though, would you continue to 45nm or tighten to 35 and call it good?

    I'm leaning toward riding it at 35Nm. If it holds, great. If it loosens then I really don't have much of a choice but to torque it to 45 and whatever happens, happens.
    Last edited by Opus51569; 1 Week Ago at 07:50 PM.
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    Quote Originally Posted by Opus51569 View Post
    Sounds like good advice, Lombard and velodog. I'll take a look tomorrow and see if there is anything amiss. Assuming there isn't, though, would you continue to 45nm or tighten to 35 and call it good?

    I'm leaning toward riding it at 35Nm. If it holds, great. If it loosens then I really don't have much of a choice but to torque it to 45 and whatever happens, happens.
    I'd try and turn the bolt into the spindle without the crankarm to get a feel of the bolt\spindle interface, then I'd have a look at the crankarm and spindle taper to see if anything is scored.

    I've never used a torque wrench to tighten down a square taper assembly so I'm not going to hazard a guess on which would be a better option, but maybe a bookie would know the odds. A question I would ask is how dear is the crank set? Are you willing to gamble that along with a generic BB?
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    A thought if you try at 35nm, make sure that the dust caps are snug up to the bolt as they may keep a too loose bolt from backing out, loosening more. Or maybe they won't matter.

    Another thought: if you push thru to 45 and the bolt doesn't snap, but gets stuck in the spindle, you're going to have another fight on your hands getting the bolt out to keep the crankarm.
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  9. #9
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    Quote Originally Posted by Lombard View Post
    I would back it out and examine the threads just to play it safe. That sudden lack of resistance doesn't sound good.
    Definitely!

    Quote Originally Posted by velodog View Post
    I'd try and turn the bolt into the spindle without the crankarm to get a feel of the bolt\spindle interface, then I'd have a look at the crankarm and spindle taper to see if anything is scored.
    Definitely2!

    I'd run just the bolt into the BB at 40Nm to stress test the threads. Taper BB's are cheap. No sense in risking riding and having the crank come off.


    It's possible the sudden lack of resistance was just the taper seating itself. Did you grease the taper? You want to make sure it slides smoothly and doesn't stick.
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    Quote Originally Posted by tlg View Post
    Definitely!

    Definitely2!

    I'd run just the bolt into the BB at 40Nm to stress test the threads. Taper BB's are cheap. No sense in risking riding and having the crank come off.


    It's possible the sudden lack of resistance was just the taper seating itself. Did you grease the taper? You want to make sure it slides smoothly and doesn't stick.
    Didn't grease the taper itself. There seems to be a pretty even division of folks who say grease vs. no grease on the exterior of the taper. But I did grease the bolt.
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    Quote Originally Posted by tlg View Post
    I'd run just the bolt into the BB at 40Nm to stress test the threads. Taper BB's are cheap.
    Good point. Even a good square taper BB is only $15:

    https://www.jensonusa.com/Shimano-UN55-Bottom-Bracket

    Quote Originally Posted by tlg View Post
    It's possible the sudden lack of resistance was just the taper seating itself.
    Now that you mention this, I would say this was what happened.
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  12. #12
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    Quote Originally Posted by Opus51569 View Post
    Yep. And I greased the bolts before the install. It's one of those things where I'm probably over-thinking it, but it's really going to suck if I end up stripping the bolt or sheering it off. I'm just not sure if 35Nm is good enough to prevent it from loosening down the road.
    Are you sure you didn't do that already? I'm not sure how else you could suddenly go from 35Nm to what feels like nothing.

    Real dirty threads could make if feel like going from a lot to a lot less but I can't imagine it taking 35Nm to break through that.

  13. #13
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    Quote Originally Posted by Opus51569 View Post
    Didn't grease the taper itself. There seems to be a pretty even division of folks who say grease vs. no grease on the exterior of the taper. But I did grease the bolt.
    Grease helps the taper seat. And it helps keep it from seizing/corroding making it easier to remove.

    Those saying not to grease are from the olden days of lightweight cnc-ed to death cranks, poor fits, and no torque wrench. Then over tightening the crank to remove creaks and cracking the crank.

    Grease and a torque wrench and you're good.
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    Quote Originally Posted by Jay Strongbow View Post
    Are you sure you didn't do that already? I'm not sure how else you could suddenly go from 35Nm to what feels like nothing.

    Real dirty threads could make if feel like going from a lot to a lot less but I can't imagine it taking 35Nm to break through that.
    I pulled the bolt and checked the threads after that and they are in good shape, so it didn't strip.

    UPDATE: After checking everything again, and taking into account the good point that a new square taper isn't expensive to replace, I decided to press ahead to 45. At 35 the torque wrench made 1/4 revolution of the bolt with the torque not changing at all. After that 1/4 revolution, though, it caught once again and continued to tighten. I was able to get to 45Nm without it stripping or sheering. Huzzah. I think it might have just needed to seat on the taper as tlg suggested.

    Thanks everyone for the opinions and advice.
    Given his penchant for nicknames, and his aversion to reading, I've decided to shorten Donald J. Trump to it's essence: Dump*

    I was "social distancing" before it was cool.

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    Quote Originally Posted by Opus51569 View Post
    I pulled the bolt and checked the threads after that and they are in good shape, so it didn't strip.

    UPDATE: After checking everything again, and taking into account the good point that a new square taper isn't expensive to replace, I decided to press ahead to 45. At 35 the torque wrench made 1/4 revolution of the bolt with the torque not changing at all. After that 1/4 revolution, though, it caught once again and continued to tighten. I was able to get to 45Nm without it stripping or sheering. Huzzah. I think it might have just needed to seat on the taper as tlg suggested.

    Thanks everyone for the opinions and advice.
    yeah I hadn't seen his post when I posted mine. That makes sense as an explanation.

  16. #16
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    FWIW, if you feel a 'sudden lack of resistance', it's not because you are 'about to shear something'; Failure, whether shear, ductile, or tensile is instantaneous and absolute. Once you've failed, it's broken. Backing off will not let the bolt 'heal'.
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    Quote Originally Posted by No Time Toulouse View Post
    FWIW, if you feel a 'sudden lack of resistance', it's not because you are 'about to shear something'; Failure, whether shear, ductile, or tensile is instantaneous and absolute. Once you've failed, it's broken. Backing off will not let the bolt 'heal'.
    Fair point. That sensation would more likely be threads stripping in place.
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    Quote Originally Posted by No Time Toulouse View Post
    FWIW, if you feel a 'sudden lack of resistance', it's not because you are 'about to shear something'; Failure, whether shear, ductile, or tensile is instantaneous and absolute. Once you've failed, it's broken. Backing off will not let the bolt 'heal'.
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    Wait, are you threading the BB into the frame, or threading the cranks onto the BB?
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    Quote Originally Posted by old_fuji View Post
    Wait, are you threading the BB into the frame, or threading the cranks onto the BB?
    Cranks onto the BB. I mentioned "bolts" in my second post, but I can see from the OP that wasn't clear.
    Given his penchant for nicknames, and his aversion to reading, I've decided to shorten Donald J. Trump to it's essence: Dump*

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    Quote Originally Posted by Opus51569 View Post
    Cranks onto the BB. I mentioned "bolts" in my second post, but I can see from the OP that wasn't clear.
    Glad you got it resolved, in any case! I had a similar thing happen years ago with an old mountain bike, though I wasn't using a torque wrench at the time. I had the same result: It got a little tight feeling, then released, then got tighter again.
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  22. #22
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    I'll just throw this out there. I worked with square taper cranks and bbs for years, still see them every now and then. In all those years I don't ever think I used a torque wrench to install a crank. Never broke a bolt or had a crank fail. Nearly all ST cranks are so overbuilt that there's just no need to be that precise.
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    Quote Originally Posted by cxwrench View Post
    I'll just throw this out there. I worked with square taper cranks and bbs for years, still see them every now and then. In all those years I don't ever think I used a torque wrench to install a crank. Never broke a bolt or had a crank fail. Nearly all ST cranks are so overbuilt that there's just no need to be that precise.
    Good to know. I e-mailed the company to ask about the spec. When they gave me 45Nm, I took them at their word and figured I should be as accurate as possible.
    Given his penchant for nicknames, and his aversion to reading, I've decided to shorten Donald J. Trump to it's essence: Dump*

    I was "social distancing" before it was cool.

  24. #24
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    Same here......I also never greased the square tapers. Back in the 80's, I remember reading that grease could cause the square taper on the spindle to enlarge the tapered hole in the aluminum crank arms (not sure if that is true)
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  25. #25
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    Quote Originally Posted by MR_GRUMPY View Post
    Same here......I also never greased the square tapers. Back in the 80's, I remember reading that grease could cause the square taper on the spindle to enlarge the tapered hole in the aluminum crank arms (not sure if that is true)
    I still have my peanut butter wrench.
    I heard that as well, but after a little while I started working w/ a really great mechanic and he used just a little on the spindle, saying it would help the crank arm slide on smoothly. He'd never seen an arm crack because of it, so that was good enough for me.

    Still have my PB wrench as well!
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