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?
Square taper torque
Opus51569
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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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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.
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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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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.
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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.
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.
Definitely!
Definitely[SUP]2[/SUP]!
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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Good point. Even a good square taper BB is only $15:
https://www.jensonusa.com/Shimano-UN55-Bottom-Bracket
Now that you mention this, I would say this was what happened.
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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.
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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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.
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yeah I hadn't seen his post when I posted mine. That makes sense as an explanation.
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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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Fair point. That sensation would more likely be threads stripping in place.
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Yup. It's already gone. As in "The pedal floored easy, without a sound."
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Cranks onto the BB. I mentioned "bolts" in my second post, but I can see from the OP that wasn't clear.
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