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vintage crank arm torque

2K views 2 replies 3 participants last post by  MR_GRUMPY 
#1 ·
For a vintage Zeus crank set with a square-taper, cotterless spindle, what is your advice for checking torque after servicing spindle bearings and reinstalling all bottom bracket components?

I've read Jobst Brandt's treatise on the subject and it seems to make sense. Lightly greasing the taper surfaces and installing the crank arms to 30 foot pounds of torque seems reasonable. What is confusing is whether to check arm tightness later, after many miles of riding.

Brandt, with an engineering degree from Stamford, points out that crank arms actually creep up the taper from use, unloading the torque on the crank-arm retention bolt. If one checks the bolt after use, one will find that the bold has lost its load due to crank arm creep. Applying more torque to re-tighten the bolt will result in the crank arm creeping further up the taper, eventually rendering the arm useless as it moves to the furthest point up the taper.

My bike's 46 years old with the cranks installed in 1971. I've never had any questions regarding this issue in the past. However, last September, the left crank arm cracked unexpectedly with no prior warning at the point where the mounting bolt inserts . Probably just metal fatigue, but now I'm paranoid and am stressing out about this crank torque issue.

Extensive internet searches just muddle the issue even further.

Advice? Suggestion??
 
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#2 ·
For a vintage Zeus crank set with a square-taper, cotterless spindle, what is your advice for checking torque after servicing spindle bearings and reinstalling all bottom bracket components?
Brandt is correct. The habit of constantly re-tightening square taper crank arm bolts back to specs (say, 40 Nm) is not a good one. After the crank has been installed and torqued to specs, all that's needed is a one-time re-tightening to specs after a few miles of use. Then as the miles accumulate and the crank moves up the taper away from the bolt head (as Brandt points out), all that's needed from time to time is to snug the bolt up just enough (5 Nm or so) so it won't unthread itself and fall out.

I don't think your crack had anything to do with re-tightening the crank bolt. For one, the four corners in a square taper crank hole are natural stress points. For the other, Zeus was known far and wide for very light components designed on the risky side of "strong." Put those two together and the crack you describe is almost certainly just due to plain old age.
 
#3 ·
Back in the day I'd use my peanut butter wrench to tighten them up as hard as four fingers could pull. After the first ride, I might have to tighten then 1/8 of a turn again.
 
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