Okay, I’m genuinely stumped. I got this old Peugeot for free, all it needed was the pedal reattached and new rubber. I thought it’s the easiest revival ever, but no matter how much I torque the nut down, it loosens up again while riding, the thread is not stripped btw.
by GlamStache
8 Comments
Threadlock?
The crank arm might need to be replaced, sometimes if ridden loose the square hole gets worn down so it can’t be tightened properly at all.
Well… take off the arm and take a picture of the “diamond shaped” opening inside.
The crankarm screw should be torqued to 30-50 Nm. If you can reach that torque without threads stripping then the only real cause left is an improper interface between the square hole on the crank arm and the square tapper axle. Inspect the square hole for damage.
Whether or not you should apply anti-size to the surfaces or degrease them is a topic of ongoing debate but it shouldn’t get undone quickly either way
Likely new crank or bb day. u/dave_coulier /uncle Joey is correct in that the square hole in the crank may be worn. The constant loosening of the crank bolt and the crank itself is a large indicator of the crank being worn. Unfortunately the only fix is new a new crank. You might be able to scour eBay to see if someone is selling just the left crank on its own.
A much more rare case is the bottom bracket spindle is worn or damaged and the crank isn’t getting a good bite on it. Highly unlikely considering the crank is aluminum and the bottom bracket spindle is likely steel.
The tapers have been stretched. Replace the crank arm.
Square taper cranks that have not been tightened properly can be damaged by the BB axle if this is what’s happened nothing you can do will keep it tight or fix it the only solution is a new crank arm.
If you take the crank arm off and post a photo of the other side where it mounts on the axle we can tell you if this is what has happened.
If there is no play after tightening it down, you might try cleaning the threads on the bolt and BB really well and then applying some lock tite.