Bottom bracket sliding

It's pretty unlikely that this just came out suddenly. The bottom bracket has probably crept out over time due to being insufficiently tightened at assembly.

To repair I would suggest complete disassembly of chainset and bb, have the bb shell threads chased at a shop and reassemble with torque wrench to correct spec (40Nm).

@airballer69 The answers above are good, but involve a bike shop and some bike mechanic skills. Since you asked, how can you repair this yourself?

1. If you don’t have tools to remove the crank and screw the bottom bracket cup back into the bottom bracket (BB), then just find/borrow a wrench like a pipe wrench that you could grab the outermost side of the bottom bracket cup with and turn it so it threads back in. NOTE: be careful not to grab or damage the threads with the wench.
2. As you are close to screwing the bottom bracket fully back in check the motion of the cranks to see if it still turns freely. If you feel the cranks start to bind then stop spinning the BB cup in. If you don’t feel it bind then wrench it “hard” once it’s flush with the frame to “torque” it down snug. Keep an eye on it overtime as it might loosen up and you’ll need to repeat this process with a little extra torque this time 😉 Good luck and let us know how it goes.

The bottom bracket in this photo is threaded, it cannot slip out like a press fit bottom bracket. It has probably been giving you multiple warning signs that it was unscrewing. Creaking noise with each pedal stroke, pedals moving in strange ways, front derailleur rubbing or not working correctly, strange thunk when rolling the bike over a thresh hold or down a curb as the crank set flopped around.

Incorrect torque value in assembly is partially at fault. I agree with the previous answer. The second part of the equation is the surface that the fixed side of the bottom bracket is tightened against. The bottom bracket shell surface was warped by the manufacturing process. Welding the aluminum tubing to the Bottom Bracket shell caused warping of the face of that little threaded tube. This means that instead of the bottom bracket firmly locking into a flat planer surface it is only touching a small bump of metal on the face.

Then to make matters worse, the red paint was applied, and gravity cause some slump in the paint making it thicker on one side of the Bottom Bracket shell and thinner on the other. Paint is not metal, as a mechanic tightens the bottom bracket into the shell, the paint will crush a little and then the force you exert into the pedal will damage the paint particles more until they fall out and leave you with a loose bottom bracket.

The bottom bracket when loose will unscrew. It will unscrew even faster if the bearings are rusty inside, increasing the friction to remove it.

The cure is a combination of refacing the shell to have a planer face that is perpendicular to the bore AND torquing the Bottom Bracket to the correct value into the fresh metal surface. Plus checking the bearing for smooth turning without corrosion.

Most bike shops can use Bottom Bracket Face cutting tools to FACE your shell. Check the BB bearings, replace if necessary. Torque to the recommended value and re-enjoy your ride!

Christopher Wallace BicycleGuild.com

@airballer69 Would you mention what make and model you bike is? I'd like to see if you'll need a special crank puller tool or if a standard tool will work to remove your crank. If you can remove the chain side crank it will make working on the Bottom Bracket easier. Also I'm wondering how you use your bike, like is it just for around town use or are you doing trails as well. Let us know if you have any other questions and how your repairs are going.