Bafang Gear Hub Disc Rotor Hole Depth Short?

Just curious if anyone experienced this issue before. Got a new rear wheel to replace one that went bad, but while attaching the rotor I twisted a screw head off. Upon examination it appears the depth of the threaded hole on the new wheel was off by 0.6mm compared to the old so the screw bottomed out before it could tighten on the rotor. I wanted to get this bike working again so I'm just using 5 bolts now which I shortened with a dremel.

Was the hole tapped before or after the hub was painted? I've seen holes get filled up with paint, zinc plating, just about any anything that's applied after tapping.

I know it doesn't help you in this case, but it's important to understand that the bolt didn't break because the hole was too short, it broke because it was over tightened.

On a side note, I'd be amazed if you were able to twist the head off one of those 5mm button head screws they use for disc rotors. Button heads always take a tiny little undersized hex wrench as compared to a regular parallel wall socket head cap screw.

Izits said:

Was the hole tapped before or after the hub was painted? I've seen holes get filled up with paint, zinc plating, just about any anything that's applied after tapping.

I know it doesn't help you in this case, but it's important to understand that the bolt didn't break because the hole was too short, it broke because it was over tightened.

Click to expand...

I'll accept some responsibility, but if the threads bottomed out on the rotor I would've used a torque wrench at that point.

Here's how I came to my conclusion.
https://m.imgur.com/a/YkrWr

Izits said:

On a side note, I'd be amazed if you were able to twist the head off one of those 5mm button head screws they use for disc rotors. Button heads always take a tiny little undersized hex wrench as compared to a regular parallel wall socket head cap screw.

Click to expand...

The most common kind of brake rotor screw is a button head using a T25 Torx key. The interface can take a lot of torque.

Yes, the Bafang front BPM motors that I've come across have all had shorter than standard tapped threads for the disc rotor mounts.

youth said:

Just curious if anyone experienced this issue before. Got a new rear wheel to replace one that went bad, but while attaching the rotor I twisted a screw head off. Upon examination it appears the depth of the threaded hole on the new wheel was off by 0.6mm compared to the old so the screw bottomed out before it could tighten on the rotor. I wanted to get this bike working again so I'm just using 5 bolts now which I shortened with a dremel.

Click to expand...

Something like half the torque required to tighten a bolt goes into the friction of the contacting surfaces. Since that friction is zero if the bolt bottoms out, it would probably break off well before it's normal torque value.

jateureka said:

Yes, the Bafang front BPM motors that I've come across have all had shorter than standard tapped threads for the disc rotor mounts.

Click to expand...

How did you get around those situations?

I don't like it, but everything I've read says 5 bolts on the rotor is fine. This is a speed pedelec, but only the rear wheel.

OP, ever used an EZ-out to pull a broken screw? Or a reverse drill bit type extractor? Both "should" be simple on a screw that was bottomed out. Use a hammer and good center punch to start the hole.

I tried with an extractor but that broke off in the drilled out bolt. I can also burn the extractor/bolt out with tap burner at work, but I'm not sure the process is safe for the motor. I've read that carbide tooling should also work.