Sheared bolt in cantilever brake post
 

I was recently attempting to switch out brakes on a beater mountain bike, and ended up shearing one of the bolts right at the base of the head. The bolt was almost completely in, and it felt like it was threading normally, though it must not have been (unless the bolt was defective--it was also off of an old bike). Either way, it left me nothing to grab on to above the brake post.

I've searched around with no real luck concerning getting that bolt out or re-threading the brake posts. Anyone ever come across this, or know any tricks? It seems that taps and re-threading devices are quite temperamental and require "ideal" conditions to work well. Any info or advice would be welcome.

Since it's a beater bike, here's how I'd handle it. Take a Dremel tool with a cutoff wheel in it and cut a slot across the the broken bolt,just cut it deep enough to us a bladed screwdriver to turn out the bolt. Use a fine file to dress off the roughness and debur the brake post. Crude but effective. Easy peazy lemon squeezy.

The 6mm bolt is big enough that there's plenty of room to drill into the remaining stud. You don;t have to drill it very deep. Then tap in a hex or torx key that jams and see if that's enough to turn it out.

If not, drill it deeper and use an EZ out to take it out.

You can drill and tap it for a left handed fastener, and then screw it out.

or tapered EZ out.. digs into the edge of a plain hole ..

older style Brake bosses were a turned piece, swaged into a sheet metal base ,
brazed onto the frame/fork , the whole boss can be replaced.

unspomer

+1

This thought crossed my mind, as I'm a bit wary of screwing up the threads completely with a tap or one of the ease-outs. Just to double check though, I'd have to cut the groove into the brake post itself as well (as there's no bolt available above that). I don't imagine it would, but could this detrimentally affect the brake post in any way (i.e. structural integrity)?

Thanks again!

You only need to cut an 1/8" deep slot, just enough to get a slotted screwdriver to fit. The slot won't affect the integrity of your brakes. It's a beater bike, remember?

WIth any luck you won't even need to cut that deep. A 1/16 to 3/32 should be enough unless the screw was binding in the threading. If so you may need to recut the slot with two blades in a hacksaw frame or widen it with the cutoff wheel of a Dremel to allow a thicker blade screwdriver.

It all depends on what was happening when the head snapped off. Did it thread in nicely and then you were just torqueing the head into contact with the brake? Or was it fighting you all the way down the threading? If the first option then you MAY even be able to pick it out using a sharp scribe to nudge it in the hole. With the head busted off there's really no load in the threads and the busted bit should turn easily. If the threads were trying to self cut their way the last little bit than that's a different story. In that case I'd say you need AT LEAST to drill and EZ out the stub. Even that may not be enough if you were putting your all into the project and the head snapped off.

But the fact that you snapped off right at the top of the boss suggests that you were just doing the last torqueing of the bolt. In which case try to just work it out with a scriber before you do anything else. Once you work it out a few turns then you can grab it with some pliers.