Brake post broke off the boss. How do I replace it?

Question

A brake post has been broken off the boss on an old BSO (bike shaped object). I'm exploring my options for reinstalling a brake.

There is a hole where the post was, some 6 mm in diameter. I bought an M8 replacement post and quickly realized that the purchase was the easy bit.

Question: what's the best way to reinstall a brake here? I am not considering a disc brake adapter.

Philosophy – my priorities

1. I totally appreciate that many would write this frame off but I do have a bit of an ambition to put this bike back in use.
2. However, I am also keen to do it on a budget, which means do it myself or with minimal help from professionals.
3. I am not massively afraid to ruin the frame in the process - I will know that at least I tried.
4. As you can probably tell, I am not one to just “go for it” – I do prefer to get a second opinion to make sure I don’t make any silly mistakes.

Assumptions

I assume that:

1. I will need to drill through the hole and somehow secure the replacement post in the boss.
2. It will be enough if I make a reasonable effort to make the replacement post parallel to the surviving one (in other words, coaxial with the missing original post). I did originally wonder whether I need to make some wooden template to slip onto the surviving post to guide the drilling for the new post. But I now assume this will not be necessary as long as I do the installation carefully.

Options considered so far

Broadly speaking, there seem to be three basic decisions to make:

1. Should I thread the hole where the old pivot was? I could use a tap and put a thread on it. Or I could just drill an unthreaded (“clean”) hole, just big enough for an M8 thread to pass through
2. Should I use a nut to secure the post on the inside of the brake boss? A normal M8 nut is slightly too big to fit in. However, I reckon I can grind a nut down slightly to make it fit, and it should then lock itself against the inside of the brake boss, without the need for a spanner.
3. Should I add a weld/braze? A car mechanic nearby will probably be happy to help as long as it’s doable. He once helped me with a car: from memory, a bracket holding the exhaust had cracked/snapped and he welded it back together, a two-minute job for some minimal token payment.

Various other things considered so far

Note, these are hard to order.

1. The threading of the hole: it doesn’t seem like it will be enough by itself. I didn’t measure the plate with the hole, but I would estimate it’s some 2mm thick. So I would need to secure the post with either a nut or a weld/braze .

2. The weld/braze: will the materials even be compatible for welding/brazing? Is it enough to just clean the area, or do I need to sand down the paintwork? Can I sand it enough with my cheap 18V cordless drill and a sanding drill bit?

3. The “clean hole and nut” scenario: I have in fact already tried grinding a nut down with my brother in law. We used an angle grinder to grind down two “edges” (think top and bottom), and one apex (so that it slots in deep enough into the boss). We did probably grind away too much though. Will that be stable though? I mean, can I tighten it enough to keep it stable?

4. The “threaded hole and nut” scenario: Is it even possible to align the thread on the hole and the nut? As far as I can tell, I would need to be able to turn the nut freely inside the brake boss, so that I can fasten it against the drilled hole in the boss. There is no room for a spanner, so I can’t see how I would do that.

Finally, I think I ought to acknowledge that I have seen Can this damage to my brake mounts be repaired? and Broken rear brake - told it was unfixable. Unfortunately the Problem Solvers Cantilever Stud Repair Kit seems to be no longer available.

I did make a four-minute video a while back capturing the situation - please note this was before I asked about proper terminology under What are these parts of the frame called?:

Answer 1

Is installing another rear wheel with a coaster pedal-activated brake an option? In this case, you will only need to fixate the brake arm to the chainstay for it to work. The "fix" will be located far away from the original brake bosses, so you won't need to think about them anymore.

Answer 2

Just threading what you have won't work because it's too thin.

If you can fashion a nut to slip in behind there and take your brake post and have the inside of the remaining braze-on actually hold the nut as you tighten the post in, that approach would be reasonable if it all works. Canti brake studs should be pretty high torque and the experiment here is whether the section of the braze-on you're hoping to use as a wrench might be weaker and softer than you're hoping. If you can get it tight enough without brazing or welding, then the fact that it's not brazed/welded shouldn't be an issue. The nut and the back side of the stud need to have a sufficient amount of contact with the top surface of the mount - it's very thin material on this one so you need the contact to be generous because otherwise it would be easy to imagine it tearing out etc.

You could braze a nut in, or you could also fabricate a little steel insert and braze that in behind there and then tap it. When brazing or welding nuts to things, it's good to start with an oxide coated nut as opposed to plated or stainless, because the oxide type protective finishes are easy to strip off chemically or mechanically while platings are much more difficult and stainless requires jumping through various hoops to do the joining with a dissimilar material. (Basic plain steel with no coating or finish also works if you can find it in the size you need). All of those solutions require proper cleaning and technique and you'll probably have a better time if you don't expect them to be too quick and dirty. It will probably also require a plug or bottoming tap to match your new post to clean it up afterward.

Because the original part looks to be low quality and is now distorted, just replacing it completely might make more sense than any other "hot" solution. The welds holding it on are minimal and would be easy to file off. You would get the seatsay filed and sanded down to round, paint-free, and clean, get an appropriate replacement complete mount, work it down so the offset is correct to the match the other one and miter it to match the stay, then you can use a simple piece of flat stock bolted to the other one to fixture it as you as braze or weld it. The person doing the welding/brazing only has to bear in mind that the tube with this kind of joint needs almost all the heat directed at it, because the edge of the boss has relatively little thermal mass and will get to temperature almost instantly while the tube takes longer - but it's not difficult once you're used to welding or brazing the edges of things to tubes.

If you can clean up the hole and get a new boss to mate with it with the right amount of clearance for brass brazing (basically slip fit with only the barest discernible movement), then that would be a reasonable approach. You would likely be best off getting a boss of that sort by unbrazing it from a scrap bike or from one of the linked ones above, but you should expect to have to work it down to a good fit as opposed to it just working. To do this kind of repair and have it work well you really need to get the paint off the entire area the heat will touch. Otherwise you're left with a mess of scorched paint afterward. That's one reason replacing the whole thing might be better - you're cutting the tube down to being round, and then removing the rest of the paint on the stay is relatively easy.

Answer 3

Frame challenge: what are you going to use the bike for?

The braze-on here is welded on, and is noticeably bent, presumably by the forces when the post came out. Moreover the rear weld might be cracking (your last picture might show an opening crack with the dirt inside once joined up. But it might not, as well). However well you secure the post, it's a weak point.

I'd happily ride that bike with no front rear* brake at 10mph to get the shops. I'd never take it down a steep descent at 30, no matter how well you secure the post.

As for putting the post back in, if you have an m8 plug/2nd tap (taper will likely be no good, there's limited clearance before you hit the tubing) I'd bend it back to square, tap the hole as square as I can, screw the post in, and then take it round to the mechanic to weld. (Of course, I'd get the paint off first...) That way I'm not relying on a few mm of failing steel to keep the post in, but I also know the alignment will be good on the weld---and that without fiddly wooden jigs which can move at the wrong moment. Otherwise, if there's clearance to put a ground nut on the back, I'd use permanent threadlock to stop it vibrating loose.

As to materials, if it's steel (almost certain) and the replacement pin is steel (ditto), it will weld just fine.

As to surface prep if you do want it welded: the mating surfaces need to be clean, and there needs to be somewhere close to get a clip on. You don't need to strip the paint off the whole thing, although depending on how the weld goes and how much you clean off, it will likely flake off. Brazing tends to heat a wider area by time the job is through: heating is less intense, and there's that flame to worry about.

All of this under the caveat that what you will have at the end is a weak point in an already weak frame. Does that matter? It depends what you want to do with it.

(N.B. there are plenty of people on here with a good deal more experience than I have.)

*got that wrong, sorry: I thought this was the front brake. It's definitely better to have a front than a rear brake, although both are even better...