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
- 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.
- However, I am also keen to do it on a budget, which means do it myself or with minimal help from professionals.
- I am not massively afraid to ruin the frame in the process - I will know that at least I tried.
- 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.
I assume that:
- I will need to drill through the hole and somehow secure the replacement post in the boss.
- 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:
- 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
- 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.
- 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.
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 .
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?
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?
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?: