Anyone here broken off a rack-mount bolt? Have remedy?
 

Right after having finished getting my newly-pieced bicycle road-ready, I broke off a #4 bolt while trying to remove it from the seatstay rack-mount. The remaining piece is .25" below the surface of the braze-on. This has infuriated me. Has anyone here ever dealt with this problem and if so, what solution worked to removed the remaining piece of bolt?

Reverse thread screw drill bit.
It will dig into the screw while counter clockwise pulling it out.

How do you ensure that the threads of the braze-on won't get damaged by the drill-bit?

Use a drill bit that is smaller than the 5mm wide hole.
I used a reverse screw drill bit to extract a broken rear dropout screw which is like 2mm wide and the threads in the frame were fine. It was probably a 1mm drill bit.

Was the screw broken off below the surface of the dropout, though? On mine, the bolt is broken off 0.25" below the surface of the braze-on, If the drill-bit slips, it can touch the threads of the braze-on.

Can you obtain a tiny length of metal tubing which will fit into the hole and allow a drill bit to pass through the middle to drill a locating hole for the extractor to bite into?

If it just touches but does not press on it with pressure, not a problem. Any metal on the threads that is removed would be minimal. It takes pressure on the bit to remove much metal.

Worst case scenario, if you have to drill it out to remove the broken off screw, you can tap it for a larger bolt for future use. Most frames use M5 bolts for racks, but you could tap it for M6. Some racks will take a M6 bolt, but some racks would need to be drilled out for an M6.

If you lack the tools to drill and tap it, I do not know if bike shops have such tools or not. Maybe someone that knows if these are common could comment?

Some people instead of installing rack bolts the common way instead install a pan head bolt from the other side of the frame, then put the rack on the bolt with a nylock nut. Thus if the rack bolt breaks, you can remove it with an allen wrench. But this does not work on the drive side if your rack bolt is close enough to the cassette to interfere with the chain.

I'd suggest the same (left handed drill bit) followed up with a screw extractor if necessary. The problem is this... If all goes well it is an easy job, but you can make things worse if you break off either in the work. The bit and extractor metal is hard and it becomes really hard to deal with a broken off extractor recessed down in the hole. Drill bits and other extractors wont touch it at that point.

So I suggest being sure to get it right the first time, especially if using an extractor. The drill bit is more forgiving than an extractor, but still use care and don't break it off in the bolt. There are factors that make this a bit tricky. Smaller bits are easier to break off and get in trouble. It might be harder to get a center mark punched and get the bit started in the center of the bolt.

Extractors are quite hard and therefore britle so they can break off in the bolt pretty easily. Try very hard to avoid that as it complicates matters greatly.

It sounds like you may not have much experience with this sort of thing. If in doubt it might be worth either enlisting experienced help or paying a shop to do it.

That's smart.
You'd think even some plastic could work, given that the main force is going downwards into the broken off bolt.

I have drilled through bolts back in the motorcycle racing days, certain bolts had to be wired closed, so I know it can be tricky getting the drill started without it moving around--plus this is a teeny tiny hole to work in.

I'd say CAREFULLY try on your own, but paying a good trusted mechanic may be the best route, especially if you are not mechanically inclined or experienced.

If, after all the things that you do to prevent it, you end up chewing up the threads, use a longer bolt and a nut on the other end of the mount.

Pay a visit to a business like an iron worker. That's what I did when I broke off a tap in the barrel piece of my Beckman rear rack. It should have a vice and a drill press to keep things straight.

I've broken bolts and also the dropout.

Use high grade bolts and bolt it from the inside.

The dropout isn't to easy to handle

As Staehpj said, use a center punch to get a good stable place to start the drill bit. also use a smaller drill bit (3/32" or even a 1/16") to make a shallow hole (<1/8") providing better guide for a 1/8" (3mm) bit. Then a 5/32" (4 mm) drill bit. A reverse cutting bit might be helpful. This will give you a " buffer between the drill bit and the threads. Let the drill do the cutting; don't put a lot of pressure on it. Once you have a small hole through the bolt it is easier to use some finesse enlarging the hole as you use the larger bits. Usually the drilling is enough to loosen it up. The real challenge is finding a center punch that will fit down a 5 mm hole without messing up the threads.

I'm assuming that the bolt broke because it was frozen in the mount or you over tightened it. It might help to put some penetrating oil in the braze-on hole and let it set for awhile before working on it.

Really good advice, and especially about penetrating oil. Let it sit overnight even, could help if it was corroded in tight. Even if you end up taking the bike to a shop, if you have some penetrating oil, put some in anyway, could very well make their job easier.

penetrating oil is a smart start. I agree with the, get it right on the first try, so using oil to loosen first gives you the best chance. Drill bit and extractor will/should work fine, just needs to be done patiently. I like the idea of using a small tube to center drill bit.

Thank you all for your input. I'm going to conclude that since this bolt broke off while trying to remove it, there must be damage to the threads somewhere that I haven't been able to see yet. I'm going to hire a bicycle specialist mechanic to do this repair. There is one in my area that my LBS referred me to. Better that than risk breaking a screw extraction bit after it's already dug into the bolt piece.

Good call. Presumably your mechanic has an M5 tap so that if he gets it out, he can run the tap through to clean it up.

And as I mentioned in a previous post, one size larger threaded hole would be a viable option if the threaded hole gets messed up beyond repair. One of my bikes uses M6 bolts instead of M5 for the racks. Your mechanic would need an appropriate bit size and a M6 tap.

Nyah, You never mentioned what bike this is, what material frame is, and if it aluminium it might have a steel insert with the threads.

I have an alu bike that I damaged a seatstay bolt hole, stripped the threads, and my lbs didn't have the tool to remove and put in a new insert thing, so I just used a p clamp.

If all else fails, rubber coated P Clamps are your friend.

Let us know the outcome.

By the way... Something like Kroil or some other really good penetrating oil is 1000 times better than the usual WD 40 for getting stuborn items loose.

I got a same problem as I broke a bolt in a brand new carbon fork about 2 years ago. I haven't had the courage to try the extraction.

Hey there Mr PW, so how did you do that?
By over tightening I presume, or had it gotten loose and so had mucho back and forth forces going on?

I personally have stripped a frame rack bolt thread by over tightening, and broken at least one seatpost bolt, but so far (touch wood) I haven't had a bolt stuck in a frame or whatever yet (mind you, I have done this on a cars brake caliper)

it's a sad subject with me haha. had a brand spanking new fork custom painted and when it came in I bought these nice anodized screws to complement it. But the screws were poor quality (as it usually is with anodized vanity crap) and I could not get them screwed all the way and one just snapped off.

darn. Thats a drag.
I suspect because the bolt was just put in, at least it is not rusted in or whatever, so hopefully with a really really careful tiny drill bit made for metal, you could start a tiny drill hole in the middle. It might be hard because the surface will be all uneven, so hard to keep it in the middle......
as mentioned by others earlier, you'd want to properly secure the fork well, and a drill press would probably be the safest so the drill stays nice and perpendicular. Have you had any suggestions from a trusted bike mechanic you know?
Certainly will require a very small drill bit, and then a very small reverse thread tapping remover thingee also, to not risk damaging the threads.
good luck