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I've had multiple times the issue of the loadbearing screw for the bike rack breaking in half, and it's always a hell to get out.

There seem to be two schools of thought:

  1. Drill a bigger hole and put in a nut and bolt (had this done in France on one side of the bike, I think the hole was increased from 5mm to 6mm; took about 5-10 minutes)
  2. Very precisely drill a hole through the half-screw that is stuck in the bike frame, so the hollow half-screw can actually come out, without making any change to the bicycle frame (in Germany they preferred this option, but they were all too busy so I had to go to a volunteer-bike-fixing center and did this myself in about 2 hours while breaking 2 drill bits in the process)

Now the side where the hole size hasn't been increased has this problem again, and being currently in Spain, I'm widely getting advised to just drill a bigger hole.

I'd like to make an informed decision, but it seems to me like the optimal decision depends on the material of the frame and its size around the screw hole and its shape.

How does one even make a choice between 1) and 2) ? Is one obviously better? Or is it a question so hard that I probably can't get it right anyway? Or it doesn't really matter ... ?

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  • 4
    Was the original bolt run loose? Or was it a cheap bolt made from cheese-steel? Or was it overly-extended and cantilevered out?
    – Criggie
    Apr 9 at 3:19
  • 1
    @Criggie Yep, I think so :)
    – Gabi
    Apr 12 at 21:37

3 Answers 3

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To get the canned advice part out of the way: it's pretty common for problems with broken rack bolts to come after the bolts have spent some time loose under a heavy load. It's possible that going again with medium strength threadlocker and/or higher torque on the bolts could solve the whole problem. But, it's also possible that's not enough for what you're doing.

It could also solve the problem completely to simply replace the rack bolts with something higher up the strength food chain. In the US these are Grade 8 bolts and in Europe they are called 10.9 as I understand it. (They're not exactly the same thing but pretty close). This will usually be a black oxide coated, very high tensile strength, non-stainless bolt. Again make sure it gets plenty of threadlocker, which will prevent corrosion too. These kinds of bolts are much stronger and tougher than the usual random stainless ones that come with bike accessories.

As for whether enlargement from M5 to M6 is a good option, here are some guidelines:

  • On plate type dropouts with lots of material everywhere you can often do it with total impunity. Examples would be many early or low-end aluminum bikes.
  • You're looking to ideally keep an amount of material equal to the nominal diameter of the bolt all around the hole. In other words, for M6 you want to be able to trace a 9mm radius circle on the frame around the center. This isn't a hard rule in every application, but if you're loading the rack hard enough to be breaking bolts than it probably isn't a good choice to go any less.
  • In a borderline case (or if the threads got stripped), one can consider going from M5 to M6 by removing just the threads to make a plain hole without actually doing much or any enlargement, and then running a nut on the back side, assuming there's room without interfering with the chain or cassette.

Your question about removing the half-screw suggests there might be something else going on; when a rack bolt shears from being overloaded, that means it will no longer be preloaded. Usually a bolt like that can be removed by either grabbing it with pliers if there's enough of a stub, or gently kissing with a dremel cutoff wheel to make a small screwdriver slot if it breaks flush with the dropout. If it's stuck from corrosion, then it has to be drilled with a bit that's appropriate for stainless.

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My preferred option is to remove the remaining bolt and preserve the frame. It should be a simple task to any engineering shop and does not need to be a bike shop. Using an Easy out screw extractor makes this a simple job provided the bolt is not ceased on place.

Drilling a larger hole and using a bolt though the frame risks crushing the frame and required a crush tube to be inserted into the hole. The sizes we are talking about are fiddly and require high precision to get this right.

You need to investigate why you are breaking bolts. Are you using high tensile bolts torqued up correctly. The bolt it self does not provide the load carry capacity, it clamps the parts and the friction between the frame and hanger take the load. If the bolt us under torqued or (comes loose) it suffers movement and fatigues. If over torqued, it is stressed past its elastic limit and is unable to provide the clamping force under vibration.

It may be worth adding a check the bolts are torqued correctly to your regular maintenance schedule, and possibly use a low strength thread locker (e.g. Loctite 222) to ensure they don't loosen off.

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Depends on the situation. For mounting a rack I would have no problems opening up an existing through-hole in the frame and using a larger bolt. Be mindful around the chain though - anything protruding near the cassette can cause issues. However over time they tend to be self-clearancing.

You would have to use either double nuts, or a nylock nut, or some other form of retention like threadlocker. The bolt should take up all of the hole without a lot of spare room, meaning I would not put a M4 bolt into a M4 hole that was drilled out, I'd use a M5.
Also check if it works better to have the bolt-head inboard or outboard.

Another option is to tap new threads into your embiggened hole. Downside of this is the thread pitch decreases with size, so there will be fewer full turns of thread in the hole effectively decreasing the total maximum load.
Or you can look at a replacement thread insert, aka a "helicoil". These can be done in the home workshop with care.

I would generally NOT drill out anywhere if:

  • The backside is unreachable. A bottle cage thread for example, the fix is to remove the old rivnut and replace it completely.
  • the hole is too close to the edge of the base part - you must have the whole bolt head supported by the metal as a minimum - any less and you risk the hole breaking out the side over time.
  • Brake componentry - they're too important to get wrong.
  • Stem bolts - again, safety critical and a minor failure can have catastrophic consequences.

Be sure to use a decent quality bolt and nuts, not those dull grey metal from a cheap multipack set.

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