Trying to get my bent hanger off and one of the bolts has rounded. It takes a 2.5mm allen key.

There’s a set of screw extractors on Screwfix

http://www.screwfix.com/prods/18643/Drill-Bits/Specialist-Drill-Bits/Screw-Extractor-Set;jsessionid=CXU3RCDLEOWEYCSTHZOCFFA?ts=79524

but the set is from 4-11mm. What do you reckon? Worth a punt? What size pilot hole shoud I drill?

Any other ideas?

If you have some Torx keys, you can try hammering a suitable size in and then extracting.

I was just about to say the same thing.

Screw extractor should be pretty much your last resort there. Torq can work but make sure you don’t hit it too hard, don’t want to bend the frame. Cutting a slot onto the bolt head is another good one (careful not to hurt the frame). Dremel with an almost worn out disc is the best tool here assuming the bolt head is flush.

What frame is it? Is the bolt threaded into the frame or just into the hanger? If it’s the latter then just drill the head of the bolt off, sorted, hanger will come off. How easy this is to do varies from frame to frame.

Screw extractors don’t work if the bolt is tight.
If it takes so much force to undo the bolt that the hex rounded first, then a screw extractor will break.
Those tapered left hand extractors aren’t particularly good anyway. They tend to spread the bolt out and jam it as they tighten.
Socket cap (Allen) bolts are pretty hard to drill as well.

Without knowing the exact details of how the bolts are fitted, I’d be looking at cutting a slot with a Dremel to use a flat screwdriver as a first choice, followed by grinding the head of the bolt off if that fails.

Some useful advice there guys, cheers. It’s an Orange 5 frame, the bolt head is countersunk into the frame and it looks like there are a couple of threads in the frame itself.

Just drill the head off, it won’t be threaded into the frame and the mech hanger, just the hanger.
A 4mm HSS drill and not too high a speed.

Screw extractors do work, in fact they are so easy and fast that they are my first resort. In particular they work well with rounded allen heads.

You are best to us them with the handle from a tap and die set.

Whether a 4mm extractor will go into a rounded 2.5mm allen I doubt but you do get smaller – try you local iron mongers (Barnitts if you’re in York as I am)

Thanks again. I’m going to try the torx trick with some wd40 first when I get home as that’s the least destructive of the options. Wish me luck

Cracked it. Tried hammering the torx in but stopped as I was concerned about damaging the dropout so I drilled off the bolt head and bingo!

Thanks for all the advice.