Removing a rounded hex bolt with 3 mm head

Any ideas on removing this bastard? It's the upper one that's completely rounded. It's partly my fault for using a disputable quality key and not stopping when it first slipped, but the bolt heads are just soft beyond belief. It hasn't seized since it has been screwed in just moments before. It has been tightened to 5 nm but I couldn't untighten it when I wanted to tweak the position. I just knew this would happen one day with this lousy ass bolts.

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Cut/file a slot in it and use a screwdriver? (assuming I can see that properly and the bolt head is actually sticking out)

Smash a torx head in there and twist.

Smash a torx head in there and twist.

@ffm Yes, that could be the option since the head is indeed sticking out. But I'm scared to mess up the handlebar that way.

@|³|MA3K I don't have a torx at hand, but might try to get it. What exactly did you mean by torx head? A screwdriwer with torx xead, an "L" shaped key or a torx bit?

Thomson has a smooth thread like mack said that works too but if you have other sizes of allen key just try it but try to undo the lower bolt first it might help

take the strain off with the other bolt as stated above.

but to get the rounded head bolt out, get some valve grinding paste in there with the Allen key, that will help it bite and should remove it with less fuss. grinding paste is also known as lapping paste. works with rounded screw heads too.

having an imperial set of keys to hand can also help, as they might fit the rounded hole slightly better.

Use this method all the time in workshop. Can confirm it works, especially with thomson stems.

Appreciate everyone's advice... I'll see what tools I can get my hands on and try something.

get some valve grinding paste in there with the Allen key

I've not heard that tip before. Do you reckon carbon grip paste would work?

It may do, give it a try. It’s a nondestructive and quick way of doing it. Plus it doesn’t bugger up your tools

Extracting tool...like tapered drill bit but threaded in reverse.
Can be had for not a lot $

I stumbled upon my neighbor who is a car mechanic. He suggested that we should try with a torx head, so I'll stop by his shop next week and give it a try.

One more thing, should I unscrew the bottom bolt completely before trying to undo the upper problematic one? I'm asking this because I've read many times that untightening one bolt actually increases the load on the other one.

should I unscrew the bottom bolt completely before trying to undo the upper problematic one

no. try and have the loads relatively even across the bolts

Someone above suggested the opposite - that the other bolt should be undone first, so I was confused.

if you slacken the other bolt off it won't tighten the one you're trying to undo, it will take the strain off.

you want them equal when doing up, won't matter when being undone.

I wouldn't be surprised if the other one was over tightened and caused stress on the rounded one, making that too tight.

One more thing, should I unscrew the bottom bolt completely before trying to undo the upper problematic one? I'm asking this because I've read many times that untightening one bolt actually increases the load on the other one.

This...

Remember this stem has two bolt only....

But i feel like you over tighten the top one....just undo the lower bolt youll be good

Slotting it with a Dremel would get my vote. But then I have a Dremel...

I wouldn't be surprised if the other one was over tightened and caused stress on the rounded one

It's an X2; the faceplate snaps before you get anywhere near the torque required to round the bolthead.

I just used this method for a rounded off Allen bolt. Worked a treat, but had to hammer for a good 10min to get it in properly.

The annoying thing is when you hammer it in good and tight, go to put the handle on only for it to fall out.

Had good results holding the bit with a vice grip (which acts as a useful handle).

Make a slot or even an indent on an edge of the bolt and use slot headed screw driver and beat anti clockwise with a hammer. Will unscrew slowly. (I did this twice on Friday with a bolt for the wheel and front brake sunken bolt - I now need to get the stuck seat post out. FML)

I used a hex key and held it on the long end, hammered on the bend.
Will fuck up my key in the end of course. Not smart..

Can you just cut through the bolt with a grinder and a steel cutting disk? Subsequently, hard maybe to get the bottom half of the bolt out of the stem, but fast and lots of sparks.