Trouble removing stripped allen bolt

[fixie541]

I've been trying to remove these two stripped counter-sunk allen bolts on my back cog for awhile now. I've managed to get the other 4 off by using a star nut and hammer. The last two bolts are really in there though.
i was curious if anyone knew what the method with the best success rate would be. I'm thinking maybe a reverse drill bit to remove the last two as a last resort.

 

[silva]

The trouble with such kinda bolt heads is that the insertion space for the tool is too small for the head of the bolt, ideally, a tool should have the maximum torque that the size of the bolt head allows.
So the goal is to give the tool more torque on the bolt head itself, meaning creating tool insertion places near the circumference of the bolt, as to achieve the max torque.
You could use a chisel to stamp (holding it straight on 90°) groves near the circumference. Then the chisel under an angle towards the losening direction. First apply penetrating oil, give it some time, and cause vibrations (hammer) / shocks. If the chisel starts to slip, create another grove and start again there.
A better method to start losening something is to first try all the bolts, just enough to see if they move, if they move, fasten them again. If you do find one that doesn't move, start with that one before all others.
If you do all the easy ones first then the mount can tilt abit, causing the remaining / harder bolts to get even harder to lose.

I also wonder why they used countersunk bolt heads at all, the cog shows 17t and it's only below 16 that bolts can start to void the space that the chain needs.

 

[Ajax Bay]

https://www.screwfix.com/p/laser-tx-extractor-bit-set-7pcs/6329F?

 

[gazza1286]

I would also suggest the method described by silva. Drilling a small pilot hole near the circumference and the using a nail punch with a hammer will do it. Examining one of the already removed bolts will give a guide as to how deep the the hole can be drilled.

 

[r04DiE]

I have been a mechanical engineer for a long time and I would first recommend this (and what @silva said):

I would also suggest the method described by silva. Drilling a small pilot hole near the circumference and the using a nail punch with a hammer will do it. Examining one of the already removed bolts will give a guide as to how deep the the hole can be drilled.

... and then this. These things have a tendancy to break on very stubborn threads and you need to drill your hole nice and straight, too!

https://www.screwfix.com/p/laser-tx-extractor-bit-set-7pcs/6329F?

 

[slowmotion]

This guy's seen it all. It's well worth watching the whole clip. Good luck.

[media]

]View: https://www.youtube.com/watch?v=WDOWPekMX44[/media]

 

[faster]

Just drill straight down the middle of the bolt with a normal drill bit slightly larger than the thread of the bolt until the bolt head comes off.

Once the head is removed, with no tension on the threads it's usually very easy to remove the remaining bit of the bolt.

The extractor bit is unlikely to work as the head of the bolt is too shallow. There will be very little for it to grip on to.

If it is tight, I doubt the chisel method would work without a lot off faffing about. It's not easy to support a built wheel in such a way that you'd be able to give the bolt a really good 'shock' to get it undone.

 

[SkipdiverJohn]

Just drill straight down the middle of the bolt with a normal drill bit slightly larger than the thread of the bolt until the bolt head comes off..

Whilst drilling heads off might work on low grade fasteners, I can't see a regular HSS type bit making much impression on the Allen bolt if it's something like a 12.9 grade hi-tensile. Chances are you'd overheat and destroy the cutting edge of the drill before getting through the fastener. I regard very hi-tensile Allen bolts as just as bad as stainless. Tool wreckers them both.

 

[Oldfentiger]

I have succeeded in removing these fasteners (as a last resort) by inserting the Allen key and securing with a couple of tack welds. Sacrificing the Allen key is still a cheap fix.

 

[Pumpkin the robot]

Whilst drilling heads off might work on low grade fasteners, I can't see a regular HSS type bit making much impression on the Allen bolt if it's something like a 12.9 grade hi-tensile. Chances are you'd overheat and destroy the cutting edge of the drill before getting through the fastener. I regard very hi-tensile Allen bolts as just as bad as stainless. Tool wreckers them both.

Its all about feeds and speeds. If you run too fast you will burn the drill out. I use a surface speed of 25m/m for stainless and a feed of 0.1mm per rev feed rate. Drills last just as long as in mild steel or aluminium if you use the correct speeds/feed.
But saying that, I have a cnc machining centre and a 9 axis robot to do this for me, not a pistol drill.

 

[Velochris]

As a first attempt I would soak in Coca Cola (or any decent loosener like PlusGas) to make them as free as possible. Then try a set of imperial allen keys. The size up from the metric key might get enough purchase to turn (or need to be tapped in and then turned).

If it does not work you have not list the chance to drill out etc.

 

[roadrash]

if you have a dremmel or similar tool , can you cut a slot in it and use a screw driver to remove it

 

[andrew_s]

Just drill straight down the middle of the bolt with a normal drill bit slightly larger than the thread of the bolt until the bolt head comes off.

I've done that with SPD cleat bolts, then just clamped down very hard on the remaining stud with mole grips.

The bolts can't be all that hard to drill, otherwise they wouldn't be mangled to the extent they are.