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I seem to have managed to round the 8mm hex bolt on my new square taper cranks on my tourer. Ive tried to drill the bolt head off but only one out of loads of my dad's drill bits worked but that was too small. Is there a technique to drilling steel as most of the bits didn't do much at all?
You need the right bit and to drill and low speed. Heat hardens it.
I wouldn't have thought drilling a crank bolt would be too successful.
Could you remove the other crank arm and get the BB out from that side?
It's the drive side so i dont think that's an option.
Any luck? Bumping it for you as I can't believe there isn't more help forthcoming!
If you're trying to save the cracks and the rest is replaceable you can try hammering a different sized allen key into the crank bolt, especially if you've got a hole there now where you drilled. Will ruin the allen key but good chance of getting the bolt out.
If you have enough room you could try cutting through the axle so you can get the cranks in a drill press and do a proper job.
New bit, slow drill speed and lots of lube is the trick.
Might be worth spending a tenner on the right sized bit, laying frame on it's side so you can lube the head as you drill and having another go?
other alternative is to use a stud extractor;
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but you still need to drill a pilot hole.
Sorry to be negative, but DON'T use a stud extractor. They always snap off and then are impossible to drill out because they're made from the hardest steel known to man.
Persevere with drilling. Buy a new drill bit, any old ones will be blunt. Medium speed, lots of downward pressure and a little oil.
I would suggest a ti nitride coated irwin jobber drill off about 5 and 8mm
Slow speed , light oil or cutting fluid. Drill first with the 5 in the centre with the bolt full of cutting fluid , then swap to the 8 when at 5/7 mm deep. Drill to same depth with 8mm and if its not enoguh - repeat.
Your not trying to drill the bolt out , only the first 3/5mm where the head meets the shaft. , then you can remove the crank and with no tension on your bolt the rest will come out with pliers.
For drilling you could try a cobalt drill or I've actually had good success with those 'drill anything' missionary type drill from the bargain shop. Was very surprised it worked but it did.
Personally I'd try to remove it as is first. Hammer in a torx bit. The hammering should free the bolt a little and the torx will cut into the hole sides and bite.
Something different if you've got the equipment:
Hammer in a long handled allen key and then mig weld it to the bolt. The heat will also help to free off the bolt.
Know anyone with a pillar drill? Decent bit and some cutting lubricant in a pillar drill would be my choice.
You could try cutting a small cut in one side so that you can tap it out, maybe a dremmel would do it?
Know anyone with a pillar drill? Decent bit and some cutting lubricant in a pillar drill would be my choice.
Trouble is the bolt is attached to a bike - so you've got to get the bike under the pillar drill!
I would also say that as the OP has managed to mangle a steel bolt designed for an 8mm allen key that he should, under no circumstances, wield either welding equipment or a pillar drill!
Trail_rat's method would be my shout - as he says, you're only trying to get to where the head/shank meet. I had to do it to three cleat bolts a few weeks ago and it took no time at all. Clearly smaller bolts, but would definitely be my method of choice.
All of the above should be tried before my suggestion.
Get an old Allen key or something similar welded to the remaining head.
Make sure it all nice and clean first so you don't have to redo it .
Get at least 2 of the right sized bits.
Nothing worse than getting half way then snapping or blunting the bit and having to go back to the shops (or wait til the next day and go back to the shops).
Buy your dad some new drill bits for the ones you've blunted too (Either to be nice to him or to shame him into keeping a decent range of tools in), and as others have said, slow drill speed and use lube if necessary.
Wow loads of advice cheers guys 😀
Will pop to screwfix in the morning for unobtainium coated bits as I'm determined to drill the ****er out now!
Success!
5 & 8mm titanium bosch drill bits with 3 in 1 oil for coolant did the trick with the drill on nice and slow.
Cheers