So as title states, over threaded a bolt and snapped it. I know you can get reverse thread to remove bolts but this wingt work as its over threaded.

Is the best thing I can do is drill it out? If so, what kind of drill bits? I've used metal ones before but snapped them in the bolt

Would help to know what bolt and where on the car

this might sound stupid . try a blade screwdriver to undo the bolt.
try it before you flame me . its worked for me before

Its one of the bolts on the flange on the muffler. Finishing it off and got impatient =(

Drill and tap it

Alternatively - weld an old spanner to it and use that for torque.

I've used this method it a few times and it works a charm. I usually just buy the cheapest spanner/wrench I can find at Super shit auto.

That wont work, its inside the flange hole.

Will try drilling it, just need some uber expensive drill bits so they don't snap

Definitely needs to be drilled and tapped. Go easy and I hope you have a good drill.

What a pita.. Last time i did this was on top clip of a para, drilled it out which melted the outside of the bolt so thread was gone, took bolt with defined thread out of my front bumper and screwed it in hard, worked a treat lol.

Use the Sutton formula cobalt series drill bit as they are designed to drill through hardened steel. Also get a can of CRC Penetr8 or Loctite Freeze & Release as they have a capillary action to really get in and lubricate the seized sections. Good idea to dab some antiseize on the flange bolts before putting them back in so they can be removed again.

If you get really stuck call this guy - http://www.yellowpages.com.au/vic/narre-warren-south/thread-doctor-the-12645464-listing.html

Good bloke - helped us out many a time at Holden back in the day.

If not, maybe look for this:
http://www.justtools.com.au/images/k12001_b.jpg
http://www.justtools.com.au/prod5511.htm

How'd you go with this?
I have snapped plenty fitting extractors to old cars and used easy outs (thick threaded strong ones). Get them from any auto store... maybe $15
Worked every time as long as you center the drill hole well, thats the tricky bit

Ended up getting some high speed tensile drill bits and some lube, drilled right out and then spent 20 minutes pushing the damn bolt out