If the stud is sheared off flush with the deck, Ezyouts (Screw Extractors) are what you need. They are easily available from tool shops. (Supercheap might have a set for super cheap if you intend to use it only once or twice)
The ezyout is shaped a little like a tapered thread tap crossed with a drill bit.
http://www.blackwoods.com.au/product_de ... ack_found=
The deal is that you :
a. Center punch the top of the sheared off bolt,
b. drill a hole into the bolt vertically maybe to a centimeter or so's depth (may need to drill further depending on the ezyout used). The size of the hole must be such that it is less than the maximum width of the ezyout so it can bite into the hole once inserted. You'll understand once you've seen one.
c. put the ezyout into the drilled hole until it touches the side of the drill hole and after one or two gentle taps try to turn the bolt anticlockwise.
You turn the bolt by putting a shifter or spanner onto the top of the ezyout like you would a nut. The shape of the ezyout bites into the bolt and pulls itself in harder as pressure is applied. Generally the bolt will undo itself fairly easily if there is nothing like threadlock in the thread. If there is you may need to heat it or tap the bolt with a hammer to break the threadlock before using the ezyout.
A machine shop will charge you about half an hour's labour for the job, depending on how stubborn the sheared bolt is to take out (maybe fifty bucks?), an easy out may cost you half of that.
Yes i have a broken bolt on the engine that goes down into the head snapped off right at the tip of where the threads start.
Make sure you clean out the holes prier to putting them back in..Also a good idea to get a full set on new ones as they may have stretched, Also possible reason 
very frustrating....
