What BLuecollarbikes said. Can you post a pic. Is it possible to get to the other side of bolt from inside of housing? Maybe a fine pick and tap with hammer in lefty direction. Maybe tack a piece of mig wire to bolt and turn out. Good luck. doug
Go buy a non Chinese, quality set of stud extrators ( other wise known as ezi-outs) a set 1/8" drill bits
and a tap & dye set for around $40 made in China, for now.
Get the expensive one's later when you practice on the cheap set and they give ya the ****es.
Use the 1/8th drill bit to drill a hole dead centre of the broken stud, use the ezi-out to remove the stub, incase you miss drilling straight dead centre with the drill, use the M6-1.0 tap from the tap & dye set and clean up the thread.
Use some oil when tapping the thread.
If you mess up and the engine is still new, send it back and ask for another, cos this is nonsense.
I had this happen to me not long ago on a rear mount. All I did was use a drill and just drill through the bolt until their was nothing left. Then bought a tap set off eBay for a fiver and retapped it to the next size up, M8 (there might not be enough parent metal to do this on the front mount though). Be very careful not too go to far when drilling or tapping though, as you will end up drilling into the crank case, not good!! Also you must pour some oil into the hole to help the aluminium filings to stick to the tap to get them out easily, or other wise they will compact and the tap will get stuck and you will never get it out!
Before you do this though, first try cutting a notch into the bolt with a dremel or whatever, and then try unscrewing out. This will save you much more time and money.
Heres some existing threads about broken mounting studs:
I just busted a back mount, about 1/4 above the engine mark. I've got lots of mounting bolts, but I know what the problem is. The chain is pulling on that side very hard, in fact it's pulling the entire engine to the left. I'm going to use better quality steel on that mount, and lengthen the chain a bit.