Now you're ****ed! 
That's turned into a bit of a disaster then!
Best you can do is get some left handed drill bits and drill VERY carefully up to the size of the threads, if the heat helps extract the remainder of the spark plug as you drill, you win. If not you may as well save yourself the grief and go straight to buying a helicoil kit for the correct size, then drill and tap the cylinder head..
This should give you an idea of what you need to do.
http://www.ebay.co.uk/itm/M14-X-1-2...074?pt=LH_DefaultDomain_3&hash=item4173f63352
Get that sort of kit, as you get the correct drill bit, the correct tap, the insert and the insert tool.
(Make sure that is the correct thread size before purchase though)
Oh and use plenty of grease on both the drill bit and the tap, (drill a bit, re-grease, drill a bit re-grease etc) as this will serve to catch the bulk of the swarf produced.
Fish what you can out afterwards with the magnet but the bulk of the swarf will be aluminium so non magnetic, best you can hope for is probably turn the engine over so that piston is as high as possible with the exhaust valve open and then blow the remainder out with an airline and brake cleaner (if you don't use some sort of solvent with the airline the swarf will just stick to the oil coating the bore)
Good luck.

That's turned into a bit of a disaster then!
Best you can do is get some left handed drill bits and drill VERY carefully up to the size of the threads, if the heat helps extract the remainder of the spark plug as you drill, you win. If not you may as well save yourself the grief and go straight to buying a helicoil kit for the correct size, then drill and tap the cylinder head..
This should give you an idea of what you need to do.
http://www.ebay.co.uk/itm/M14-X-1-2...074?pt=LH_DefaultDomain_3&hash=item4173f63352
Get that sort of kit, as you get the correct drill bit, the correct tap, the insert and the insert tool.
(Make sure that is the correct thread size before purchase though)
Oh and use plenty of grease on both the drill bit and the tap, (drill a bit, re-grease, drill a bit re-grease etc) as this will serve to catch the bulk of the swarf produced.
Fish what you can out afterwards with the magnet but the bulk of the swarf will be aluminium so non magnetic, best you can hope for is probably turn the engine over so that piston is as high as possible with the exhaust valve open and then blow the remainder out with an airline and brake cleaner (if you don't use some sort of solvent with the airline the swarf will just stick to the oil coating the bore)
Good luck.
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