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			Actually... heli-coil is pretty do-it-yourself-able.  You just need to drill out and tap the existing hole, which is a breeze if it's aluminum, and not all that hard if it's iron/steel.  Then the insert goes in with some thread-locker, and you're pretty much done.
 All you need for tools are the proper sized bit and tap, which aren't expensive unless we're talking about a 1"+ hole.  In fact, there are heli-coil kits that come with the bit, tap, locker and insert... and I would think a spark plug hole repair would be the most common repair as well.
 
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