previously helicoiled spark plug hole

[jnorman]

Hello,
I've recently purchased a 2001 towncar with the Ford 4.6 motor. I've been having consistent problems with misfires since purchase. Despite replacing all plugs and coils. Most recently cylinder 3 has blown a plug, and that hoke has already been helicoiled once. One mechanic has already told me that most likely the head needs to be replaced. Can the hole be rethreaded again, to a larger size? I've found a website online that sells oversized threading tools but the kit is over $400. I'm hoping to get out a little cheaper than that. Any help? Thanks.