Pull the heads and have steel inserts installed in all the plug holes by a machine shop with experience doing so. They need to be GLUED in with the proper type of compound to ensure they do not come out on you.

BTW...this is a good example why you always use anti-sieze on all spark plugs going into aluminum heads...ALWAYS.

 

Heli-Coil it! Some aftermarket heads are Heli-Coiled from the factory to prevent just this. Me being cheap, and quick, I'd Heli-Coil it, put the plugs in it and drive it! ::
HTH
--Kyle
PS: I agree. Anti-seize, always! I cracked the ceramic on 4 of the 6 sparkplugs I took out of my Nissan because the last genius didn't use anti-sieze 75k ago.

 

It gets snug but it isn't right. Did edelbrock honestly make these without the heli coils? Pure aluminum threads? Should I try it while the plug is snug or not chance it and just pull the head. Can't really hurt anything if it doesn't work right? Plug might just tick if it comes loose.

 

I've used a helicoil on some other parts, the final step in the process is to BREAK OFF the tab that is used to screw the helicoil in. So, if you don't remove the head from the block, you will wind up with a small but destructive piece of metal in the cylinder.
That could really ruin your day! :horror:

 

As mentioned above, the Helicoil Kit is the way to go.

Edelbrock uses inserts in the rocker stud, intake and exhaust mounting holes on their heads. Other head makers offer the option.

 

The plug will not only tick, but probably blow out
and cause more damage. I had a loose plug in an iron head
blow out and dent the shock tower.

 

The plug will not only tick, but probably blow out
and cause more damage. I had a loose plug in an iron head
blow out and dent the shock tower.

I know someone who had a spark plug from an old Toyota fly out and embed itself in his jaw. Still has the scars to prove it. :shocked2:

 

I have the HeliCoil kit for 14mm spark plugs if you need to borrow it. I've done plenty on heads while in the car, though never in a Mustang. Clearance might be an issue.

IIRC, the spark plug thread insert don't have the tang.

 

Always remember to use an anti seize compound on the spark plug threads when the head has aluminum threads. Old info for most of us, but if it helps 1 person it is worth posting.