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A 1997 Ford F-150 came in from Beaumont running rough, with a loud ticking that got worse under load. It was not a lifter. A spark plug had backed its way out of the cylinder head, taking the aluminum threads with it. Here is what that looks like and how it gets fixed.

What We Saw

Why This Engine Does It

The 4.6L and 5.4L 2-valve Triton engines use an aluminum head with a short spark plug thread, only a handful of threads deep. Aluminum is soft. Heat cycles loosen the plug a little at a time, and once it is loose it hammers itself the rest of the way out.

It usually announces itself the same way: a tick that turns into a pop, a misfire on one cylinder, and a smell of exhaust under the hood. Some people drive it for weeks. The longer it runs like that, the more of the head gets chewed up.

The Repair, Step by Step

• Pull the coil and what is left of the plug, then look down the hole with a borescope before touching anything.
• Clean the chamber. Every shaving has to come out, not get pushed in on top of a piston.
• Cut new threads square to the original hole and set a steel insert. Square matters more than anything else here.
• Torque a new plug to spec, run it, and re-check. A repair that leaks is worse than no repair.

What It Is Not

It is not a new cylinder head, and on most of these it is not an engine-out job either. We did this one in place. A shop that quotes you a head for a blown plug thread is quoting the easy answer, not the right one.

It is also not a job for the aluminum-filler kits sold in parts stores. A steel insert cut square is the repair that lasts.