Spark Plug Hole Thread Repair

Theoretically, routine spark plug replacement should be a simple task. Crank out the old plug, thread in a new one, and you're done. Fiendish engine gremlins often work against you though, and can turn an easy job into a hand-wringing nightmare. Debris can build up in the threads on the end of the plug, or an overzealous mechanic may have previously cross-threaded the plug. Either way, there's a good chance the abused plug will take out the soft aluminum threads in your cylinder head as the plug is removed. It's not necessarily the ruin of the engine, however, as several spark-plug-hole thread-repair kits are available at nearly any auto parts store. With the right kit and a few hours, the damage can usually be repaired without even removing the cylinder head.

Tools
Each kit typically consists of a special tapping tool, a selection of stainless-steel thread inserts, and an insert installation tool. You'll need to provide a standard t-handle to turn the tap, an extension to go between the tap and the t-handle (if necessary to reach the spark plug hole), needle-nose pliers, some heavy grease, rags, long-stem cotton swabs, no-residue brake cleaner, and a small flashlight. The tapping tool is designed to drill and tap your spark plug hole to a slightly larger size, while the inserts are coils of square wire sized to thread into the newly enlarged spark plug hole. Once in place, the inserts spring outward against the hole's walls, restoring the perfect internal dimensions to accept your original spark plug.

New Threads
Begin by disconnecting the vehicle's battery so that the engine cannot start during your work. Using the flashlight or other means, carefully determine and adjust the piston location to prevent contact with the tap. Apply generous amounts of grease to the tapping tool to help catch the aluminum shavings, and slowly begin cutting new threads in the aluminum head, taking care that the tool is following the angle of the original hole. Some tapping tools have a dual-stage thread, and will actually pull themselves through the damaged hole. Remove the tool every rotation or so, to remove the contaminated grease and shavings. The more often you clean and re-grease the tool, the less likely you'll drop aluminum shavings into the cylinder.


Once the hole is fully tapped, clean the area using cotton swabs wet with the brake cleaner. You may be able to rotate the crank and lift the piston to its highest position. Then, using the flashlight, look for shavings or grease on the top of the piston. Small aluminum shavings will probably burn up and not do any internal damage, but clean up as best you can just to be safe. Compare the hole depth to your insert widths and select the widest insert that will still fit completely within the aluminum threads. Slide the insert onto the installation tool and thread it into the new spark plug hole. Thread the insert until flush with (or just below) the hole opening so that the new spark plug will properly seal against the head.


Now for the most critical step: Once the insert is in place, reach through the insert with the needle-nose pliers and carefully remove the short "tang" on the bottom of the insert. Generally, no twisting is required; just work the pliers in and out a few times and the tang should snap cleanly off. Be extremely careful that the steel tang doesn't fall into the engine cylinder, as it MUST be removed before the engine can be started. Sometimes the inserts are slightly magnetic (check ahead of time) and you may be able to remove a wayward tang with a magnetic retrieving tool as a last resort. Once the tang is removed, apply some anti-seize to your new spark plug threads, carefully install and tighten—and you're done!