How to Rethread a Spark Plug Hole

by Gus Stephens
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A number of events may conspire to damage the spark plug threads in a car's cylinder head. Simply leaving a plug in place over too many years and miles without changing it may bond the plug to the cylinder head threads in a way that it cannot be removed without damaging the thread. Conversely, plugs may gradually loosen over time and eventually be blown out of the head by cylinder compression, mauling the threads. Finally, there's that sickening feeling when you're changing plugs and inadvertently cross-thread a plug while installing it. Fortunately, a couple of fixes are available to the experienced home mechanic.

Step 1

Run a commercially-available spark plug "thread chaser" tap of the correct size into the existing damaged spark plug threads and try to clean them up. If the damage isn't too extensive, maybe you'll get lucky and additional options will not be required.

Step 2

Purchase a threaded insert kit and install the insert into the spark plug hole if the threads are too damaged for repair with a thread chaser. A typical commercially-available threaded insert kit includes an oversized threaded tap, a threaded insert, and an installation mandrel.

Step 3

Coat the supplied oversized threaded tap with bearing grease to catch the shavings when you thread it into the cylinder head.

Step 4

Thread the tap into what remains of the existing spark plug threads, cutting new, oversize threads as you turn the tap increasingly tighter. Back the tap out after you have threaded to a depth equivalent to the threads on your spark plug.

Step 5

Install the threaded insert onto the installation mandrel. Use the mandrel to screw the threaded insert into the new oversize threads.

Step 6

Install the spark plug into the threaded insert and tighten.

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