Spark Plug Thread Chaser
A specialized tap used to clean carbon buildup and repair minor thread damage in cylinder head spark plug holes.
A spark plug thread chaser is a specialized cutting and cleaning tool designed to restore and clean the internal threads of a spark plug hole in the cylinder head. Unlike a standard thread cutting tap, which cuts new threads and removes metal, a thread chaser is designed to reform, straighten, and clean existing threads, removing carbon deposits, rust, and old threadlocker without removing parent metal.
Over time, spark plug threads can become packed with carbon, especially in engines with deep plug wells or if a plug was loose. Trying to install a new spark plug into dirty threads can cause it to cross-thread or bind before it is fully seated, leading to improper heat transfer and combustion leaks. Thread chasers are run down the plug hole to clean the threads, ensuring the new plug can be torqued accurately.
To use a thread chaser, the tool's flutes are coated with heavy grease to catch any dislodged carbon and metal shavings, preventing them from falling into the combustion chamber. The tool is carefully hand-threaded into the hole to ensure it follows the original thread path, then turned slowly with a wrench. If threads are completely stripped, a thread repair insert (like a Helicoil) must be installed instead.