How to fix a stripped spark plug hole

Learn how to fix a stripped spark plug hole with a spark plug insert repair kit

If not addressed promptly, stripped spark plug threads can lead to poor engine performance, misfires, and even engine damage. This guide will walk you through how to fix a stripped spark plug hole, detailing the required tools, the step-by-step procedure, and essential tips for a successful repair.

Fixing stripped spark plug threads requires the installation of a solid double-threaded spark plug insert. Some people use a Helicoil. You can use the Heilcoil brand insert, but a standard Helicoil wire insert is not a proper repair for a stripped spark plug hole. A standard coil-type insert won’t hold the pressures generated during engine operation.

Start with a spark plug thread chaser

If you can’t start the spark plug threading into the hole,

Spark plug thread chaser

you may just have deformed starter threads. You can often reform those threads using a spark plug chaser, a tap designed especially for spark plug threads.

Purchase a spark plug thread chaser to match the size of your spark plugs. Apply engine oil to the chaser’s threads. Slowly turn the chaser into the spark plug threads, making sure to hold it perfectly in line with the hole. Once it starts, run the chaser down the threads.

If the threads are too damaged and can’t be reformed with a thread chaser, you’ll have to install a spark plug insert.

Tools and materials needed to fix a stripped spark plug hole

• Spark plug thread repair kit— The kit comes with the proper sized drill bit, a tap, insert, and insert installer tool
• Drill— To enlarge the hole, drill the spark plug hole larger to accept the insert (not needed for all applications).
• Socket and Ratchet— to turn the tap and insert installer
• Torque wrench— To install the insert and spark plug
• Compressed air— To clean the spark plug hole and remove metal chips
• Grease— To coat the tap to retain cutting chips

Step-by-step process to install a spark plug insert

The spark plug thread kit contains a stepped oversized tap and turning handle, a seat cutter, an insert installation tool, and a solid insert.

Time-Sert spark plug thread repair kit with tapping and installation tools

The stepped tap is critical because it centers the re-tapping tool in the spark plug holes. The solid bushing-style insert prevents combustion gases from seeping through the coils. The bushing inserts are available for washer-style spark plugs and tapered seat-style plugs.

Cut away view of a solid bushing style spark plug insert by Time-Sert

Time-Sert bushing style insert for spark plugs with a washer seal.

Time-Sert tapered seat style sold bushing spark plug insert

Refer to the instructions in the kit, but most follow this general procedure:
1) Rotate the engine by hand until the piston is close to top dead center.

2) Soak cotton rope in engine oil and feed it into the cylinder through the spark plug hole. The oil-soaked rope will catch any metal chips made during the re-taping process.
3) Apply grease to the tap to capture and hold the metal chips generated by the thread tapping.
4) Cut new threads into the cylinder head. Leave the tap in place.
5) Slide the seat-cutting tool over the tap and rotate it to create a seat for the bushing insert. The seat prevents the bushing from inserting too deep into the head.
6) Remove the seat cutter
7) Lubricate the insertion tool and install the bushing insert
8) Apply sealant to the bushing and install the threaded bushing into the newly tapped hole. Continue turning the installation tool until it meets resistance. The bottom edge of the insert is cold rolled. As the installation tool hits the rolled edge, resistance is created. Continue turning to force the rolled edge outward. This will bend the rolled edge against the newly cut threads and prevent the insert from backing out of the hole. NOTE: If the spark plugs have a tapered seat, you must apply the provided sealant to the bushing before inserting it.
9) Remove the oil-soaked rope from the cylinder and use a shop vac and small tubing to suck any remaining chips from the hole and cylinder.
10) Install the spark plug

©, 2015 Rick Muscoplat