Rick's Free Auto Repair Advice

Spark plug thread repair kit

How to repair stripped spark plug threads with a spark plug thread repair kit

Some engines are prone to seized spark plugs that damage the threads when removed, so you have to perform a spark plug thread repair technique. Lots of people will tell you to install a Helicoil. Nope. A standard Helicoil CANNOT be used for spark plug thread repair. A standard coil type insert will not hold the pressures generated during engine operation. So must use a solid threaded bushing-style INSERT instead, a spark plug thread repair kit designed just for spark plugs.

Here are a few ways to do spark plug thread repair:

1) If the threads are still there but you can’t get the spark plug started,

spark plug thread chaser

Spark plug thread chaser

buy a spark plug thread chaser. Apply a light coating of thread cutting oil and slowly screw the chaser into the old threads. The chaser might reform the stripped threads, allowing you to reuse them. If the spark plug now screws in but won’t tighten properly, you’ll have to move on to this next spark plug thread repair technique.


2) Purchase a spark plug thread repair kit to fit the spark plug threads for your vehicle. The kit contains and a stepped over sized tap and turning handle, a seat cutter, insert installation tool and a solid insert.

Spark plug thread repair kit

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 gasses

spark plug thread repair kit, spark plug thread repair

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

from seeping through the coils.

The bushing inserts are available for washer style spark plugs and tapered seat style plugs.

Spark plug thread repair, spark plug thread repair kit

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








spark plug thread repair kit, spark plug thread repair

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. Then 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.
  2. Apply grease to the tap to capture and hold the metal chips generated by the thread tapping.
  3. Cut new threads into the cylinder head. Leave the tap in place.
  4. Slide the seat cutting tool over the tap and rotate to create a seat for the bushing insert. The seat prevents the bushing from inserting too deep into the head. Remove the seat cutter
  5. Lubricate the insertion tool and install the bushing insert
  6. Apply sealant to the bushing and the threaded bushing into the newly tapped hole and 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 it creates resistance. 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. If the spark plugs have a tapered seat, you must apply the provided sealant to the bushing before inserting it.
  7. 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.
  8. Install the spark plug

©, 2015 Rick Muscoplat

Posted on by Rick Muscoplat

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