SPARK PLUG "TWIST OFF"
Federal-Mogul Document #1517
Let's say you are working on a simple tune-up and during spark plug removal, one breaks off below
the hex, leaving the threaded "shell" stuck fast in the head (Figure 1). That can happen with any
spark plug but most frequently with the smaller 5/8" hex size. It may never happen to you, but if it
does, don't panic. You're probably asking what caused it and how do you get the broken piece
removed without pulling the head.
Spark Plug Shell
stuck in the head
Spark Plug Shell
shears off here
Figure 1
The construction of a spark plug shell is like a "hollow bolt". If you exceed the recommended
torque, the shell can shear off below the hex. If excessive force is required to remove the plug, it
could be traced to one or more of the following causes:
1. Someone installed the plug with excessive force.
2. Someone cross-threaded the plug during installation.
3. Someone installed the plug in dirty threads.
4. The plug was left installed for too long a time.
Unless you installed them, the twist-off is not your fault. However, you're the one stuck with the job
of removing it and you don't want to take all day.
Assuming adequate clearance, in most cases, plug removal can be accomplished with an "easy-out"
and a little patience.
You must have the proper tools. We recommend a "square cut" type of easy-out rather than
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the "spiral cut" design (Figure 2). A spiral cut design has a greater potential of causing the shell to
expand, locking even tighter to the cylinder head as removal pressure is applied.
Spiral Cut Easy-Out
(Not Recommended)
Recommended Square
Cut Easy-Out
Sharp Gripping
Flutes on Easy-Out
End
View
Figure 2
On the other hand, the square cut type with its sharp flute will dig into the shell metal to provide a
strong gripping surface without expanding shell metal.
To remove the plug, move the piston to Bottom Dead Center, and make sure the engine is cold,
even if you have to wait for cool-down. Then soak the broken plug shell with a generous amount of
penetrating oil. Give it a few minutes to work, then tap an appropriately sized easy out firmly into the
empty shell (figure 3). Using an appropriate size socket and ratchet, brace yourself, and while
holding the socket squarely on the easy out, apply a firm, steady counterclockwise pressure.
Square Cut Easy-Out
tapped into plug
shell in head
Figure 3
Although the required installation torque on taper seat spark plugs is only 7 to 15 foot pound, we
have heard reports of successfully removed spark plugs, without head damage, at torque values
exceeding 180 foot pounds!
If, because of cross-threading or thread fatigue, the cylinder head is damaged during removal, be
sure to install a quality thread repair kit. A good name brand such as Heli-Coil when properly
installed will form a good as original thread. Carefully follow their installation instructions.
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