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. 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.
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:
Someone installed the plug with excessive force
Someone cross-threaded the plug during installation
Someone installed the plug in dirty threads
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 do not recommend a "spiral-cut" (Figure 1) type of easy-out; rather we recommend a "square-cut" type design. 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.
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 . Using an appropriate size socket bar, brace yourself, and while holding the socket squarely on the easy out, apply a firm, steady counterclockwise pressure.
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!
