The mod motor's fatal flaw lay in its spark plug threads.
Ford's 6.8L (413 cubic inch, found in E-Series and F-Series trucks) V10 is part of the company's "modular" engine family (1991 to present), and represents the largest evolution of the design. One way to think of the V10 is as a 4.0L V6 with four extra cylinders or as a 5.4L V8 with two more cylinders.
Basic Design
Ford's truck V-10 is an iron-block, overhead-cam aluminum head design that uses centrally mounted spark plugs and direct fire ignition. Almost all valvetrain and accessory components are shared with the 5.4L, making the design fairly stout and easy to repair. However many 1997-2008 engines have a fatal flaw in the cylinder head.
Design Flaw
The aluminum heads on two-valve 4.6L and 5.4L V8s and 6.8L V10s (built in the company's Windsor, Ontario plant, not to be confused with 2005 and up 3-valve heads) have a fatal flaw in the spark plug threads. Being a soft metal, aluminum doesn't have much inherent strength in the first place; it also expands at a different rate than the steel spark plugs threaded into it. Ford put the V10's spark plugs at the bottom of a five inch deep well, and only left enough room to allow for about four threads worth of spark plug engagement in the cylinder head.
Failure
After repeated heat cycling, the spark plug essentially welds itself to the threads, weakening the material and changing the load-bearing center. The end result of this design flaw is that the rear-most spark plugs on the engine's heads tend either randomly shoot out of the block while driving, or get stuck while in the process of removal. When hard steel sticks to soft aluminum, the aluminum will inevitably give way first. The end result is that the head's spark plug threads strip clean out, leaving a smooth hole and no way to re-install the plug.
Factory Repairs
Ford covers this engine problem under warranty (Technical Service Bulletin TSB 07-21-2). However, if your engine fails after the warranty expires, you are responsible for the repairs. Ford issued a repair kit called the Lock-and-Stitch that allows the owner to install an aluminum insert where the spark plug threads should be. The kit comes complete with tools, materials and special installation procedures.
Advice
Ford's kit will do the job, but installing aluminum thread inserts in the place of already-failed aluminum threads is a lesson in repeating history. Aftermarket steel-insert kits cost about the same, and are likely to outlive Ford's aluminum band-aids.
Tags: spark plug, plug threads, spark plug threads, spark plugs, fatal flaw, cylinder head
