Spark Plug Purgatory

Written by Don Stewart | 31 July 2008

Thanks go out to Ed Milano for providing initial information on this topic. At the time, neither of us knew that there was a big can of worms behind the scenes. But big it was and research about it led to the Pony Tricks article that follows.

Something as simple as spark plug removal, spark plug installation, and confidence that spark plugs will stay put once they’re installed should be—well—simple. At least, I think it should be. Sometimes it’s hard to reach all of the plugs but that issue aside, the rest of the procedure should be simple. You know, put ‘em in, torque ‘em to spec, and forget about ‘em until it’s time to do it again. But our favorite automobile manufacturer found a way to confound the process. Actually, two ways.

Sometimes component design harbors unintended consequences that can cause major headaches. As you’ll see, Ford aced this one when they created a bona fide spark plug purgatory for owners of many of their vehicles, including Mustangs. In one case, thin-cast aluminum cylinder heads were the culprit and in the other case, spark plug design collaborated with head design to cause trouble.

First we’ll cover the problem caused by thin cast heads. In that case, the 2V modular 4.6L, 5.4L, and 6.8L engines were at risk for “spitting” spark plugs out of the heads, including those in 1997 through 2004 Mustangs with the 4.6L engine. There were only five threads holding the plug in the head, making it relatively easy to strip the threads. And if the threads stripped, the plug would make an untimely exit. This unintended consequence could be caused by any number of conditions including an over-torqued plug, an under-torqued plug, a power adder that caused higher than normal combustion chamber pressure, or even removing a plug from a hot engine. To get a feel for the appearance of stripped threads, check out the picture showing the condition as viewed from inside the combustion chamber.

If a plug was ejected while the engine was running, it could take off like a small rocket, tearing-up anything that got in its way. Sometimes it would even punch right through the hood. And as you can imagine, vehicle owners weren’t amused when the failures led to expensive repairs. Ford’s recommended repair was to replace the head(s) with the same thin cast design. During the warranty period, that method was accepted by owners because Ford ate the cost but failures that occurred after the warranty expired had to be paid out-of-pocket.

To lower out-of-pocket costs, it wasn’t unusual for post-warranty repair techniques to avoid replacing the heads. Probably the most popular technique was to put a helicoil insert in the stripped hole. But helicoils are made of stainless steel and steel expands much less than aluminum, meaning that the inserts were loose when the engine was at operating temperatures. A loose helicoil could move enough to strip its threads leading to both the helicoil and the plug being ejected from the head. And if a power adder was bolted to the engine, there was a higher risk of failure. Finally, helicoils were prone to unscrew themselves from the head the next time the plugs were removed. But fail or not, unscrew or not, the difference in expansion meant steel inserts allowed combustion pressure to leak causing things like loss of power, poor gas mileage, and the potential for fire. In addition, plugs mounted in helicoils ran hot because first of all (to state it again) the inserts were loose, resulting in poor heat transfer to the head. In addition, steel is a poor conductor of heat compared to aluminum.

So it’s apparent that steel inserts weren’t a good way to fix aluminum heads. But how about aluminum inserts? Are aluminum inserts even available and, if so, who makes them? As it turns out, aluminum inserts are available and one in particular has been approved by Ford. It’s called LOCK-N-STITCH and it’s made by Full Torque. Ford TSB 07-21-2 dated 10/5/2007 covers the use of LOCK-N-STITCH inserts.

Online at locknstitch.com/sparkpluginserts.htm, you can read about LOCK-N-STITCH but a short description would include such points as more threads for the spark plug, heat transfer equivalent to an undamaged head, and a mechanical lock by way of a pin that holds the insert in place after it’s screwed into the head. Because there are more threads for the plug, torque can be increased by about a factor of two thereby minimizing the potential for a loose plug and maximizing heat transfer. And the mechanical locking pin eliminates the tendency for inserts to come unscrewed when the plugs are removed. All of these things make you wonder if it wouldn’t be wise to just bite the bullet and treat all eight holes to inserts as a prophylactic measure, i.e., fix-it now so it won’t break later when you’re 500 miles from home. The picture with this article shows a cross-sectional view of a head repaired with the insert.

While LOCK-N-STITCH is a much cheaper repair than installing new heads—heads that might also fail—it’s still a significant investment at something over $600 just for the kit. But on the plus side, the engine doesn’t have to be disassembled and the repair can be made with the engine in the car.

Now let’s look at the second situation. It’s a condition where spark plug design collaborated with head design to create the potential for another unintended consequence. The 3V versions of the same modular engines (4.6L, 5.4L, 6.8L) were involved this time and the problem was centered on a high likelihood that spark plugs would break while being removed after several thousand miles of use. Included in the group of affected vehicles were the 2005 through 2008 Mustangs equipped with 4.6L engines having engine-build-dates before 11/30/2007.

If you haven’t seen the plugs that caused the problem, look at the picture. Autolite calls them Revolution HT (high thread). Note that the ground electrode we’re all familiar with has been replaced by a so called extended ground electrode shield extending downward from the sealing ramp.

The plug doesn’t look like it would be hard to remove but after carbon has built up on the shield, there isn’t enough clearance for it to pass through the hole in the head. If forced, the shield separates from the threaded part of the plug and stays in the hole. It separates because it’s a press fit in the plug body rather than being an integral part of it. And just to make things really interesting, the porcelain insulator frequently breaks flush with the top of the shield or slightly higher, leaving part of itself inside the shield.

Broken plugs became such a pervasive problem that on April 1, 2008 Ford finally issued TSB 08-7-6 to describe the correct removal procedure. The procedure first cautioned against removing plugs unless the engine was at room temperature. Then it specifically stated that no power tools were to be used, only hand tools. But it gets better. The TSB said to start the removal process by loosening the plugs no more than 1/8 to 1/4 turn. The next step was to put 1/2 to 3/4 teaspoon of Motorcraft Carburetor Tune-Up Cleaner around the body of the plug and leave it there for at least 15 minutes.

The cleaner was supposed to work its way down the threads to the shield and then dissolve the carbon. That was the intent but sometimes it didn’t work. If the plug broke anyway, the TSB explained how to extract various modes of failure using special Rotunda Tool 303-1203 (cost about $240). Finally, when plugs were put back in the engine, it was recommended to coat the shield with a light film of nickel anti-seize compound.

Referring back to 4.6L 3V engines built on 11/30/07 and later, Ford enlarged the spark plug bore diameter slightly, making it much less likely that plugs would stick and break while being removed. A similar revision took effect on 10/9/2007 for the 5.4L and 6.8L 3V engines.

Consternation is defined as a sudden, alarming amazement or dread that results in utter confusion or dismay. I think that adequately describes how I’d feel if I was surprised by either one of these spark plug related issues. But the blue oval proved that even simple things might have unintended consequences that could be complicated and discouraging.

I suspect that Ford wished that both issues could have been discovered before production was authorized but since they weren’t, it’s fortunate that there are procedures and tools to save the day. The tools are rather expensive but much less so than disassembling the engine and possibly replacing the heads.