Crown Victoria Project Update - Building a Panther, Part 2

With my tuned-up engine and fresh trans, I really began driving the car hard on the Malibu canyon roads. One Sunday morning in March 2009, I tossed a spark plug driving back from one of those outings. This picture shows some of the carnage. The spark plug had been backing out over time. Once it let go, the plug bounced like a pogo stick in its hole, destroying the boot and the threads in the cylinder head. I had to phone a friend for a ride home.

After some research, I discovered that this is a common problem for the modular engine family. Look closely at the threads in this picture. I brushed antiseize onto them prior to screwing the plug into the cylinder head. Notice how only the bottom five threads have been exposed? The rest of the threads are untouched. There are only five threads holding the spark plugs into the heads, and I’ve seen early, non-power-improved heads with only three spark plug threads. Supposedly, the 2004 and later versions of the 4.6 and 5.4 SOHC engines have seven threads. There is some debate online about whether you should use antiseize on new plugs when installing them. I do and will continue to do so because of modern spark plugs’ long change intervals. I’d rather have to retorque my spark plugs periodically than have them freeze up in the cylinder head. The spec in my factory service manual is 13 ft-lb.

Most shops will tell you to replace the cylinder head after a spark plug blowout, and you can imagine the outrageous price quotes I got from some of them. Ford has a service bulletin on this problem: the company’s recommended solution is the Lock-N-Stitch thread insert. Lock-N-Stitch loaned me the repair kit, which was very cool of them, because it is expensive to buy and way more than I could afford.

We won’t document the entire process in this article for a couple of reasons: Most of our readers may not care about 4.6 Ford spark plug blowouts (though there is good information here about thread repair in general) and the Lock-N-Stitch kit comes with very thorough and easy to follow directions. I doubt we could do a better job than they did.

For those who are interested, we’ll hit the high points of the repair. Basically, you drill out the damaged threads, tap new threads for the insert, and install the insert and roll pin that holds it in place. I was able to do the entire job with the engine still in the car over a couple of days. This tool cuts a seat for the thread insert to rest on. The red stuff is TransJel transmission assembly lube I spread on to catch some of the aluminum shavings.

The next tool drills out the stock spark plug threads. This kit is designed specifically for 4.6 and 5.4 SOHC engines—the dark collar around the cutter exactly fits the spark plug hole and prevents the cutter from wobbling or drilling off center.

Put more grease or assembly lube on this tool as well. There will be an alarming amount of metal shavings in this process. Stop often to clean the cutter and add fresh grease.

Next, I tapped the head to accept the thread insert. Again, I used TransJel to catch some of the aluminum chips.

It is impossible to keep all the aluminum shavings out of the engine, but I fished nearly all the debris by winding a pipe cleaner around a bent-up piece of welding rod. Adding some TransJel to the tip, I swabbed the top of the piston and was able to clean out the junk that fell into the cylinder.

The last few steps are to thread in the insert and drill one more hole for the roll pin that retains the insert in the head. The thread insert itself is hard-anodized aluminum rather than stainless steel. That’s why it’s the Ford-recommended repair. The aluminum insert has the same expansion and heat transfer properties as the cylinder head. The reasoning is that a stainless insert repair can cause overheating in that cylinder because it transfers less heat from the plug to the coolant than a repair with an aluminum insert does.

The repair was successful and here is that same cylinder two years and at least 10,000 miles later. The heads were off the engine recently for a performance upgrade (rather than a failure), and I was happy to see that there was no damage to the piston or cylinder walls either. You’ll read about the cylinder head swap in a later update.