Tom Wilson
April 1, 2003
Few people know the FRPP heads have fully threaded spark-plug holes. As you can see, trying to seal the stock half-threaded plug using its shoulder instead of tapered seat doesn't work well. Our first clue was when the spark-plug boots kept coming loose. We couldn't figure it out until we realized combustion pressure was doing the dirty deed.

While working with our 4.6 GT open-track project car, we came across one of those "gotcha" situations begging for a bit of explanation. Admittedly, it's a simple matter of using the correct spark plug, but when knowing what the correct spark plug is seems to be some sort of secret, things become a little more interesting.

The story begins with 4.6 modular V-8 cylinder heads. Both the Two-Valve GT and Four-Valve Cobra heads use the same half-threaded, shouldered, tapered-seat spark plugs. That's basi-cally the end of the story unless, like us, you've upgraded your GT with Ford Racing Performance Parts M-6049-D46 High-Performance Cylinder Heads. These Two-Valve heads are shipped from the FRPP warehouse complete, including spark plugs. When it comes time to change those spark plugs, you may not notice they're different from the stock variety. Ah, but they are.

Anticipating higher combustion pressures with these sporty heads, FRPP engineers specified a full-threaded-reach spark plug. The one they ended up using is nothing dramatic-it's a platinum-tipped unit that can be found in several stock Ford applications, namely 3.0 V-6 Ranger engines. However, if you don't notice the longer thread reach on the plugs in your FRPP heads, then chances are excellent you'll install a set of stock 4.6 spark plugs in your FRPP castings at replacement time.

What happens is the stock plug will thread in until its shoulder hits the cylinder head. Thinking you've hit the plug's seat, you give it a cinch down and move on to the next plug. As the modular spark plugs fit way, way down in wells in the cylinder heads, it's impossible to see anything is amiss. But in reality, the spark plugs are only halfway down the FRPP head's spark-plug hole, meaning the electrodes are masked way up in the spark-plug hole. What this does for flame-front propagation and compression ratio is-well-not good.

Having inherited our project car sans paperwork of any sort, we had no idea its FRPP heads didn't use stock plugs. Later we broke a plug-the center porcelain came right out of the steel shell-and we found all the rest finger-tight. Thinking that odd, we removed the stock-type 4.6 plugs someone before us had installed and blithely installed a new set of stock 4.6 GT plugs. And we wondered why they all came loose again.

Knowing something was up, we called FRPP and the situation was explained to us. As the FRPP heads were never installed on the car, the parts system has never heard of these heads or their spark plugs. But now you have. The correct plug is Motorcraft AGSF32C. Give 'em a dab of antiseize and thread 'em in.