[James]

It look like sometime in the truck past a head was change out.You might want to make sure the threads in the heads is the same length as the spark plug reach. If the long reach plug is place in a hole design for a head with a short reach the expose threads could cause a couple of problems. It could have a build up of carbon making it difficult to remove the next time and/or could create a hot spot in the combustion chamber and pre-ignite the fuel charge (cause spark knock and loss of power).

My truck heads required the long reach but only the tip half of the plug is threaded into the head. They use to make plugs this way but decided to do away with them to cut their inventory (see picture). I also have two spare set of heads. One is made like the one in my truck the other is machine for a short reach plug.