Spark Plug Replacement
Not many years ago, most vehicles needed a “tune up” every 12,000 miles. This included spark plugs, points & condenser, air & fuel filters, and much more. With the introduction of electronic ignition, that interval was stretched to 30,000 miles, and points and condenser were not needed. With platinum plugs, many cars would make it to 60,000 miles without a tune up. But something else started happening – the spark plugs were seizing in the cylinder heads. When removing spark plugs after 60,000 miles, the threads in the aluminum cylinder head would just come out with the spark plugs. This necessitated either a new cylinder head or extensive machine work to install new threads into the spark plug hole. Big expense!
Now many manufacturers are stating that their engines never need spark plug replacement; or some say after 100,000 miles they might be needed. We have seen many Ford 5.4 liter and 4.6 liter engines that have blown the spark plugs with their threads right out of the engine. Some of the cylinder heads are designed so that only 3-4 threads are all that the spark plug has to thread into. Really, really expensive!!
Another concern with extended spark plug replacement intervals is the stress placed on other, much more expensive ignition components. If the spark plug gap is worn, it makes the ignition coil work much harder to fire across the increased gap. Since electricity always tries to find the easiest path to ground, the high resistance in the spark plugs could make the spark burn through spark plug wires, back through ignition coils, and even into ignition modules and engine control computers.
But these problems are, for the most part, easy to avoid. When S&S recommends a minor ignition tune up at 60,000 miles, all of these things are considered. Even on an engine with platinum tipped spark plugs, the average price per mile for spark plug replacement comes out to approximately $.0036, or $3.60 per 1000 miles driven. This is pretty cheap maintenance.