Ford Pick-Ups/Bronco 1976-1986 Repair Guide

Spark Plugs


See Figure 1

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Fig. Fig. 1: Cross section of a spark plug

A typical spark plug consists of a metal shell surrounding a ceramic insulator. A metal electrode extends downward through the center of the insulator and protrudes a small distance. Located at the end of the plug and attached to the side of the outer metal shell is the side electrode. The side electrode bends in at a 90 degree angle so that its tip is even with, and parallel to, the tip of the center electrode. The distance between these two electrodes (measured in thousandths of an inch) is called the spark plug gap. The spark plug does not actually produce a spark but merely provides a gap across which the current can arc. The coil produces anywhere from 20,000 to 40,000 volts which travels to the distributor where it is distributed through the spark plug wires to the spark plugs. The current passes along the center electrode and jumps the gap to the side electrode, and, in so doing, ignites the air/fuel mixture in the combustion chamber.


See Figure 2

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Fig. Fig. 2: Spark plug heat range

Spark plug heat range is the ability of the plug to dissipate heat. The longer the insulator (or the farther it extends into the engine), the hotter the plug will operate; the shorter the insulator the cooler it will operate. A plug that absorbs little heat and remains too cool will quickly accumulate deposits of oil and carbon since it is not hot enough to burn them off. This leads to plug fouling and consequently to misfiring. A plug that absorbs too much heat will have no deposits, but, due to the excessive heat, the electrodes will burn away quickly and in some instances, preignition may result. Preignition takes place when plug tips get so hot that they glow sufficiently to ignite the fuel/air mixture before the actual spark occurs. This early ignition will usually cause a pinging sound during low speeds and heavy loads.

The general rule of thumb for choosing the correct heat range when picking a spark plug is: if most of your driving is long distance, high speed travel, use a colder plug; if most of your driving is stop and go, use a hotter plug. Original equipment plugs are compromise plugs, but most people never have occasion to change their plugs from the factory-recommended heat range.


See Figures 3, 4, 5 and 6

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Fig. Fig. 3: Twist and pull on the rubber boot to remove the spark plug wires; never pull on the wire itself.

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Fig. Fig. 4: Removing a spark plug; 8-302 shown

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Fig. Fig. 5: Always use a wire gauge to check the electrode gap; a flat feeler gauge may not give the proper reading.

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Fig. Fig. 6: Adjust the electrode gap by bending the electrode

A set of spark plugs usually requires replacement after about 20,000 to 30,000 miles, depending on your style of driving. In normal operation, plug gap increases about 0.001 inch; for every 1,000-2,500 miles. As the gap increases, the plug's voltage requirement also increases. It requires a greater voltage to jump the wider gap and about two to three times as much voltage to fire a plug at high speeds than at idle.

When you're removing spark plugs, you should work on one at a time. Don't start by removing the plug wires all at once because, unless you number them, they may become mixed up. Take a minute before you begin and number the wires with tape. The best location for numbering is near where the wires come out of the cap.

Removal & Installation

On models equipped with electronic ignition, apply a small amount of silicone dielectric compound (D7AZ-19A331-A or the equivalent) to the inside of the terminal boots whenever an ignition wire is disconnected from the plug, or coil/distributor cap connection.

  1. Twist the spark plug boot and remove the boot and wire from the plug. Do not pull on the wire itself as this will damage the wire.
  3. If possible, use a brush or gag to clean the area around the spark plug. Make sure that all the dirt is removed so that none will enter the cylinder after the plug is removed.
  5. Remove the spark plug using the proper size socket. Truck models use either a 5 / 8 inch; or 13 / 16 inch; size socket depending on the engine. Turn the socket counterclockwise to remove the plug. Be sure to hold the socket straight on the plug to avoid breaking the plug, or rounding off the hex on the plug.
  7. Once the plug is out, check the plug to determine engine condition. This is crucial since plug readings are vital signs of engine condition.
  9. Use a round wire feeler gauge to check the plug gap. The correct size gauge should pass through the electrode gap with a slight drag. If you're in doubt, try one size smaller and one larger. The smaller gauge should go through easily while the larger one shouldn't go through at all. If the gap is incorrect, use the electrode bending tool on the end of the gauge to adjust the gap. When adjusting the gap, always bend the side electrode. The center electrode is non-adjustable.
  11. Squirt a drop of penetrating oil on the threads of the new plug and install it. Don't oil the threads too heavily. Turn the plug in clockwise by hand until it is snug.
  13. When the plug is finger tight, tighten it with a wrench. If you don't have a torque wrench, tighten the plug until snug (do not over tighten).
  15. Install the plug boot firmly over the plug. Proceed to the next plug.