See Figures 1 through 12
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 90º angle so that its tip is just past and parallel to the tip of the center electrode. The distance between these two electrodes (measured in thousandths of an inch or hundredths of a millimeter) is called the spark plug gap.
The spark plug does not produce a spark but instead provides a gap across which the current can arc. The coil produces anywhere from 20,000 to 50,000 volts (depending on the type and application) which travels through the wires to the spark plugs. The current passes along the center electrode and jumps the gap to the side electrode, and in doing so, ignites the air/fuel mixture in the combustion chamber.
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 closer the electrode is to the block's cooling passages) 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 might possibly lead to preignition or other ignition problems. Preignition takes place when plug tips get so hot that they glow sufficiently to ignite the air/fuel mixture before the actual spark occurs. This early ignition will usually cause a pinging 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 generally a good compromise between the 2 styles and most people never have the need to change their plugs from the factory-recommended heat range.
REMOVAL & INSTALLATION
Except 390 and Larger V8s with Thermactor Controls and Air Conditioning
See Figures 13, 14 and 15
- If the spark plug wires are not numbered as to their cylinder, place a piece of masking tape on each wire and number it.
- Grasp each wire by its rubber boot on the end and remove it from the spark plug. If the wire sticks to the plug, a slight clockwise turn should help to loosen it. Do not attempt to remove the wires from the spark plugs by pulling on the wires themselves, as this will damage the wires.
- Using a 13 /16 inch spark plug socket, loosen each plug by making several counterclockwise turns. If your car is equipped with a 351 Cleveland engine, Boss 302, Boss 429, or 429 SCJ, a 5 /8 inch spark plug socket should be substituted for the 13 /16 inch size.
- Using compressed air (if available), blow the area around each spark plug clean. If no compressed air is available, wipe these areas clean with a rag. Make sure that no foreign matter enters the cylinders through the spark plug holes.
- Remove the spark plugs from the engine.
- Insert the plugs into their holes, and hand-tighten them, taking care not to cross-thread the plugs.
- Using the spark plug socket on a torque wrench, torque the plugs to the proper specifications. (See the table in the general tune-up information.)
- Install the spark plug wires, each on its respective plug. Make sure that the wires are firmly installed on the plugs. Check the firing order illustrations for the proper positioning of the wires.
390 and Larger V8s with Thermactor Controls and Air Conditioning
It is advisable to have the following group of tools in order to remove the difficult-to-reach spark plugs of the above-mentioned engines:
- Swivel ratchet handle
- Flexible spark plug wrench of the proper size for the spark plugs in your engine.
- Remove the carburetor air cleaner assembly and the braces which connect the spring towers to the cowl.
- Disconnect the ignition wires from the distributor cap.
- Remove the rocker arm covers, carburetor choke tube, and vacuum hoses.
- Disconnect the ignition wires from the spark plugs.
- If compressed air is available, blow the dirt from around the spark plugs. If no compressed air is available, wipe the areas around the spark plugs with a rag.
- Using the recommended tools, remove the spark plugs from the engine, making sure that dirt does not enter the spark plug holes.
- Insert and tighten the spark plugs, using the same combination of tools as used during removal.
- Install the ignition wires to the spark plugs. Make sure that they are installed firmly and in their proper positions. Refer if necessary to the firing order illustrations.
- Thoroughly clean the gasket surfaces on the rocker arm covers and the cylinder heads.
- Position new rocker arm cover gaskets, install the rocker arm covers, tighten the bolts, and torque them. Torque for all engines, except Boss 429 and 429 SCJ, is 3-5 ft. lbs. Torque for the 429 SCJ is 2 1 /2-4 ft. lbs. For the Boss 429, proper torque is 12-15 ft. lbs.
Connect and/or install:
- Choke heat tube
- Vacuum hoses and emission system hoses
- Spark plug wires to the distributor cap, referring to the firing order diagrams to make sure that the wires are properly positioned.
- Spring tower-to-cowl braces
- Carburetor air cleaner and vacuum hose-to-heat stove fitting
INSPECTION & GAPPING
Check the plugs for deposits and wear. If they are not going to be replaced, clean the plugs thoroughly. Remember that any kind of deposit will decrease the efficiency of the plug. Plugs can be cleaned on a spark plug cleaning machine, which can sometimes be found in service stations, or you can do an acceptable job of cleaning with a stiff brush. If the plugs are cleaned, the electrodes must be filed flat. Use an ignition points file, not an emery board or the like, which will leave deposits. The electrodes must be filed perfectly flat with sharp edges; rounded edges reduce the spark plug voltage by as much as 50%.
Check spark plug gap before installation. The ground electrode (the L-shaped one connected to the body of the plug) must be parallel to the center electrode and the specified size wire gauge (please refer to the Tune-Up Specifications chart for details) must pass between the electrodes with a slight drag.
NEVER adjust the gap on a used platinum type spark plug.
Always check the gap on new plugs as they are not always set correctly at the factory. Do not use a flat feeler gauge when measuring the gap on a used plug, because the reading may be inaccurate. A round-wire type gapping tool is the best way to check the gap. The correct 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. Wire gapping tools usually have a bending tool attached. Use that to adjust the side electrode until the proper distance is obtained. Absolutely never attempt to bend the center electrode. Also, be careful not to bend the side electrode too far or too often as it may weaken and break off within the engine, requiring removal of the cylinder head to retrieve it.