GM Chevy Mid-Size Cars 1964-1988 Repair Guide

Spark Plugs


See Figures 1, 2 and 3

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° 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 20,000-25,000 volts (the HEI transistorized ignition produces considerably more voltage than the standard type, approximately 50,000 volts) 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 fuel/air mixture in the combustion chamber. All plugs used since 1969 have a resistor built into the center electrode to reduce interference to any nearby radio and television receivers. The resistor also cuts down on erosion of plug electrodes caused by excessively long sparking. Resistor spark plug wiring is original equipment on all models.

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Fig. Fig. 1: Spark plug heat range

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Fig. Fig. 2: Cross section of a spark plug

Spark plug life and efficiency depend upon condition of the engine and the temperatures to which the plug is exposed. Combustion chamber temperatures are affected by many factors such as compression ratio of the engine, fuel/air mixtures, exhaust emission equipment, and the type of driving you do. Spark plugs are designed and classified by number according to the heat range at which they will operate most efficiently. The amount of heat that the plug absorbs is determined by the length of the lower insulator. The longer the insulator (it extends farther into the engine), the hotter the plug will operate; the shorter it is, the cooler it will operate. A plug that has a short path for heat transfer 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 has a long path of heat transfer will have no deposits but, due to the excessive heat, the electrodes will burn away quickly and, in some instances, pre-ignition may result. Pre-ignition takes place when plug tips get so hot that they glow sufficiently to ignite the fuel/air mixture before the spark does. This early ignition will usually cause a pinging (sounding much like castanets) during low speeds and heavy loads. In severe cases, the heat may become enough to start the fuel/air mixture burning throughout the combustion chamber rather than just to the front of the plug as in normal operation. At this time, the piston is rising in the cylinder making its compression stroke. The burning mass is compressed and an explosion results producing tremendous pressure. Something has to give, and it does; pistons are often damaged. Obviously, this detonation (explosion) is a destructive condition that can be avoided by installing a spark plug designed and specified for your particular engine.

A set of spark plugs usually requires replacement after 10,000-12,000 miles depending on the type of driving (this interval has been increased to 22,500 miles for all 1975-79 models and 30,000 miles for all 1980 and later models). The electrode on a new spark plug has a sharp edge but, with use, this edge becomes rounded by erosion causing the plug gap to increase. In normal operation, plug gap increases about 0.001 in. (0.0254mm) for every 1,000-2,000 miles. As the gap increases, the plug's voltage requirement also increases. It requires a greater voltage to jump the wider gap and about 2-4 times as much voltage to fire a plug at high speed and acceleration than at idle.

The higher voltage produced by the HEI ignition coil is one of the primary reasons for the prolonged replacement interval for spark plugs in 1975 and later cars. A consistently hotter spark prevents the fouling of plugs for much longer than could normally be expected; this spark is also able to jump across a larger gap more efficiently than a spark from a conventional system. However, even plugs used with the HEI system wear after time in the engine.

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Fig. Fig. 3: Spark plug diagnosis

Worn plugs become obvious during acceleration. Voltage requirement is greatest during acceleration and a plug with an enlarged gap may require more voltage than the coil is able to produce. As a result, the engine misses and sputters until acceleration is reduced. Reducing acceleration reduces the plug's voltage requirement and the engine runs smoother. Slow, city driving is hard on plugs. The long periods of idle experienced in traffic creates an overly rich gas mixture. The engine does not run fast enough to completely burn the gas and, consequently, the plugs become fouled with gas deposits and engine idle becomes rough. In many cases, driving under the right conditions can effectively clean these fouled plugs.

To help clean fouled plugs in a running engine, first accelerate you car to the speed where the engine begins to miss and then slow down to the point where the engine smooths out. Run at this speed for a few minutes and then accelerate again to the point of engine miss. With each repetition this engine miss should occur at increasingly higher speeds and then disappear altogether. Do not attempt to shortcut this procedure by hard acceleration. This approach will compound problems by fusing deposits into a hard permanent glaze. Dirty, fouled plugs may be cleaned by sandblasting. Many shops have a spark plug sandblaster and there are a few inexpensive models that are designed for home use and available from aftermarket sources. After sandblasting, the electrode should be filed to a sharp, square shape and then gapped to specifications. Gapping a plug too close will produce a rough idle while gapping it too wide will increase its voltage requirement and cause missing at high speed and during acceleration.

There are several reasons why a spark plug will foul and you can usually learn what is at fault by just looking at the plug. Refer to the spark plug diagnosis figure in this section for some of the most common reasons for plug fouling.

The type of driving you do may require a change in spark plug heat range. If the majority of your driving is done in the city and rarely at high speeds, plug fouling may necessitate changing to a plug with a heat range one number higher than that specified by the car manufacturer. For example, a 1970 Chevelle with a 350 cu. in. (300 hp) engine requires an R44 plug. Frequent city driving may foul these plugs making engine operation rough. An R45 is the next hottest plug in the AC heat range (the higher the AC number, the hotter the plug) and its insulator is longer than the R44 so that it can absorb and retain more heat than the shorter R44. This hotter R45 burns off deposits even at low city speeds but would be too hot for prolonged turnpike driving. Using this plug at high speed would create dangerous pre-ignition. On the other hand, if the aforementioned Chevelle were used almost exclusively for long distance high speed driving, the specified R44 might be too hot resulting in rapid electrode wear and dangerous pre-ignition. In this case, it might be wise to change to a colder R43. If the car is used for abnormal driving (as in the examples above), or the engine has been modified for higher performance, then a change to a plug with a different heat range may be necessary. For a modified car it is always wise to go to a colder plug as a protection against pre-ignition. It will require more frequent plug cleaning, but destructive detonation during acceleration will be avoided.


See Figures 4, 5, 6, 7 and 8

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, or they're going to get mixed up. On some models though, it will be more convenient for you to remove all the wires before you start to work on the plugs. If this is necessary, take a minute before you begin and number the wires with tape before you take them off. The time you spend doing this will pay off later when it comes time to reconnect the wires to the plugs.

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Fig. Fig. 4: Twist and pull on the rubber boot to disconnect the spark plug wires; never pull on the wire itself

  1. Disconnect the negative battery cable from the negative battery terminal.
  3. Twist the spark plug boot slightly in either direction to break loose the seal, then remove the boot from the plug. You may also use a plug wire removal tool designed especially for this purpose. Do not pull on the wire itself or you may separate the plug connector from the end of the wire. When the wire has been removed, take a wire brush and clean the area around the plug. An evaporative spray cleaner such as those designed for brake applications will also work well. Make sure that all the foreign material is removed so that none will enter the cylinder after the plug has been removed.

If you have access to a compressor, use the air hose to blow all material away from the spark plug bores before loosening the plug. Always protect your eyes with safety glases when using compressed air.

  1. Remove the plug using the proper size socket, extensions, and universals as necessary. Be careful to hold the socket or the extension close to the plug with your free hand as this will help lessen the possibility of applying a shear force which might snap the spark plug in half. If the cylinder heads on the engine are original, then for all 1964-69 engines, and all V8s 1964-71, use a 13 / 16 in. spark plug socket. 6-cylinder engines for 1970 and V8s from 1972 and later are equipped with tapered seat plugs which require a 5 / 8 in. socket.
  3. If removing the plug is difficult, drip some penetrating oil on the plug threads, allow it to work, then remove the plug. Also, be sure that the socket is straight on the plug, especially on those hard to reach plugs. Again, if the socket is cocked to 1 side a shear force may be applied to the plug and could snap the plug in half.

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Fig. Fig. 5: Grasp the wire boot, twist it, and pull it from the spark plug

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Fig. Fig. 6: Label the spark plug wires to assure proper installation

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Fig. Fig. 7: Remove the spark plug using a suitable socket and driver

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Fig. Fig. 8: Once the plug is loosened, finish unthreading it by hand and remove it from the cylinder head


See Figures 9, 10 and 11

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%.

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Fig. Fig. 9: Spark plugs that are in good condition can be filed and re-used

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Fig. Fig. 10: Always use a wire gauge to check the electrode gap on used plugs

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Fig. Fig. 11: Adjust the gap by bending the side electrode very slightly towards or away from the center electrode

Check and adjust the spark plug gap immediately 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 gauge (see Tune-Up Specifications) should pass through the gap with a slight drag. Always check the gap on new plugs, too; since they are not always set correctly at the factory.

Do not use a flat feeler gauge when measuring the gap on used plugs, because the reading may be inaccurate. The ground electrode on a used plug is often rounded on the face closest to the center electrode. A flat gauge will not be able to accurately measure this distance as well as a wire gauge. Most gapping tools usually have a bending tool attached. This tool may be used to adjust the side electrode until the proper distance is obtained. Never attempt to move or bend the center electrode or spark plug damage will likely occur. Also, be careful not to bend the side electrode too far or too often; if it is overstressed it may weaken and break off within the engine, requiring removal of the cylinder head to retrieve it.


  1. Inspect the spark plugs and clean or replace, as necessary. Inspect the spark plug boot for tears or damage. If a damaged boot is found, the spark plug wire must be replaced.
  3. Using a feeler gauge, check and adjust the spark plug gap to specification. When using a gauge, the proper size should pass between the electrodes with a slight drag. The next larger size should not be able to pass while the next smaller size should pass freely.

Do not use the spark plug socket to thread the plugs. Always thread the plug by hand to prevent the possibility of cross-threading and damaging the cylinder head bore.

  1. Lubricate the spark plug threads with a drop of clean engine oil, then carefully start the spark plugs by hand and tighten a few turns until a socket is needed to continue tightening the spark plug. Do not apply the same amount of force you would use for a bolt; just snug them in. If a torque wrench is available, tighten the plugs to 11-15 ft. lbs. (15-20 Nm).

A spark plug threading tool may be made using the end of an old spark plug wire. Cut the wire a few inches from the top of the spark plug boot. The boot may be used to hold the plug while the wire is turned to thread it. Because the wire is so flexible, it may be turned to bend around difficult angles and, should the plug begin to crossthread, the resistance should be sufficient to bend the wire instead of forcing the plug into the cylinder head, preventing serious thread damage.

  1. Apply a small amount of silicone dielectric compound to the end of the spark plug lead or inside the spark plug boot to prevent sticking, then install the boot to the spark plug and push until it clicks into place. The click may be felt or heard, then gently pull back on the boot to assure proper contact.
  3. Connect the negative battery cable.


Every 15,000 miles, visually inspect the spark plug wires for burns, cuts, or breaks in the insulation. Check the boots and the distributor cap tower connectors. Replace any damaged wiring.

Every 45,000 miles or so, the resistance of the wires should be checked using an ohmmeter. Wires with excessive resistance will cause misfiring and may make the engine difficult to start in damp weather. Generally, the useful life of the cables is 45,000-60,000 miles.

To check resistance, remove the distributor cap, leaving the wires in place. Connect one lead of an ohmmeter to an electrode within the cap; connect the other lead to the corresponding spark plug terminal (remove it from the spark plug for this test). Replace any wire which shows a resistance over 30,000&omega. Generally speaking, it is preferable that resistance be below 25,000&omega, but 30,000&omega must be considered the outer limit of acceptability. It should be remembered that resistance is also a function of length; the longer the wire, the greater the resistance. Thus, if the wires on your car are longer than the factory originals, resistance will be higher, quite possibly outside these limits.

If all of the wires must be disconnected from the spark plugs or from the distributor at the same time, be sure to tag the wires to assure proper reconnection.

When installing a new set of spark plug wires, replace the wires one at a time so there will be no mix-up. Start by replacing the longest cable first. Install the boot firmly over the spark plug. Route the wire exactly the same as the original. Connect the wire tower connector to the distributor. Repeat the process for each wire. Be sure to apply silicone dielectric compound to the spark plug wire boots and tower connectors prior to installation.