Volkswagen Air Cooled 1949-1969 Repair Guide

Spark Plugs


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 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.


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

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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 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.


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Fig. Fig. 3 To remove the spark plugs, detach the cables from the plugs ...

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Fig. Fig. 4 ... then use a socket and ratchet to loosen the plug

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Fig. Fig. 5 Once the plug is loose, unthread it by hand to prevent damage to the threads in the cylinder head

Spark plugs should be cleaned and gapped at 6,000 mile intervals and replaced every 12,000 miles.

In normal operation plug gap increases about 0.001 in. (0.025mm) for every 2500 miles (4000 km). 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 the plug at high speeds than at idle. The improved air/fuel ratio control of modern fuel injection combined with the higher voltage output of modern ignition systems will often allow an engine to run significantly longer on a set of standard spark plugs, but keep in mind that efficiency will drop as the gap widens (along with fuel economy and power).

When you're removing spark plugs, 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.

  1. Disconnect the negative battery cable, and if the vehicle has been run recently, allow the engine to thoroughly cool.
  3. Carefully twist the spark plug wire boot to loosen it, then pull upward and remove the boot from the plug. Be sure to pull on the boot and not on the wire, otherwise the connector located inside the boot may become separated.
  5. Using compressed air, blow any water or debris from the spark plug well to assure that no harmful contaminants are allowed to enter the combustion chamber when the spark plug is removed. If compressed air is not available, use a rag or a brush to clean the area.

Remove the spark plugs when the engine is cold, if possible, to prevent damage to the threads. If removal of the plugs is difficult, apply a few drops of penetrating oil or silicone spray to the area around the base of the plug, and allow it a few minutes to work.

  1. Using a spark plug socket that is equipped with a rubber insert to properly hold the plug, turn the spark plug counterclockwise to loosen and remove the spark plug from the bore.

Be careful when using a flexible extension on the socket. Use of a flexible extension may allow a shear force to be applied to the plug. A shear force could break the plug off in the cylinder head, leading to costly and frustrating repairs.

To install:

  1. 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 wire feeler gauge, check and adjust the spark plug gap. 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.
  5. When installing spark plugs, care should be taken not to overtighten them. The Volkswagen cylinder heads are made of a relatively light alloy and can be cross-threaded more easily than the more common cast iron heads in domestic automobiles. For this reason, a few drops of light oil or anti-seizing compound should be placed on each plug before it is inserted by hand (using the extension holder supplied with the VW tool kit) and turned cautiously for several turns before tightening down to a torque of roughly 22-29 ft. lbs. New gaskets should be used. When the proper torque is used in tightening the plugs, only the compression ring of the new gaskets will be crushed-this is the desirable condition

Do not use the spark plug socket to thread the plugs. Always carefully thread the plug by hand or using an old plug wire to prevent the possibility of crossthreading and damaging the cylinder head bore.

  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.


After the spark plugs are removed from the engine, they should be inspected for outside appearance which gives valuable information on the running condition and mixture adjustment of the engine. 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%.

In the dual-carburetor type 3 engine, each carburetor feeds two cylinders. It is not unusual to discover that one bank of cylinders is running either richer or leaner than the other bank. When this situation occurs, the carburetors should be adjusted properly.

The spark plugs in the Volkswagen should be gapped to between 0.024-0.028 in. (0.6-0.7mm). In general, the best idling will result from a wide spacing of the spark plug gap, but a wide gap will also cause starting to be slightly harder. A wide gap at high speeds will also lead to missing. On the other hand, a plug with a smaller gap will miss less at high speeds, give easier starting, but will cause poor idling. In order to improve starting ability when outside temperatures are very low, the plug gap may temporarily be reduced to 0.020 in. (0.5mm). Because of the wide range of conditions under which the Volkswagen operates, it is probably best to gap plugs in the middle of the 0.024-0.028 in. range, in other words 0.026 in. However, if one places more emphasis on good idling than on good high-speed performance, or vice-versa, one may wish to choose gaps in either the higher or the lower portion of the recommended range. The Volkswagen will operate equally well at any setting in the recommended range, and any differences would probably be so slight as to be unnoticeable.

It is advisable to adjust only the side electrode by bending it either toward or away from the center electrode with a special spark plug gapping tool. In the normal operation of the engine, spark plug gaps increase due to natural burning, so used plugs must have their gaps reduced in order to achieve the correct adjustment.