The Spark Plug

Cloudy Joe thinks only the big things are important. He spends most of his time shining up his airplane, but he never pays any attention to little things like spark plugs. But since the proper functioning of your engine, as well as your own safety, depends upon these small units, I am going to take time right now to tell you how these little "big" things are kept in first-class condition. As a rule, spark plugs should be taken out, cleaned and inspected once in every sixty - or at very most, seventy-five-flying hours, but they must be handled with the greatest care to prevent damage to any part of the plug.

If you do some of the mechanical work on your plane yourself, here are the various steps to be followed in the proper inspection and maintenance of spark plugs:

(1) After removing the core assembly from the shell and taking out the gasket, place the cores in a bench work tray.

(2) Clean both the core and core electrode, and wash and dry the core.

(3) Measure electrode shoulder diameter; inspect tip diameter and core; lubricate threads and install core assembly gasket.

(4) Clean and inspect shells; form shell electrodes; assemble plugs and adjust electrodes.

(5) Test plugs and replace in engine (or prepare for storing).

Figs. 189 and 190. To remove the core assembly of a two-piece spark plug, such as the BG, from its shell, place the plug in a vise socket (Fig. 189), or a split hinge socket in a vise (Fig. 190), and use a wrench as illustrated. Never use an open-end wrench, and never place the shell or core in the jaws of a vise.

Fig. 191. Here you see how a special punch is used to remove the copper core assembly gasket from a shell of the internal gasket type. On some of the newer shells, you will find the gasket between the top of the shell and the bottom of the coupling nut.

Fig. 192. Spark plugs are easily damaged. Don't let the cores lie around in a box or in piles. Put them into a bench work tray, such as shown here, except when you are actually working on them.

Fig. 193. A core-cleaning collet and 00-grade sandpaper or #150 Aloxite cloth are used to remove all traces of carbon, oil, lead, etc., from the core. You will get the best results if the motor shaft to which the collet is attached turns about 1750 r.p.m. Never use emery, carborundum cloth, paper, steel wool, file or metal buffing wheels for cleaning cores.

Fig. 194. Measure the shoulder or head diameter of the electrode with a micrometer or BG gauge, as shown in the illustration, taking the measurement at the point of the metal electrode where the mica insulation starts. It you find that the diameter is less than .260", throw the core away.

Figs. 195 and 196. The most important part of the whole maintenance process is the forming of the shell electrode, for on the way this is formed depends the efficiency with which the plug will operate in the engine, and the rate of burning of the electrodes. For this work you will need an electrode-forming and adjusting tool mounted on a solid base, and a set of specially shaped dies. The tool shown in the illustration comes with the female die recessed in the lower end of the rack, and five male dies to cover the range of BG plugs. Be sure to set up your equipment so that the base of the tool is at a suitable working height, and proceed as follows:

Take the electrode-adjusting foot out of the rack, see that the screw at the lower end of the rack clears the rack guide, and put the proper shell electrode die into the bed of the tool frame, fastening it with the set screw. Remove the handwheel and pinion from the frame. When you have placed the shell over the male die as shown in Fig. 196, lower the rack until the female die fits firmly over the shell electrode and strike a sharp blow with a one-pound hammer at the center of the upper end of the rack - but don't try to prove how strong you are. Remember that spark plugs and everything about them must be carefully and gently handled. When the electrode conforms to the shape of the shell, raise the rack and take out the shell.

Fig. 197. The same tool shown in the preceding illustrations is here set up for adjusting electrodes. Bushings are provided to bring spark plugs of varying shell-thread length into position for adjusting. In making the adjustment, be careful not to bend the electrodes to far.

Specifying an electrode setting of .012" instead of .015" will lengthen periods between plug overhauls, particularly in high-power-output engines, longer periods between overhauls are not always worthwhile, since small gaps are apt to result in poor idling characteristics.

Figs. 198 and 199. The possibility of insulation failures can be largely eliminated by testing plugs in a test bomb and with the portable equipment shown in these illustrations. (Cloudy Joe thinks that this part of the proceedings may be worth learning, since it does not require much headwork.) The test bomb may be manually operated (Fig. 198), or connected to a tank of carbon dioxide under pressure (Fig. 199), or connected to a compressed air line if provision is made to extract the moisture from the air.

Because plugs which sometimes spark at atmospheric pressure may not spark at all under operating conditions in your engine, you must be sure to test the plugs under pressure. You will find that a plug which sparks regularly under 125 lbs. of pressure re square inch with the gap adjusted to /015" generally performs satisfactorily in any engine. For best results, keep the spark gaps properly adjusted. Check and reset them each time the plugs are removed for cleaning. If the gaps are not checked at regular intervals, you may have difficulty in starting your engine; and the actual running of the engine may be ragged and irregular if the gap growth is allowed to proceed too far.

For testing as pressures up to 150 lbs., use either a booster or service magneto in good condition. Current from spark coils or transformers gives misleading results and may damage the insulation.

Fig. 200. A container of the type shown here is convenient for storing or shipping spark plugs, and is widely used by the airlines.

Before you do any work on spark plugs, study a service manual carefully. Be sure to use the right kind of tools, and don't use too much force. Spark plugs may be a small part of your aircraft, ut they are important both to the smooth running of your engine and to your own safety.