If by any reason the mica core becomes saturated with oil, it is best to obtain a new one, as it is almost impossible to remove the oil by simple means open to the average operator.
The chief value of a mica plug lies in its toughness and mechanical strength, a good mica plug being practically indestructible.
When heated, porcelain does not expand at the same rate as the metal sleeves, hence in poorly designed or imperfect plugs, heavy strains are thrown on the delicate porcelains which causes them to crack. When a crack develops it provides a lodging place for soot and carbon which of course causes a short circuit. Should a compression leak occur through faulty packing between the porcelain and sleeve, it should be immediately tightened up for eventually it will leak enough to destroy the plug or reduce the output of the engine.
When ordering a plug be sure that you know the size and type required by your engine. Some engines require a longer plug to reach the combustion chamber than others. Never install a shorter plug than that originally furnished with the engine. Be sure that the plug is not too long as it may interfere with the action of the valves or may be damaged by them. Plugs are furnished with several threads and taps, i. e.:
- ½ inch pipe thread (Generally used on stationary engines).
- Metric Thread (Generally used on imported autos).
- ⅞ inch A. L. A. M. Standard (Used on Domestic automobiles).
Using a plug in a hole tapped with the wrong thread will destroy the thread in the cylinder casting and cause compression leaks.
(96) Care of Spark Plug.
Porcelains are often broken by screwing the plug too tightly in a cold cylinder, as the cylinder expands when heated and crushes the frail plug. A plug installed in this manner is difficult to remove as the expanded walls grip the thread. The plug should be screwed in just enough to prevent the leakage of gas. A short thin wrench should be used in screwing the plug home such as a bicycle wrench. A wrench of this type is so short that it will be almost impossible to exert too much force, and will be thin enough to avoid any possible injury to the packing nut. Bad leaks may be detected by a hissing sound that is in step with the speed of the engine, small leaks may be detected by pouring a few drops of water around the joint. If a leak exists bubbles will pass up through the water and show its location.
Plugs are more easily removed from a cold cylinder than a hot. If the plug sticks when the engine is cold and is impossible to remove with a moderate pressure on the wrench squirt a few drops of kerosene around the threads. Never exert any force on the porcelain or insulation. The high tension cables should be connected to the plugs by means of some type of “Snap Terminal,” such terminals may be had from automobile dealers.
These terminals make a firm contact with the plug and do not jar loose from the plug by the vibration of the engine. They are easily disconnected when the inspection of the plug becomes necessary, and are generally a most desirable attachment.