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    Reading Spark Plugs by Mike Chilando

    Three Parts to the Spark Plug

    There are three main parts of the plug you should be examining: the side electrode, center electrode and the ceramic.

    Starting with the center electrode, it should be the same color as a new one out of the box. If it is blue or purple, it’s been overheated. Its shape should be that of a cylinder with sharp corners at the top. “Round” corners and erosion of the electrode will more often be seen on overheated plugs and those run for a long period of time. Sharp corners are desired because it takes less energy for a spark to jump from a corner than a rounded edge.

    The side electrode is more difficult to diagnose than the center electrode. The color of the side electrode should be similar to what it looks like out of the box. Green is slightly hotter than normal, blue is way too hot. The fuel mixture should be adjusted as soon as possible to fix  this problem. On the side electrode there should be a “heat line,” a ring around the electrode generated at the tip. The hotter the chamber, the farther the ring will travel toward the base of the electrode. If the ring is in the first third, the chamber is too cold. If this ring is in the final third, almost to the base of the electrode, the chamber is too hot. When the ring is found somewhere in the middle of the electrode, the chamber is in the correct temperature range. The final area to examine is the ceramic. The color should be a like-new white, but, to be on the safe side, a light tan will do. If the ceramic is shiny black, the plug was probably fouled by oil. Dark gray or black is a good sign of an over-rich condition. Glazing, shiny white, yellowing or pitting on the ceramic is a sign of overheating. A white fuzz between the center electrode and the ceramic is known as “cement boil,” which is what you get when the material that holds the electrode and ceramic together gets too hot.

    Additionally, on the ceramic there may be specks of material. The two most common types are pepper specks and shiny balls. They show detonation, but the pepper specks also can mean a slight oil-control problem. The specks will attach themselves to the ceramic and the outer shell of the plug. The shiny balls are tiny pieces of aluminum from the top of the piston. Similar to the side electrode, the ceramic also should have a ring around it. This is called the fuel ring. It should be a brown-gray color and half-way up the height of the ceramic. If it’s too deep on the ceramic, the chamber is again too hot. If its too close to the electrode, the chamber is too cold.


    Once you have decided what each plug indicates, it’s time to look for patterns in the readings. The simplest patterns affect all plugs in the rack. All are lean, rich or detonating.

    Now that you have taken an accurate plug reading and corrected any unusual patterns, it’s now time to analyze and solve the indicated problems. If the chamber is too hot, this is most likely a sign of a lean mixture and/or too much timing. This also could be insufficient cooling of the engine, detonation or too high a heat range on the plugs. To solve this condition, you will need to start adding some fuel or removing some timing. If these remedies do not provide the answer, look into the cooling and plug heat-range options.

    If the chamber is too cold, it is likely the inverse of the above: the mixture is too rich and there’s not enough timing. Try leaning out the injection  , or try a plug with a hotter heat range. If you see traces of oil in the chamber, this is an indication of a piston ring that has not sealed or has broken, worn valve guides or excessive crankcase pressure. These should be fixed before you go any further.

    You are looking for are plugs that show correct temperature and sufficient oil control in the combustion chamber. The electrodes should be the same color as when they came out of the box and not eroded. The ceramic should be white and have a heat line half-way up its height. There shouldn’t be any black specks or shiny balls anywhere.


    Always use the coldest plug you can use without fouling it.

    Don’t expect much of an ability to read a methanol fueled spark plug before possibly 4 to 6 runs-  even then  you may not see enough pattern to form a conclusion.

    Methanol depends on making a lot of heat in the combustion chamber to produce power-  if you believe the plug is burning “Rich”  make adjustments to the engine to lean it out at high rpm with a high speed lean out valve.