Last updated: January 14, 2013

Firing end appearances

The appearance of the firing-end of a used spark plug graphically reflects the condition of an engine, the suitability of the spark plug heat range, and whether or not the fuelling and ignition systems are correctly set.

Normal condition

An engines condition can be judged by the spark plugs firing end appearance. If the firing end of a spark plug is brown or light gray, the condition can be judged to be good and the spark plug is functioning correctly.


The accumulation of deposits on the firing end is influenced by oil leakage, fuel quality and the engines operating period. Deposits come from:
Fuel: C (carbon), Pb (lead), Br (bromine)
Lubricating oil: C, CA (calcium), S (sulphur), Ba (barium), Zn (zinc)
Others: Fe (iron), Si (silicon), Al (aluminium), etc.

Dry and wet fouling

Although there are many different causes, if the insulation resistance between the center electrode and the shell is over 10M ohms, the engine can be started normally.

If the insulation resistance drops to 0, the firing end is fouled by either wet or dry carbon.

Lead fouling

Lead fouling usually appears as yellowish brown deposits on the insulator nose and this cannot be detected by a resistance tester at room temperature. Lead compounds combine at different temperatures; those formed at 370 420 degrees Celsius having the greatest influence on the resistance.


After having overheated, the insulator tip is glazed or glossy, and deposits which have accumulated on the insulator tip have melted. Sometimes these deposits have blistered on the insulators tip.


Breakage is usually caused by thermal shock due to sudden heating or cooling.

Normal life

A worn spark plug not only wastes fuel but also strains the whole ignition system because the expanded gap requires higher voltages. Worn spark plugs also reduce the engines efficiency, resulting in reduced fuel economy and increased exhaust emissions. The normal rate of gap growth is about 0.01 0.02mm/1,000 Km for four stroke engines and about 0.02 0.04mm/1,000 Km for two stroke engines.

Erosion, Corrosion, Oxidation

The material of the electrodes has oxidized, and when the oxidation is heavy it will be green on the surface. The surfaces of the electrodes are also fretted and rough.

Abnormal erosion

Abnormal electrode erosion is caused by the effects of corrosion, oxidation, reaction with lead, all resulting in abnormal gap growth.

Lead erosion

Lead erosion is caused by lead compounds in the gasoline which react chemically with the material of the electrodes ( nickel alloy) at high temperatures. Crystals of nickel alloy fall off because of the lead compounds permeating and separating the grain boundary of the nickel alloy. Typical lead erosion causes the surface of the ground electrode to become thinner, and the tip of the electrode looks as it it has been chipped.


Melting is caused by overheating. Mostly, the electrode surface is rather lustrous and uneven. The Melting point of nickel alloy is 1,200 1,300 degrees Celsius.