Spark plug

A spark plug comprising:

• an insulator including at least 99% by weight Al2O3 and having a density of 3.95 g/cm3 or greater and wherein said insulator includes 10-1000 ppm La2O3, and approximately 10-1000 ppm of at least one selected from the group consisting of Yb2O3, MgO, transition metal oxides from Group III B of the periodic table, and oxides of the Lanthanide Series.

A spark plug comprising a polycrystalline insulator having a core nose and a terminal portion and wherein said insulator is capable of allowing passage of light from said core nose to said terminal portion, and wherein said insulator has a porosity of 0.3% or less and includes greater than 99.9% alumina and at least one material selected from the group consisting of MgO, Y2O3, La2O3 and oxides of the Lanthanide series, and a density of 3.96 g/cm3 or greater.

A spark plug comprising an unglazed insulator, wherein said insulator has an outer surface and said outer surface has an average roughness of 0.40 micrometers or less, and a density of 3.95 g/cm3 or greater, a dielectric strength of approximately 17 kilovolts (RMS) per millimeter, and wherein said insulator is capable of allowing the passage of light.

A spark plug comprising an insulator includes a large shoulder and a small shoulder between said core nose and a terminal portion and wherein the thickness of the insulator at said small shoulder portion is less than 2.54 mm, and a dielectric strength of 17 kilovolts (RMS) or greater per millimeter.

A spark plug comprising an unglazed insulator, wherein said insulator has an outer surface and said outer surface has an average roughness of 0.20 micrometers or less, a density of 3.95 g/cm3 or greater, a porosity of 0.3% or less and is capable of allowing the transmission of light.