Spark plug

A spark plug comprising:

• a tubular ceramic insulator having an axial bore extending therethrough in an axial direction;
• a center electrode disposed at a forward end portion of the axial bore;
• and
• a metallic shell provided around the ceramic insulator;
• the spark plug being characterized in that:
• the metallic shell has:
• a tool engagement portion projecting outward and having a polygonal orthogonal-to-axis sectional shape;
• a gas seal portion projecting outward;
• and
• a groove portion formed between the tool engagement portion and the gas seal portion and having an orthogonal-to-axis sectional area of 36 mm2 or less;
• the metallic shell is covered with a nickel plating layer and has a chromium-containing layer formed on the nickel plating layer;
• and
• as measured at a forward end of an inner circumferential surface of the groove portion, the nickel plating layer has a thickness of 0.2 μm to 2.2 μm.

A spark plug comprising:

• a tubular ceramic insulator having an axial bore extending therethrough in an axial direction;
• a center electrode disposed at a forward end portion of the axial bore;
• and
• a metallic shell provided around the ceramic insulator;
• the spark plug being characterized in that:
• the metallic shell has:
• a tool engagement portion projecting outward and having a polygonal orthogonal-to-axis sectional shape;
• a gas seal portion projecting outward;
• and
• a groove portion formed between the tool engagement portion and the gas seal portion and having an orthogonal-to-axis sectional area of 36 mm2 or less;
• the metallic shell is covered with a nickel plating layer and has rust prevention oil applied onto the nickel plating layer;
• and
• as measured at a forward end of an inner circumferential surface of the groove portion, the nickel plating layer has a thickness of 0.2 μm to 2.2 μm.

A spark plug comprising:

• a tubular ceramic insulator having an axial bore extending therethrough in an axial direction;
• a center electrode disposed at a forward end portion of the axial bore;
• and
• a metallic shell provided around the ceramic insulator;
• the spark plug being characterized in that:
• the metallic shell has:
• a tool engagement portion projecting outward and having a polygonal orthogonal-to-axis sectional shape;
• a gas seal portion projecting outward;
• and
• a groove portion formed between the tool engagement portion and the gas seal portion and having an orthogonal-to-axis sectional area of 36 mm2 or less;
• the metallic shell is covered with a nickel plating layer and has a chromium-containing layer formed on the nickel plating layer, and rust prevention oil applied onto the chromium-containing layer;
• and
• as measured at a forward end of an inner circumferential surface of the groove portion, the nickel plating layer has a thickness of 0.1 μm to 2.4 μm.