Spark plug having an oil film on an intermediate portion of the insulator or intermediate portion of the metallic shell

A method of manufacturing a spark plug for an internal combustion engine comprising, in combination, the steps of

• providing an insulator having an axial bore for holding a terminal electrode at a rear end portion and a center electrode at a front end portion and an intermediate body portion defining a surface flanked by a first transition region and a larger body portion proximate said rear end portion and a second transition region and a smaller leg portion proximate said front end portion,
• providing a metallic shell having a larger portion for receiving said larger body portion of said insulator and a smaller intermediate hole portion defining a surface,
• inserting one end of a center electrode into such axial bore of such insulator to be held in a sealed manner,
• forming an oil film on at least one of:
• a) such surface of such intermediate body portion of such insulator;
• and b) such surface of an intermediate hole portion of such metallic shell that faces such surface of such intermediate body portion of such insulator, and
• attaching such metallic shell to such intermediate body portion of such insulator.