Spark plug having a noble metallic firing tip bonded to an electric discharge electrode and preferably installed in internal combustion engine

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

• disposing a Ni-containing central electrode at least at one end of a through hole of an insulator held in a housing;
• providing a Ni-containing grounded electrode on said housing in a confronting relationship with said central electrode to form a spark gap between said central electrode and said grounded electrode;
• bonding a noble metallic firing tip on at least one confronting surface of said central electrode and said grounded electrode by applying a laser beam to a boundary between said noble metallic firing tip and a base electrode member of said confronting surface, said noble metallic firing tip having a diameter “A” in the range from 0.3 mm to 0.6 mm;
• and
• hardening the laser welded portion so as to form a fused junction layer at the boundary between said noble metallic firing tip and said base electrode member,
• wherein said fused junction layer contains 40˜70 wt % noble metallic firing tip component,
• said noble metallic firing tip has a non-fused portion axially extending from its top to an upper end of said fused junction layer by an axial length “L”, where 0.2 mm ≦L≦0.7 mm,
• said fused junction layer axially extends from the upper end to its lower end by an axial length “M”, where 0.2 mm ≦M≦0.7 mm, and
• the relationship B≧1.3A is established
• where “B” represents the maximum width of a contact area which is formed by projecting the boundary between said fused junction layer and said base electrode member in an axial direction of said noble metallic firing tip.