Spark plug having an erosion resistant tip

In a spark plug electrode which includes a metallic shell having a tubular insulator in which a center electrode is provided, a front end of the center electrode forming a spark gap with an outer electrode extended from the metallic shell, the spark plug electrode comprising:

• the center electrode being made of a heat-conductor core cladded by a nickel-alloyed metal;
• a recess provided on a front end surface of the nickel-alloyed metal;
• a columnar tip made of a precious metal, a rear end of the tip being fitted in the recess in such a manner that a front end of the tip is somewhat protracted from the recess;
• an outer surface of the tip being bonded to an inner surface of the recess all through their circumference by means of laser or electron beam welding;
• a dimensional relationship of A,B,C,D,E,F and G being given as follows:
• 0.3 mm.ltoreq.A.ltoreq.0.8 mm, 1.2A.ltoreq.B.ltoreq.3A, 0.1 mm.ltoreq.(C-A)/2.ltoreq.0.5 mm, D.ltoreq.(C-A)/2, E.gtoreq.B/4, 0 mm.ltoreq.F.ltoreq.0.5 mm and A/5.ltoreq.G.ltoreq.A/2
• where
• A:
• a diameter of the columnar tip,
• B:
• a length of the columnar tip,
• C:
• a diameter of a front end of the nickel-alloyed metal,
• D:
• a length of a front end of the nickel-alloyed metal,
• E:
• a length of the front end of the tip which is protracted from the recess,
• F:
• a distance between a rear end of the tip and a front end of the heat-conductor core,
• G:
• a distance of a welding portion penetrated from the outer surface of the tip to the inner surface of the recess.