(57) In a method of making a spark plug centre electrode, a billet (12) of a first metal
which has good thermal conductivity is placed within a cup-shaped billet (10) of a
second metal having good corrosion resistance, the first metal being melted within
the cup-shaped billet of the second metal and, subsequent to cooling, the composite
billet is extruded through an extrusion die (17) to form a composite centre electrode
in which a core of the first metal is enclosed substantially entirely within a sheath
of the second metal.
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