If the metal is 1/16 inch or less in thickness it is only necessary to
circle along the crack, the metal itself furnishing enough material to
complete the weld without additions. Heat both sides evenly until they flow
together.

Material thicker than the above requires the addition of more metal of the
same or different kind from the welding rod, this rod being held by the
left hand. The proper size rod for cast iron is one having a diameter equal
to the thickness of metal being welded up to a one-half inch rod, which is
the largest used. For steel the rod should be one-half the thickness of the
metal being joined up to one-fourth inch rod. As a general rule, better
results will be obtained by the use of smaller rods, the very small sizes
being twisted together to furnish enough material while retaining the free
melting qualities.

[Illustration: Figure 36.--The Welding Rod Should Be Held in the Molten
Metal]

The tip of the rod must at all times be held in contact with the pieces
being welded and the flame must be so directed that the two sides of the
crack and the end of the rod are melted at the same time (Figure 36).
Before anything is added from the rod, the sides of the crack are melted
down sufficiently to fill the bottom of the groove and join the two sides.
Afterward, as metal comes from the rod in filling the crack, the flame is
circled along the joint being made, the rod always following the flame.

[Illustration: Figure 37.--Welding Pieces of Unequal Thickness]

Figure 37 illustrates the welding of pieces of unequal thickness.