Substitutes or Subs as they are known as are used as adaptors where threads of one size or type need to be coupled with threads of another type or size.
This means that subs have to be manufactured from selected bars of alloy steel, heat-treated to provide the strength and toughness required to carry the entire weight of the drill-string or to withstand high torque differentials.
To act as cross-over connections or used at points of heavy wear to provide a readily replaceable thread and include:
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