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An underreamer having an adjustable arm extension includes a cam for urging the arm outwardly in an opened position, and a stop pin which engages a slot in the arm for limiting the maximum arm extension. An insert for the arm has a surface which overlaps a portion of the slot to establish an effecti

An underreamer having an adjustable arm extension includes a cam for urging the arm outwardly in an opened position, and a stop pin which engages a slot in the arm for limiting the maximum arm extension. An insert for the arm has a surface which overlaps a portion of the slot to establish an effective length for the slot. By changing the configuration of the surface on the insert, the arm extension can be adjusted.

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An underreamer comprising: a tubular body; means for connecting an upper end of the tubular body to a drill string; at least one cutting arm hingedly connected to the body for movement between a retracted position essentially within the body and an opened position in which an arm outer end is dispos

An underreamer comprising: a tubular body; means for connecting an upper end of the tubular body to a drill string; at least one cutting arm hingedly connected to the body for movement between a retracted position essentially within the body and an opened position in which an arm outer end is disposed outwardly of the body; a slot in a side of such an arm; means for actuating the arm from the retracted position toward the opened position, the arm at the opened position being urged outwardly toward an over-opened position; a stop pin mounted in the body having an end fitting into the slot in the arm; and a plate on the arm having a surface engageable with the stop pin at the opened position for limiting the maximum outward travel of the arm to the opened position, wherein said plate is removable from the arm for interchange with another such plate having a surface of different configuration from the first plate for changing the effective length of the slot.