Hole Opener : A larger device (usually of a fixed diameter) that enlarges the wellbore to a diameter equal to or less than the upper casing drift). Compare to an under-reamer or watermelon mill or string mill
George E. King Engineering, 5000 Oilfield Terms: A Glossary of Petroleum Engineering Terms, Abbreviations and Acronyms, US 2010. http://gekengineering.com/Downloads/Free_Downloads /Glossary_of_Petroleum_Engineering_Terms_25_August _2010.pdf, (Oil - Amer) {593}