Abstract: A drilling tool has a body with a main drilling region and a pilot drilling region, eccentric to the main drilling region. The main drilling region has main blades and the pilot drilling region has pilot blades. The main drilling region has a main pass-through diameter, centered upon a first axis, one of the main blades extending to the main pass-through diameter, the pilot region having a pilot gauge diameter, centered upon a second axis of rotation at least one of the pilot blades extending to the pilot gauge diameter, wherein part of a circle of diameter equal to the pilot gauge diameter and centered upon the second axis of rotation extends outside of a circle of diameter equal to the main pass-through diameter and centered upon the first axis.

Abstract: A bi-center drilling tool comprises a main reamer region located eccentrically to an axis of rotation of the tool, and a leading cutting structure located ahead of the main reamer region, in the drilling direction. The leading cutting structure is located so as to apply a side loading to the drilling tool. The side loading is in direction to counter the loading which, in use, is due to the main reamer region being eccentric to the axis of rotation.

Abstract: A drilling tool has a body with a main drilling region and a pilot drilling region, eccentric to the main drilling region. The main drilling region has main blades and the pilot drilling region has pilot blades. The main drilling region has a main pass-through diameter, centered upon a first axis, one of the main blades extending to the main pass-through diameter, the pilot region having a pilot gauge diameter, centred upon a second axis of rotation at least one of the pilot blades extending to the pilot gauge diameter, wherein part of a circle of diameter equal to the pilot gauge diameter and centred upon the second axis of rotation extends outside of a circle of diameter equal to the main pass-through diameter and centred upon the first axis.

Abstract: A bi-center drilling tool comprises a main reamer region located eccentrically to an axis of rotation of the tool, and a leading cutting structure located ahead of the main reamer region, in the drilling direction, the leading cutting structure being located so as to apply a side loading to the drilling tool, the direction in which the side loading acts being such as to counter a side loading applied, in use, due to the main reamer region being eccentric to the axis of rotation.