Abstract: A tubing bit opener is disclosed having upper and lower cutters with the lower cutters being carried by mounting blocks having bevelled edges which are received in matching undercut grooves in the tubing bit opener. The upper cutters are attached to arms which are slidable within inclined pockets of the tubing bit opener for inward and outward movement relative to the tubing bit opener. Both the mounting blocks carrying the lower cutters and the arms carrying the upper cutters are easily removable for maintenance and repair. The lower mounting blocks are held securely in place by the drill body. Because the surfaces of the lower mounting blocks extend laterally along a considerable extent of the adjacent lowr cutters, the cutters are supported for stable rotation without vibration. The tubing bit opener is typically used to sink six inch diameter or larger caissons in any rock formation on land or under water.