Abstract: A tool for use in the line boring of diesel engine cylinder blocks. The tool includes a mounting pad having a pair of holes for fitting over dowel pins projecting from the end of the cylinder block. A central opening of the pad is then centered in alignment with the bores of the engine block to receive a bearing providing rotative support for a shaft. The shaft extends through the center of each bore to permit accurate cutting of damaged bore areas with a cutting tool on the shaft. The bearing can be adjusted in limited fashion by an adjustment mechanism on the pad to assure that the shaft is centered in any undamaged bores that are present. An adapter plate permits mounting of the pad on engine blocks having locating openings instead of dowel pins.