Reamer bit for raise boring

In a reamer bit for raise boring in rock, of the type comprising a body on which a number of rolling cutters with inserts are rotatably mounted for contact with and disintegration of the rock, and a detachable stem mounted in a central bore on the body and adapted to be received in a pilot hole drilled in advance in the rock, said stem having two cylindrical portions, an upper one of which having a smaller diameter than a lower one thereof, said bore in the body having corresponding upper and lower cylindrical portions surrounding the stem, the interface between said upper cylindrical portions of said body and said stem defining upper stem restraining surface means, and the interface between said lower cylindrical portions of said body and said stem defining lower stem restraining surface means, the improvement wherein the stem is provided with a flange located below said lower restraining surface means, said flange abutting the body from below and providing substantially all axial support for said body, means releasably connecting said flange to said body, said upper and lower restraining surface means together comprising a substantial axial portion of that part of the stem surrounded by the bore, the corresponding surrounding portions of the bore in the body forming a close fit with the cylindrical portions of the stem, said upper and lower restraining surface means being separated by an annular relief portion on at least one of said stem and body, the diameter of the portion of said stem located at an upper end of said relief portion being no larger than the diameter of said upper cylindrical portion of said stem.