An improved offset saw blade mounting assembly for use on a reciprocating power saw tool includes a shim member that is used to fill a central slot of the tool drive shaft. The shim member fills the slot and serves to reinforce the drive shaft and thereby reduce the likelihood of the drive shaft failing. The shim member may be a separate piece, or formed as part of a separate tubular insert, or integrally formed with the mounting block of the offset mounting assembly. A finger guard is also provided that substantially closes off any opening which may occur between the mounting assembly and the power tool.