Abstract: A fastener driving tool is provided which employs a drive housing subassembly, preferably in the form of opposed cast metal housing sections, which encloses the flywheel and suitable bearings for rotating the same at high speed. This subassembly also supports the associated idler wheel positioned on the other side of the fastener driving ram from the flywheel and mounts a toggle mechanism for adjusting the idler wheel from an inoperative position, in which the idler is spaced a substantial distance away from the ram, to an operative position close to the ram in response to movement of the tool into engagement with a workpiece. The drive housing subassembly also provides a top opening recess or well which is adapted to receive and position a removable cartridge which includes the ram and an elastic cord return mechanism, this subassembly also defining a vertically extending slot communicating with said recess which is adapted to receive the vertically positioned ram between the opposed flywheel and idler.