Underground drilling apparatus

A drilling apparatus is adapted for connection to a drill string such that the drill string rotates the drilling apparatus in a rotational cutting motion. The drilling apparatus includes a sonde housing having a distal end and a proximal end. The sonde housing includes a top platform positioned adjacent the distal end of the sonde housing and further includes a tongue positioned on the top platform. The drilling apparatus further includes a rotary cutting tool mounted to the distal end of the sonde housing. The rotary cutting tool defines first and second pin openings that co-axially align with first and second pin openings of the sonde housing. The rotary cutting tool also includes a proximal extension defining a groove that receives the tongue of the sonde housing. The drilling apparatus also includes first and second pins having first ends respectively received in the first and second pin openings of the sonde housing and second ends respectively received in the first and second openings of the rotary cutting tool. A cross pin is mounted for limiting relative movement between the sonde housing and the rotary cutting tool.