Electromagnetic gap sub assembly

A compact, robust, wear resistant, and high performance electrically insulating gap sub for use with borehole EM Telemetry is disclosed. In one embodiment, the gap sub may include an externally threaded mandrel separated from an internally threaded housing by a dielectric material. Additionally, some embodiments may include an external gap ring for separating the upper and lower electrical halves of the sub which offers structural support, acts as the primary external seal, and provides an abrasion resistant non-conductive length on the exterior. Some embodiments may include torsion pins to prevent the possible unscrewing of the dielectric filled threaded sections should the dielectric material become damaged or weakened.