Top: The sealing surface for the o-rings is deeper than the threads. The threads are 'wetted' (and lubricated) by the mud in the oil well. The sealing surface is protected from hammer blows and rough handling, which may deform it's geometry and roundness os the surface. Specially the o-ring 'ramping' surface. The housing's wall thickness is larger at that point, and therefore the connection can sustain higher operating pressures. This is the modern configuration for sub-surface connections, and very adaptable for using swivel rings and keying features.

Middle: The threads are deeper than the sealing surface and therefore are 'dry'. This is an old design. It lacks all the advantages mentioned above. It is normally incorporated into new connections only because: either the engineer has limited exposure to high performance connections, or he had a need to match some existing old sub.

Bottom: This type of connection uses radial screws. Typically 3 or 4. It is the cheapest connection, and used almost exclusively for expendable gun perforating equipment. Note: New perforating techniques may dictate the use of more sturdy connections (such as the Top one) because the higher accelerations experienced in the higher pressure wells are shearing them off, leaving portion of the equipment in the bottom of the well.

National=K=Works specializes in the design and testing of modern and advanced end-connections. This is because every Thermohousing leaving our plant is destined to operate at the limits of today's drilling capabilities, and it carries our seal of excellence engraved on it. Our reputation and survival depends on the reliability of our designs. The present temperature record for one of our sub-surface sealing connections is 658 degF [348 degC] recorded at the bottom of a Japanese Geothermal well (WD-1) http://www.gerd.co.jp/  during March 1996.