Abstract: A coupler system for downhole wellbore service tools comprises an elongated main body having a longitudinal bore, a plurality of release blocks disposed in slots in the main body wall, and an exterior spline about its circumference below the release blocks. The release blocks are movable radially inward and outward, and are biased inward by springs. A piston having a longitudinal bore is movably disposed within the main body bore. The piston has spaced apart larger diameter sections and a central smaller diameter section. A cylindrical washpipe body fits over the main body and has an internal spline for engagement with the main body spline, and shoulders (on the main body and on the interior of the washpipe body) limit downward movement of the washpipe body.