A well tool, such as a paraffin scraper or centralizer, installable on a rod of a pump rod string, which is composed of two longitudinal sections longitudinally movable relative to one another on a rod interposed therebetween into clamping locked position on the rod and a resilient longitudinal member compressed between one of the sections and the rod, the two sections having first co-engageable means for moving the two sections laterally relative to each other to compress the compressible member against the rod upon relative longitudinal movement of one section toward the other and a second co-engageable lock means for holding the two sections in longitudinally telescoped locked position on the rod.