Abstract: A telescoping torque transmission apparatus including an inner mandrel of upper and lower sections, a sleeve surrounding the mandrel and slidable thereon, a splined drive connection between the sleeve and mandrel with drive keys interposed between all drive surfaces of the spline drive, cartridge seals sealing above and below the spline drive and also defining upper and lower balance chambers between said mandrel and said sleeve, a port through said sleeve communicating with one of said chambers, a port through said mandrel communicating with the other of said chambers, a keyed lock ring having teeth to engage teeth on the upper end of the lower mandrel section and keyed to engage in the spline key ways of the upper mandrel section, a lock ring threaded on the lower end of the upper mandrel section to retain the keyed lock ring in engagement with the lower section teeth to prevent unthreading of the lower mandrel from the upper mandrel and a pressure equalizing valve communicating with the spline drive chambe