Blow-off valve for a hydraulic dashpot

A blow-off valve for a hydraulic dashpot comprising a fluid charged cylinder ( 1 ) and a piston ( 3 ) that travels back and forth therein on the end of a piston rod. The piston partitions the cylinder into two compartments ( 6 & 7 ) and is provided with breaches ( 4 ) and shock-absorbing valves. The piston rod ( 2 ) or an extension ( 8 ) thereof is hollow and accommodates operating components ( 13 ). The object is a valve that can be accommodated along with the hydraulic lines that serve it outside the piston rod or an extension thereof. The outer surface of the piston rod is accordingly provided with at least one axial inwardly undulating groove ( 14 ) at least as long as the piston, including its fastenings and operating components, is high. Alternatively, the bore that extends through the piston rod or its extension and its fastenings and operating components is provided with a similar radially outward groove that can be closed off at the compression end by a spring-loaded valve.