Abstract: The adapter extension of subject invention provides for the coaxial extension of a plunger or rod whose effective length has become shortened by weakness or failure of a back-up spring which causes the plunger or rod to lose its effectiveness as a strut, for example, a situation that occurs primarily in nuclear reactors where the plunger engages a grid, whereas the grid provides lateral support to the upper end of the dry tube. The adapter is thimble-like and fits tightly over the end of the plunger that engages the grid. The adapter contains a spring which has the effect of substituting for or supplementing the original spring behind or below the plunger. The body of the adapter includes a retainer for holding the interior spring captive during assembly and operation of the adapter.