Abstract: The cylinder of a fluid actuated, extremely high pressure ram has an insert in its interior surface which carries a packing seal. The interface between the insert and the cylinder portion which receives the insert can also receive the pressurizing fluid which operates the ram so that the insert inner diameter will not change as the fluid pressure is increased to operate the ram and so that the gap between the interior diameter of the insert and the ram will not change even though the main cylinder body has its interior diameter increased due to the ram operating pressure.