An intensifier unit includes a pair of sequentially operated reciprocating ram assemblies, each powered by a cylinder motor. Extension of the cylinder rods is accomplished by application of pressurized hydraulic fluid to the pistons. The pistons are returned quicker than they are extended by means of continuous application of pressurized air to the back side of the pistons. The cylinder motors are precompressed prior to their respective forward strokes by means of an accumulator which continuously bleeds from the main hydraulic fluid supply line. Circuitry is provided to insure that precompression continues even if the ram and its driving piston move from their retracted position. Also disclosed is a safety interlock to shut down the intensifier if both cylinder rods become accidentally simultaneously extended.