Air driven pump units designed and constructed to be fail safe, user friendly, lightweight and compact. They work on a simple but efficient principle, power is magnified through differential piston areas. A large pneumatic piston drives a smaller hydraulic piston, which provides fluid flow at high pressure. Pump assemblies can be supplied in many combinations, a choice of air motors and pressure gauges are available to ensure that the correct assembly is specified to meet the bolt tensioning requirements. The pump is fitted with a safety on off valve.