Abstract: An adjustable wrench assembly for installing and removing a threaded cap member of an hydraulic cylinder assembly includes a head member adapted to partially surround the piston rod of the cylinder assembly. A cover plate attaches to the head member to completely surround the piston rod. A plurality of adjustable contact pins accommodate various sizes of piston rods and are engagable with the piston rod to keep the wrench square and prevent it from twisting. Various sizes of adapter plates are connectable to the head member and include a plurality of engagable pins to engage mating slots or holes in the cap member. The adjustable contact pins include non-metallic plastic contact portions to prevent marring of the piston rod.