heyas; you want to make the sleeve of the piston a child of the body, and the bar of the piston a child of the lower leg. make sure they stick out at right angles (along the x-axis), make sure you have the proper rotation ordres (xyz), and then tell the piston sleeve and the piston rod to "point at" each other. that should work fairly well. make sure your rod is long enough so it doesn't fall out of the sleeve when you extend the leg.