I vaguely recall that I was able to drain them in place, by loosening the plug for the operating valve. But I also seem to recall deciding that next time I would remove them from the frame. You have to disconnect the arm anyway, to work the pistons (both to expel the old oil and to work the new oil into the cylinders). After that, it's only two bolts to remove them, so you can properly clean them (and not take a chance on introducing dirt).