I 'think' 8 parts is the min... though the more I think about it the less sure I am... Either the combo shown here, or instead of a second piston and anime, you can just have the other two inputs, though that means one arm and slower... which I did try... it wasn't that much slower; only about 100 cycles or so, which leads me to think that 2 arms could be faster than this... meh!
