I could be wrong but from what I found with IK you want your main motion piece to be last on the chain....Ie on an arm model you want the hand last on the chain..that way when you move the hand everything else moves with it. If you just move the shoulder everything else will still move but it wont move properly. So for an engine I would think you would want the chain to be piston > Rod > Crank. That way when you move the crank everything else should move. What I am curious about is how other objects can interact...IE can you hollow out a tube for the pistion and have the tube be solid enough to constrain the piston? This could be useful for my truck animation..right now I am just spinning the wheels myself. In the Carrara bible it showed how to use force to do it, but it still didnt do things properly from what I saw.