This is still unfinished, but it has the animation in place for pistons moving on the end of con rods, moved by a camshaft.
I have seen a few posts asking how trhis is done, so here is a small 242kb divx5 anim and the 88KB zip file containing the .blend.
I hope this is useful to somebody.
I used parenting and track To constraints, no armatures, just Key frames for the camshaft rotation.
Click for anim of working engine, divx v5 242KB
Blendfile at the end of this link, 88KB zipfile.
Could do a tutorial if this method works for you guys.
Sonix.,