rigging help

[made_a_lamp]

I was following this tutorial

http://en.wikibooks.org/wiki/Blender_3D:_Noob_to_Pro/Advanced_Tutorials/Advanced_Animation/example/_Piston%2C_Rod_and_Crank

it all seems to work except for there is a bit of unwanted movement from the rod joint attached to the piston.

Here is my .blend file
http://tinyurl.com/6b4jaw

and here is an .avi vid showing my problem
http://tinyurl.com/6g3nhh

Has anyone tried that tut and been successful?

thanks for help…

[made_a_lamp]

anyone?