Model A Engine Noise Video
Attached is a video of a Model A engine with Piston Knock in #4 cylinder. Notice the noise abruptly stops when the #4 spark plug is grounded. The #4 piston and rod were removed and wrist pin and crank clearances were found to be well with in tolerances. Some have suggested the knock is being caused by misaligned (bent) connecting rod.
Grounding the spark plugs with an insulated screw driver is a good method for tacking down Model A engine knocks.
Click the play icon in the middle of the video to start.[youtube=http://www.youtube-nocookie.com/watch?v=JZExk84htyI&w=640&h=400&rel=0&showinfo=0]