When I'm using a known good spark plug to test spark at each wire, do I need to ground the threads of the plug to the engine block like the guy in this video is doing?
What happens if I don't ground the threads and just let it jump through the air to the block? I'm asking cause that's what I did once; I could see an ark going from roughly the threads of the plug to the engine block and then also the regular spark at the end of the plug.
Yes you should ground the spark plug. In your case the spark plug was close enough to the block to still jump the gap. If the plug was far enough away from metal nothing would have happened.
On a separate note.
When testing for spark it is recommended to use a spark tester. Using a spark plug is a poor test of the ignition system. In free air the spark plug gap poses a very small load to the coil. The spark plug loads the coil when it is in a compressed mixture of air and fuel. The spark tester is either rated for a specific Kv or is adjustable to different Kv. By adjusting the tester to the Kv rating of the coil and then watching the quality of spark the health of the coil can be assessed. The spark should be bright and blue not dim and orange.
