Originally Posted by Sludgeroo
For indexing (great idea by the way), I think you would actually put the black line opposite the side where the j strap meets the plug. You would put it on the gap side, that way when the black line is at 12 o clock (perpendicular to the ground) the gap will face comb chamber.
I guess it depends on where you want the flame front facing. I'm not very familiar with the geometry of the Subaru engine yet.
Here are some examples:
Those pics have lines on plugs opposite of each other so depends of engine design.
Here is a Google search for spark plug indexing:
I learned of the concept from a Big Daddy Don Garlits interview on tv
about 20 years ago.