How the shield works. Optics 101. Go find a Mag light or similar flashlight that you can focus the beam. Now this trick will work just the opposite(mirror) of the H4 shield but it will demonstrate the principle. Turn on the light and focus a tight beam on the wall. Now slide a card over one half the lens. As the edge approaches the center line the spot stays a circle. Now slightly defocus the spot so that there is a small dark spot in the middle of the spot (a donut). Now slide the card over the lens notice now that the ring will turns into a semi circle. If the dark side of the semi circle is opposite the shadow then refocus the beam and defocus it in the other direction till there is a dark spot (donut) in the middle. Slide the card over the lens this time the shadow and the dark side of the ring will meet. You are now at perfect cutoff. Have fun with it. So in the H4 system the low beam is NOT at the focus point but it sits just in front of it. So if you added a Spacer (o-ring) to improve the focus to supposedly "improve" cutoff then the shield will not work and your high beam will be down in the dirt, not above the low beam. If you look at any h4 headlight with a halogen bulb in it you should notice that the low beam light only comes from the top half of the reflector. The bottom half of the lens should be dark. Only adjust focus on the H4 system to get the high beam just above the low beam cutoff line. Which is where you want it. Now you know. Q.E.D.