T-3 Headlight Aiming Procedure (Screen Method)
Background: This procedure supplements the GM headlight aiming procedure in that it enables the user to compensate for an aiming "range" that is not level. Since most driveways and garage floors are pitched to shed water, this procedure should help the typical DIY user when a perfectly level surface is not available. This procedure also provides a set-up method for ensuring that the car is "square" to the aiming screen in the absence of a helper. Please read through the whole procedure since there is one "preparation" step involved to complete Step 3.
Tools / Materials:
25' Measuring Tape
Sheetrock or Carpenter's Square
2 Sawhorses or equivalent
Laser Level (optional but worth it)
String Level
Yardstick
Chalk line
Wheel Chocks
Floor Jack
Scissors or Bottle Jack
2 foot Level
1 foot Level
Sheet of Cardboard 72" x 36" (a 24" packing box unfolded works well)
Sharpie Pen
Duct Tape
The aiming screen method is provided in the Chevrolet Passenger Car Shop Manual as illustrated below:
This method is pretty self-evident except that it assumes that the surface the of the "aiming range" is level over 25 feet plus the wheelbase of the car. It also assumes that the car is "square" to the aiming screen. Rather than searching for a site this level, one can follow 3 steps to compensate for the conditions you are more likely to have at your home.
Step 1: Correct for Driveway Pitch
If you are using your garage door or the front wall of your garage for your aiming screen, you must account for where the horizontal centerline of the headlights is, relative to the screen when the car is 25' away from the screen. On my 1961 Corvette, the centerline of the headlights (distance X in the above diagram) is 27.5 inches above the ground. However, my driveway and garage floor are pitched at about 1.1 degree to shed water so that when my car is 25' back from where I placed the screen, a line projected level from the center of the headlight to the screen actually hits the screen at 21.5 inches above the floor.
To find this correction, I attached a laser level to a yardstick at 27.5 inches above the ground. I taped the yardstick to a sawhorse sited 25' from the screen and used the laser spot to find the apparent horizontal centerline on the screen (i.e. 21.5 inches above the floor.) You could also do this with a mason's string level.
Once you have the apparent centerline height, use that for the dimension "X" when you make your screen.
Step 2: Centering and Squaring Up the Car
In order to aim the headlights, the car must be "square" to the screen. If you have a helper, this is fairly simple. If not, you need a reference that lets you pull up to your 25' mark square to the screen. To do this set up a "visual range." I used 2 sawhorses with driveway markers taped to them (call these markers) and my laser level. First you need to establish a line that is perpendicular to the screen. You can do that with a sheetrock square and a chalk line or with a laser level. Next align your two markers along the perpendicular line about 10 feet apart. Now you can drive up to the 25' mark using the two markers in front of you to align the car square to the screen. Since your vantage point in the car is not on the centerline of the screen, this only gets you "square." To center-up, you simply move the screen left or right to center it on the car.
I used some wheel chocks to spot the car at 25' from the screen. Pull up to where the headlights are near the 25' line, place the chocks then roll up to the spot.
Step 3: Correct for Car Attitude
Since the driveway is pitched, the car sits with a "nose up" attitude. While the car is still sitting in the garage, find a spot on the car where the surface is essentially parallel to the floor and where you can rest a 2 foot level. I tried the door sill but actually had better luck with a spot on the front fender, 3 inches outboard from the hood and about 1.25 inches forward from the back edge of the hood. Since you will be jacking up the rear end, you may also need a spot for "cross level." On my car that was the cowl vent.
After you have pulled up to the 25' line, place your 2 foot level on the spot you identified previously. Use a soft cloth under the level to protect your paint. Do the same thing with the cross level spot using a 1 foot level. Chock the front wheels then put a floor jack under the differential housing and raise the rear of the car until the level shows that it is level. Since it may not be level from side to side, put another jack under the low side of the frame midway between the wheels to adjust the car so that it is level from side to side. Now your car is sitting as if on a level surface, 25 feet from the screen and you can follow the Chevrolet aiming procedure as shown above. You could also do Step 3 with a laser level to "shoot" a level line through the center of the front wheel and jack the car to raise the center of the rear wheel to the line.