The use of racing "billet" fuel doors has become a popular upgrade to both street and off-road vehicles. There are two versions of the billet fuel door available for the Early Bronco. The first is a large billet "door" that covers a recessed stock fuel cap. A great tech article on this style of door is available on Jim Price's website (luvmybronco.classicbroncos.com). The other alternative is an actual billet door and funnel mounted to the fender.      This tech article will guide you through the installation of the Mocal Aero 300 fuel filler door for the main and auxiliary tanks on 73'-76' Bronco's. This racing quality British made door is used as standard equipment on the McClaren, Ferrari, and Jaguar. The forged aluminum cap sits in a pressed aluminum flange sealed with a fuel resistant o-ring. Cap and flange are polished and anodized silver (colors available as special order). Caps are supplied with nuts, bolts, and fiber gasket. Locking version includes two keys. I used the locking version with matching keys for both doors (available upon request). Non-locking doors are also available at a lower cost.      To start the install, first remove the left rear wheel. It will make the install much easier. Unclamp and remove the main and overflow hoses between the filler tubes and the tanks. Now remove the three retaining screws at the filler neck and the retaining bolt at the bottom of the fender. The filler tube should now drop out through the bottom of the rear fender.           Next use a metal burr bit to slightly ream out the existing filler hole to fit the larger Aero Filler neck. Be careful not to cut past the existing screw holes. It will not take much, so test fit as you cut.       After reaming out the hole to fit the Aero filler, mark your mounting holes. With one hole positioned at twelve o'clock, the ten, two, and six o'clock holes will almost align perfectly. Drill out the 12, 4, and 8 o'clock holes first, then just mark and ream out the existing holes as seen in the photo.      Now take out your trusty sawzall with a long metal bit and notch out the ¾" inside fender lip to allow the 90deg hose room to fit back into the fender well. Be careful not to cut through the fender while you are notching out the back plate.       Once you cut two to three notches, hammer the lip up flush with the fender well top.           Now you can install the Aero 300 funnels in the fender. Do not fully tighten down the bolts at this time. Take your stock filler tubes, at this time, and prep them for final installation. As you can see in the picture, the main filler tube is cut right after the weld of the upper and lower section. The auxiliary tube is cut a little lower down the neck. You will need to cut 1 " off the 90deg hose that you will attach to the main filler tube. Note: If you own a 66'-72' the overflow tube attaches higher on the filler neck. You will need to cut the hose more or move the overflow tubes further down.       If you wish to vent your system, you will need to install ports into your filler tubes. I used ¼ " barbed to 1/8 " threaded npt brass fittings. I drilled an 11/32" hole just to the right of the overflow tube on the main filler and just to the left of the overflow on the auxiliary tube. These locations will allow the tubes clearance in the fender wells. I reamed the hole out enough to thread the fitting into the tube. After screwing the brass fittings in, I used some JB Weld to secure the fittings a little better to the tube.       After installing the vent fittings, finish the assembly of your filler tubes. It is a tight fit, but the 90deg elbows will go over the filler tubes. A little soap solution will help. Be careful and wear gloves to prevent slicing your fingers on the filler tube. You should be able to slide the elbow down the tube approximately 1½" inches, almost to the top of the overflow tube, by working it back and forth. If you can't get it to go, slightly notch the top of the filler tube in 4 places and bend in slightly. Tighten down with 2 ½" hose clamps. Attach 3ft of 7/32" vacuum hose to the auxiliary pipe and about 1ft of hose to the main pipe.       You will find the Aero 300 filler neck has a two step barbed end. The first step is appx. 2" and the second set of barbs are 2 1/4" O.D.. The 90deg elbow (1 13/16" I.D.) will not go over the second set of barbs without notching the rubber elbow about ¼" in four to six spots around the edge of the elbow. Due to the aero mounting bolts, the rubber elbow will not slide all the way on the barbed base.       It's now time to reinstall the stock filler tubes. Slide a second loose hose clamp on the elbow to use on the aero 300 fitting. Carefully slide the tube up the inside fender into the notch you cut earlier in the fender well. If the 90deg. elbow does not align up correctly remove the filler tube and adjust the elbow to the right direction. This should be about in line with the overflow tube on the main and slightly to the right of the auxiliary. Coat the elbow and aero fitting with some soap solution and work the elbow on to the aero fitting mounted in the fender. You can now fully tighten down the allen screws on the Aero 300 door and firm up and clamp the elbow to the main fitting.            Before clamping the lower pipe support back in, connect new main fuel and overflow hoses. I found both hoses by the foot at my local NAPA store. The main hose is 1 ¾" I.D. and the overflow is 19/32" I.D.            Now route both vent tubes over the aero filler necks to a ¼" plastic barbed "Y". I connected the "Y" to a ¼" plastic fuel filter to catch any slosh or overflow. The output of the fuel filter runs over the rear side marker lamp, down the rear fender and out the back of the car. I also attached a check valve in the end of the vent hose to prevent debris or water from entering the hose. Make sure the vent does not exit around an exhaust pipe.       I have found the aero doors to work fine in daily use. You will notice that you have to feed the gasoline pump handle fully into the elbow or gas will slosh back out of the neck. Also stop filling after the pump handle kicks off the first time. Do not top off repeatedly or you will flood the filter and vent hose. I am looking into fabricating an attaching chain, (similar to what boats use) so that I won't accidentally leave a $90.00 gas cap at the gas station. Questions? Email John@edgecomb.net Tech article by John Edgecomb (edge4)