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These cars use a non vented cap because the fuel tank itself is vented. If you look at the filler neck of the non station wagon 66-67 cars you are aware of how low the opening is compared to other cars. If the cap is replaced with a vented cap and the fuel level is high or under hard acceleration fuel will escape through the vent hole onto the ground.

Ford designed a vent system for these fuel tanks that rises several inches above the level of the filler neck in order to prevent fuel spillage. The vent is a 1/4" steel line that comes off the filler neck at the tank and rises under the trunk seal lip extending to the right side of the car behind the tail light assembly. Once in the tail light area the vent tube makes a "U" turn upward and then straight down through the floor of the trunk. The end of the vent tube is just ahead of the rear bumper and extending from the truck floor. Open at the end you may ask how this tube keeps out dirt and moister from the fuel tank? Ford knew this could be a problem and installed a small check valve in the vent line, being just above the trunk floor along the right side of the filler tube. Because the end of the vent tube is exposed, it can cause problems that mimic a bad fuel cap. If plugged with dirt the tank can not release its pressure build up and will push fuel out around the cap. Inspect the vent for debris build up, in my area of the country we have a little critter know as a "mud dauber" that loves to built egg case storage units in open vent lines.

Most auto parts stores have a very limited amount of information about old cars. Many of the discount parts places only have a computer data base that covers back to 1980. The professional stores will have paperback catalogs that extend their capability back into the twenties. The main problem with the 66-67 fuel cap is that in all the listings there is a "black hole" saying nothing is available to replace them with. If you are a concourse car owner than the reproduction caps from Autokrafters are the way to go. The correct replacement cap will have the wings on top to grip it with while turning on and off. If you just want your everyday driver/cruiser to stop leaking fuel there are some other choices. Stant Manufacturing Inc., the leading maker of thermostats, fuel, and radiator caps offers a part number 10702 that will perform as required for our 66-67 cars. This cap does not have the wings on top to grip it, only having a knurled outside diameter. Being chrome plated is another plus that I like with this number. Another number for the same cap is the NAPA 703-1064 and should your parts man not be able to cross these numbers over to his brand, ask him for one of these caps: 71-76 E-Van

73-76 F-pickup with style side bed

71-76 Bronco

These caps have the same engagement depth and locking lug diameter to seal properly.

I hope this clears up some of the mystery about 66-67 fuel caps for you. The last thing I would like to add is that I learn about this type of thing by going to my parts man and asking to go through the books to find things that fit and work properly for my needs.

Most parts catalogs contain small drawing with sizes to help find what you are looking for. Many professional parts stores change their catalogs at least once a year, try to work up to a relationship with the parts man and he will sometimes give you the old paper catalogs rather than throw them away. Your professional auto parts store is worth the small extra cost in their skill and knowledge alone.