They have a small .040" hole for venting with a replaceable brass vent fitting installed on the inside to reduce liquid sepage. These filler necks are used commonly for gas tanks, oil tanks that need to be vented, or any other fluid container that does not need to be completely sealed to the atmosphere. It's small diameter keeps it from drawing to much attention but the filler hole is plenty large enough to fill with any type of nozzle.

• 1.250 x 12 threaded bung
• 1.50 O.D. Aluminum cap
• Cap seals against the bung by an O-ring seal that is replaceable
• The O.D.of the flange on the bung is 1.75" and the stepped down section measures 1.5"
• Weld-In Bung available in Mild Steel 6061-t6 Aluminum or 304 Stainless Steel