Check the listing for details. Battery Vent Cap Adapter 12V Volt. Condition: New. Listed at 1.70 USD. Battery Vent Cap Adapters from Flow Systems Vent caps for 4D Batteries, 8D Batteries, 12 Volt Batteries or Marine Batteries are extremely difficult to find in the aftermarket. This Battery Vent Cap Adapter manufactured by Flow Systems, will convert the existing vent cap or vent cap plate on any 12V, 4D, 8D or Marine battery into a more commonly used bayonet style vent cap. This is the push-in version of the adapter. We also offer a screw-in version if your battery's vent hole uses threads to attach the vent to the battery cover. The vent cap adapter works in conjunction with any standard bayonet vent cap manufactured by Flow Systems. Bayonet vent cap sold separately. The orange vent cap in the pictures is there to show you how the adapter works in conjunction with the vent cap. Before ordering the vent cap adapter, please measure first. The diameter of the part that pushes into the battery is 0.76" inches. The hole that the vent cap adapter fits into should be slightly bigger. See pictures for dimensions. Flow Systems vent caps are also available on Ebay. Flow Systems is the largest produced of battery vent caps around the globe. If you have questions about the adapter or batteries in general, send us a note and we would be happy to try and help.