Don't always want to dive doubles, but your gear is set up for doubles. The Hollis Single Tank Adapter gives you the versatility of single cylinder use when you have a dual cylinder set up. All you need is the adapter and this set of stainless steel bolts with butterfly nuts which make installation and removal a breeze. Small parts can get misplaced or lost, so having a spare set of hardware can get you out of a bind in an emergency. Bolt Kit makes a great Save-A-Dive Kit item. Bolt Kit comes complete with a set (2 each) of the following: Carriage Bolts 5/16"-18 x1.25", Flat Washers 5/16" .75" OD, Lock Washers 5/16" MD and Wing Nuts 5/16"-18. All components are stainless steel for durability and long life. Kit makes a great Save-a-Dive Kit item.