These enable you to fit an outlet to your tank by giving a standard BSP thread into which you can screw another fitting, e.g. tap or hose tail. They are inserted and screwed into a tank and are secured at the back using the back nut. More detailed instructions can be found below.
The boss has three pieces. The main plastic body that has an external male thread and a female inside thread, a large rubber seal, and a back nut for securing the boss onto the tank. Both the outside male thread and the inside female thread come in BSP sizes. The female thread size is what determines the size of the fitting you can screw into it.
In the drop down menu below a range of sizes are indicated. The larger number is always the male thread (the size of hole you need in the tank) and the female thread is the smaller (the size of fitting that can be fitted).
To fit a boss into a tank, simply drill a hole into the tank where you wish to site it. Please note the hole has to be bigger than the male thread size. The seal will prevent leakages. Make sure that the location you choose can be reached manually as the back nut will need to be tightened.