These boss fittings 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.
They will give you a female thread on one side of the tank and a male thread on the other side where the back nut os screwed up to tighten.
The boss has three pieces. The plastic body with a male thread and female thread, a thick rubber washer to push up against the wide should of the body and a back but. 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.
The usual way you would fit these boss fittings is to have the male thread on the inside of your tank or container. So you would need to tighten up the back nut internally. The washer stays on the outside of the tank on the shoulder of the fitting. You can however fit the boss the other way round so you have the male thread on the outside od the tank. Washer will then be in the inside of the tank still up against the shoulder of the boss.
To fit a boss into a tank, simply drill a hole into the tank where you wish to site it. Push the fitting through the hole and tighten the back nut on the inside the tank. If fitting the other way round or you can't get inside your tank or container drill the correct size hole in the side of the tank and link the opening of the top tank with the hold you just drilled with a length of string. It can be quiet fiddly, might require a bolt on the end of the string to help locate the hole or a large stick to poke it through. Once the two holes are linked with the length of string put the bulkhead fitting threads first with the piece of string going through the female threads, and while making the string taught let go of of the fitting and it should drop down near the hole. Pull the fitting through the hole and screw the backnut on and then tighten. the washer will create the seal on the inside of the tank.
Size of hole required for the boss fittings:
¾ inch Boss - 42 mm
1 inch Boss - 49 mm
1½ inch Boss - 68 mm
The fittings have a very thick shoulder with the washers having a big surface area as well so if your hole is a few mm bigger this should still be fine.