Tank adapter designed to connect a 3/4" or 1/2" NPT pipe fitting to a rain barrel.

A few years back I converted a used 220 litre pickle barrel into a rain barrel.
As my vegetable garden has grown, a single barrel of water is not enough to get through more than a few days without rain. The simple solution was to add more barrels.

After purchasing two more barrels, I discovered that the tank adapter was no longer available at my local hardware store and the cost was $45 or more on Amazon.
Since I needed two adapters for each of the new barrels plus one for the existing barrel, the cost was quickly becoming prohibitive.

After a few (many) hours with OpenSCAD and a few hundred metres of filament for tests and prototypes, here is a serviceable tank adapter. Cost is under $5.

You will need to print (in PETG or ABS):
1 x tank_adapter
1 x tank_adapter_nut
and (in TPU):
1 x tank_adapter_seal

The tank adapter and nut are best printed in PETG. It has excellent layer adhesion (cut back or turn off the part cooling fan while printing), durability and heat tolerance. ABS can also be used. Because the material properties and shrinkage are a bit different from PETG, there is a version of the tank adapter nut especially for ABS. Just make sure you print both the tank adapter and the nut with the same material.

The seal requires flexible material. I used black TPU.

The tank adapter comes in two sizes. One for 3/4" NPT pipe fittings and one for 1/2". Both use the same nut and seal.
Use a 1 5/8" hole saw to make the hole in the barrel.
It is designed for a wall thickness up to about 1/4".

Installation is straight forward.

1. Place the seal over the threads on the tank adapter. Ensure that the bevelled side of the seal is facing the bevelled surface of the adapter.
2. Apply Teflon plumbers tape to the adapter threads. This helps ensure a good seal and also helps prevents the threads from binding or jamming.
3. Push the tank adapter through the hole in the barrel FROM THE INSIDE.
4. Carefully holding the adapter, place the nut onto the threads and tighten BY HAND.
5. Apply Teflon plumbers tape to the threads on the faucet (tap) or other NPT fitting.
6. Screw the fitting into the tank adapter. Again, only tighten by hand. Do not over tighten, remember that PETG or ABS is a lot softer than the brass or steel the faucet is made from.

CAUTION: HAND TIGHTEN ONLY
This should be sufficient to form a water tight seal. Wetting the seal (or the barrel wall) before installation helps. If there is still some seepage, leave the parts for 24 hours to settle in and tighten again by hand.
Using a wrench, spanner or other leverage on the nut or adapter should not be necessary and may weaken the adapter. Be very careful if you do.

NPT thread library from https://dkprojects.net/openscad-threads/