Step 3: Converting the Drain to Garden Hose Connection
Rather than use a number of PVC pipe adapters to get down to a garden hose, we reused the 2" cap and some spare plumbing parts we had kicking around.
To start with, we had a brass spigot with 3/4" NPT (National Pipe Thread) male connection on it. Standard threaded pipe is tapered, so the farther you screw it in, the bigger it gets. This helps make solid, water-tight connections.
We drilled a 3/4" hole through the center of the 2" drain cap.
Next, we stuck the pipe end of the brass spigot through the cap, added some sealant at the joint, and then threaded on a nut from the back side. Since the hole was the right size to start with, the sealant and tightened nut made a solid connection on the cap.
We then headed back outside. Using plumbers putty (you could also use teflon tape) we threaded the drain cover with spigot onto the drain.
We decided to have the brass spigot rotated clockwise part-way, because the open big drain valve made it harder to grab the knob on the spigot.
Then we connected a garden hose, ran it down-hill, and tested our flow from the rainwater collection system.