If an inlet on your rainwater harvesting tank is too small, what do you do?  Turns out, you can drill out a larger hole "over" the smaller hole with a little ingenuity.  When a hole is drilled in a tank the first time, typically you drill a pilot hole, and then use a hole saw mounted on an arbor.  The hole saw arbor has a center guide drill bit that follows through the pilot hole.  Well, lets say that hole fits a 2" bulk head adapter, but you need a 3" bulkhead adapter to manage the volume entering the tank.  The following pictures give you an idea on how to do this making a template out of an old piece of plastic, attaching it to the tank, and using it to guide the hole saw with a new pilot hole.  I use an old plastic food grade barrel to practice on before I do this at a customer site.  Holes saws are dangerous tools so please take precautions to use it in a safe manner!

Another way is to cut out a wooden template the size of your desired hole and use a hot glue gun to attach it to the tank over the old smaller hole.  Wait for the glue to dry and then cut the new hole with your hole saw.  See the pictures below.  In this case Daniel re-cut a 3" hole that was fitted with a 1.5" bulk head adapter into a 3.75" hole for a 3" multitite gasket.