I have a couple of rain barrels to help with watering the garden. They are old food barrels, and the openings on the top are about 2" in diameter, threaded. I use a siphon pump from the auto store to pull water out when I need it.

What I would like to do is rig it so that the pump can stay in the hole all the time. Unfortunately, it has a much smaller and finer thread, so it cannot be screwed in (which is probably to stop someone from hooking a fuel line to a food line, which makes a lot of sense). And it needs to be a tight seal, or we are going to breed mosquitoes. It also has to be removable to clean the barrel.

So, does anybody have any suggestions on how to match the food-grade thread? Here's the catch -- I don't have a lathe, otherwise I would experiment. Any threading taps I have seen are nowhere near big enough.

Thanks in advance,

Chris