I installed one of these units and it works well. The one I bought has a plastic drain plug with an O-ring. Unlikely to hold up and looks wrong. I've been unable to find out what the thread is. It doesn't seem to be an NPT, or a metric thread, at least not one available at the hardware stores. I took the plug to Lowes and tried it in their thread examples mounted in the hardware section - both SAE and metric - and it didn't fit any. Also tried NPT fittings over in plumbing, also without luck. I asked the seller what the thread was and he replied "don't worry about it". It doesn't look to be tapered and it is between 1/4 and 3/8 diameter. I'd like to fit an adapter and/or brass petcock. Any ideas?