I'm drilling my sump, I need 42mm bulkheads according to the chart on marine depot. do you know where I can get them? I've been to makuman, sun-sea, and a plumbing hardware strore near my house. I'm sure they have them, but I dont know what to ask for. I orderer my bits online for a great price, they should be here anyday. What is the differnce between double threaded and standard bulkheads?