Hi again,
There are three sets of drill bits: Fraction, Letter, and Number. They are not duplicate sizes. You use a different size drill when tapping holes and you want the threads to have the quality you want. Fine threads or different percent of contact between bolt and the hole you are tapping. I will try to attach a Tap Drill Chart to this posting if unable to do this I will email it to you. When setting up your drill press, chuck up a drill bit that just fits your coupling and lower it into the coupling with the drill press turned off then clamp down your work piece. Remove the test bit and install your #8 bit and drill your aligned hole. I hope this will get you on the water sooner. I hope to post some photos of my project as soon as I figure out how to get them from the camera to the PC. These old dogs are hard to teach. Chuck