The $40 master cylinder, right? What nut are you talking about? What bore? The ad says it is a 7/8 bore..... So what exactly are you asking? The 7/16 nut that goes where?
Also, please don't start multiple threads asking the same question. It's frowned upon here.....
The majority of us are here to help others. But in order to give the help, we need a clear understanding of what you're asking. Your thread title says bore size, but then you start talking about a 7/16 nut...... I'm lost.....