Finally starting on the heater install and have a question on where to install the "hot" hose. I have a sensor on the top where Al installed his so am thinking the location Jester shows in the pics here would make the most sense.

The problem is that the bolt at that location has a square head and nothing in my tool box fits it. I thought just a 3/8" extension would work but it's a little too big. Ideas? Maybe they did that so guys like me don't remove it?