55.2-59 12 bolt rear end filler plug

[Iceman_70]

My 56 stepside came with a 12 bolt rear end. I’ve rebuild it some time ago, and today I wanted to finally clean it, put the gasket and fill it up. Problem is that I can’t find back the filler plug... I even don’t remember if I lost it, misplaced it or if there even was a plug when I bought the truck...

I measured the hole (3/4 inch) and google told me I need a 1/2 NPT plug. To verify I took the 3/8" NPT to 1/2" NPT adapter that came with my water temp sensor, but great was my surprise that it didn’t fit. The threads in the filler hole look finer then those on the adapter... Or there different 1/2 NPT sizes ?
I keep on having difficulties with these inch measurements... Can someone please clarify what I need, or how I can make sure I buy/order the right part. Don’t have a lot of specialized shops in Belgium, so I mainly order these parts in the US.