I would like to test the high pressure fuel pump.
 

I need to attach a pressure gauge to the fuel rail or high pressure pipe. I was going to make a T pipe but I need to know the size of the flair fitting barrel nut. I've googled it to death but no luck. I tried to borrow a kit from Autozone but there did not seem to be any fitting in the kit that were even close to the 12 mm barrel nut in the car. Is the an easier way to tap into the fuel rail? There are a couple of videos but none of them detail how they attached the gauge. Any help is greatly appreciated. Frank

I should mention that I have a 2010 Convertible S. I walnut blasted the valves but the symptoms of running rough and difficulty idling continued.