Heater Union Banjo Bolt Thread

[frax]

I was wondering if anybody knew the thread style(?) size of the heater union banjo bolt. I need to know for a tap to clean up the threads in the head. If anyone has anything please let me know.

 

[sneakypete]

not sure of the threads, but ive never seen a tap that big..... but im sure you can get one special order, its gonna be very expensive
are the threads striped?
-pete