This one?https://www.gatewaytoairguns.org/GTA/index.php?topic=127547.20
I use a milling attachment in my lathe.... The end mill is in the headstock chuck, and the valve is mounted in the milling vice and adjusted to an angle of 20 degrees to the vertical.... I mill it out with a 5/32" end mill....Bob
I only mentioned modifying a drill bit because if you live in a small town (like I do), you can't buy an end mill.... anywhere.... .... so it's a work-around....Bob