The hole in the middle of the clack, I bore in 11mm with a 7.3mm drill (tapping size of 5/16 X 32) then, bore through with a 1/8" and finish the ball seat with a 7mm endmill - I've never found the need for this business of making the ball seats conically pointed towards the ball - a flat seat and a sharp "donk" with a brass dolly on a spare ball seems to work perfectly well. The seat for the spigot for the boiler is also shaped after drilling the 1/8" hole, with the 7mm endmill.