It's a fairly common method of cleaning carbon build up on valves and much easier than having to use chemicals and manually agitating with different sized brushes.Gotta admit I've never heard of this process. Why would you do this? Better flow after blasting, valves carboned/gunked up?
I "think" its 21mm cant remember which isn't much help to you. You can easily see the valves close as you turn the engine over manually looking down the inlet.