This is a valve case with a servo adapter to create an adjustable valve for air / water with a servo, you may have to loosen the tension on the valve before mounting the servo if you have a valve like mine. There is a bolt inside to take it apart, you just loosen it a little so the servo can easaly opereate it. http://images.biltema.com/PAXToImageService.svc/product/large/2000017915 This is the kind valve i am using, the outer dimentions is 21mm on the valve. http://i.ebayimg.com/00/s/MTE3NFgxNjAw/z/H2sAAOSwzrxUsJR7/$_12.JPG This is the kind of servo im using.