Unions have two types of through holes. One type lists a “bore” which is the inner diameter of the hole in the wall in the center of the device from one end to the other of the wall.

The other type will not have a wall in the middle of the device and is considered a “butt to butt” connection for which there is no bore. In these types of devices, one end of the tubing lines up with another tubing’s end, hence butt to butt. In this case, there will not be a bore size listed.

The unions and adapters that have a bore listed, produces a very small dead volume and the butt to butt does not.