The invention relate generally to a tightening mechanism for a bottle filling and capping system, and a bottle neck configuration. More specifically, the invention includes a machine for tightening plastic caps onto the threaded necks of plastic bottles, and bottle neck configuration that facilitates the capping process, both of which result in a simple and effective capping operation. A conveying assembly moves the bottle in a circular path where the neck of the bottle and the cap are brought together by moving the bottle under a cap that is loosely held at a particular elevation and angle by a feeder mechanism. The conveying system constricts the rotational movement of the bottle, and a neck support stabilizes the neck of the bottle vertically, as the cap is pressed onto the neck initially by a roller. A capping station in a bottle filling line is comprised of a spring biased toothed finger that engage the peripheral edge of the caps as the bottles are conveyed through the machine. Depending on various factors such as the number and circumferential length of the threads on the cap and bottle a second roller and second toothed finger will be used to complete the tightening process. When a capped bottle exits the capping station, the cap is fully engaged with the bottle in that it is tightened and in its final position relative to the bottle.