I recently ordered a pair of used ball lock Cornelius kegs from a website. One of the posts of one of the kegs is slightly bigger than the other. In order to get the beer out line onto the keg in the first place, I needed a mallet. I could not get the out line off even with a monkey wrench and a screwdriver. I eventually chopped the ball lock connector in half with a dremel tool.
The problem post is 15.81 mm. I was able to measure 4 other posts, on my various other kegs. They measure 15.53mm, 15.63mm, 15.47mm, 15.83mm. This is about 0.3mm of difference between the biggest and smallest parts. I have never put a connector on the 15.83mm post.
For my day job, I am an engineer. When two parts are supposed to snap together, it is important to machine those parts to a precise size, or you will find yourself jamming things together and prying them apart. 0.3 mm sounds very small, but it makes a huge difference in whether two parts can fit.
On my old keg, I have always had trouble with one of the posts. I always have to fight to get connectors on and off of it. I suspect that I have an issue with an incorrectly sized post or an incorrectly sized connector.
How big is a ball lock post supposed to be? Are sticky, difficult connectors a common problem?