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Ball-lock or pin-lock: which Corny keg posts do I have?
Ball-lock posts use a keyed groove and pin-lock posts use physical pins, and their disconnects do not fit each other.
Two families of Cornelius keg
Cornelius (Corny) kegs come in two post styles left over from the soft drink industry. Ball-lock kegs, once used by Pepsi, have smooth posts that a disconnect grabs with internal bearings. Pin-lock kegs, once used by Coca-Cola, have lugs or pins on the post that the disconnect engages by pushing on and twisting.
The two are mechanically incompatible. A ball-lock disconnect will not lock onto a pin-lock post, and vice versa, so the first job is to identify which family your keg belongs to.
Telling gas from liquid
On a ball-lock keg the gas post and the liquid post differ in diameter, and the gas post usually carries notches around its collar so you cannot connect the wrong line. The gas and liquid disconnects are keyed to their posts, which is why a gas disconnect physically will not seat on a liquid post.
On a pin-lock keg the difference is the number of pins: the gas post has two pins and the liquid post has three. Count the pins and you know which is which even when the keg is unlabelled.
Disconnects follow the keg
Because the posts differ, so do the disconnects. Ball-lock disconnects come as a keyed gas half and a keyed liquid half. Pin-lock disconnects are shaped to drop over the pins. If you mix keg styles in one setup you will need both types of disconnect, which is a common reason a new line refuses to connect.
Servicing the posts
Both post styles unscrew for cleaning and hold a poppet and an O-ring inside. Watch the thread size when you buy spares, because Cornelius posts appear in more than one thread and two that look identical can differ. Keep gas and liquid poppets and O-rings matched to their posts so the keyed system keeps doing its job.
Get the right fitting
Open the KegFitFinder toolMatch a disconnect to your postsFind ball lock keg disconnect set on eBayaffiliateRelated
Sources: American Homebrewers Association