A Cornelius keg or Corny keg is a type of keg originally manufactured by the Cornelius company for use in the soda industry. Now obsolete, they are easy to find used and popular with home brewers. These kegs are generally preferred by home brewers because they are well suited for storage in converted refrigerator or freezer kegerators and hold the typical 5 gallon batch of beer. These kegs can be found in used condition (often filled with residual CO2 pressure and a bit of old syrup) for a few dollars more than their worth in stainless steel scrap. 2 and 3 gallon versions are also available, although they are rarer than full-sized 5-gallon kegs.
Two different disconnect/coupling systems (ball-lock and pin-lock) were widely used and are NOT interchangable. The ball lock style is most commonly used by homebrewers and, unless stated, all information here is for ball lock kegs. There were three primary manufacturers of these kegs; Firestone, Cornelius, and John Wood.
 Keg Anatomy
 Posts and Poppets
Ball Lock kegs were originally used by Pepsi while pin lock kegs were originally used by Coca Cola. This refers to the means by which the tubing is connected to the keg. Ball lock connectors have several small balls on the quick-connect which hold the post into the connector. Pin lock kegs have pins on the keg post which mate with the collar of the disconnect.
All posts have an internal poppet valve, which is a spring with a small O-ring seal on top. The poppet prevents the pressurized liquid from flowing while the tubing is disconnected. Poppets from different styles of kegs are not interchangable since the springs will be of different lengths based on the height of the post, the heights of the keg post to which the ball lock post is threaded, and the presence or absence of a plastic insert between the dip tube and the poppet.
The end of each post has a rubber 7/16"ID x 5/8"OD O-ring
 Lids and Relief Valves
The keg will have a lid which will either be egg-shaped or more squared. The later is known as racetrack lids and usually have a flat oval gasket. The more common lids have a 3 1/2"ID x 4"OD O-ring gasket.
There have been three major types of relief valves on the keg lids. The oldest type is a metal valve. Next came the dome type relief valve. These were a bulkhead design, meaning the lid has a flat hole in it through witch a threaded piece is inserted from the top, and a collar is threaded on from the inside. The bulkhead has an O-ring seal on the outside of the keg. The O-ring is a #020, 1"OD x 7/8"ID. The most recent style of valve is like a poppet valve. This one threads in from the top and has a spring with a rubber washer on the end to seal. The O-ring is a #007, 9/32 OD x 5/32 ID.
 Dip Tubes
Each keg has two dip tubes, one for gas and one for liquid. The top of each tube is flared, and a 5/16"ID x 1/2"OD O-ring is located beneath the flare.
 Reconditioning Cornelius Kegs
At a minimum, all 5 O-rings should be replaced and the kegs should be filled with hot water/detergent over night (followed by sanitizer) before the first use.