Cornelius five gallon "soda" kegs are commonly used by home brewers to keg homebrewed beer, and are typically used with kegerators. It is generally easier to keg beer than it is to bottle beer. Used Cornelius kegs are commonly available, quite affordable and very easy to clean and maintain. There are a decent amount of components that make up a Cornelius keg, and whether one is rebuilding a used keg or putting one together from spare parts, it is helpful to know what goes into assembling a Cornelius keg.
Collect the keg components. The components consist of the keg tank, a long liquid dip tube and o-ring, a short gas dip tube and o-ring, one lid and o-ring, a pressure relief valve for the lid, one gas plug (post) with o-ring, one liquid plug (post) with o-ring, and two small poppet valves that fit inside the plugs (posts). There is a total of five o-rings. There are different types of Cornelius kegs, and different brands of similar five gallon soda kegs are generally referred to as Cornelius kegs. Be sure that the components that you have for a specific keg all correspond with a specific keg design. The pin-lock type of keg is generally not used, and it is far more common to find and use the ball-lock variety. If rebuilding a keg, all of the o-rings are usually replaced with new ones, as they can absorb soda odors, and possibly harbor bacteria and the like.
Clean and sanitize all of the components if necessary. There is little reason to assemble a dirty keg! Used kegs usually still have soda residue in and on them, and must be completely and thoroughly cleaned as well as sanitized. Completely disassembled kegs can be soaked overnight in a solution of Powdered Brewery Wash or similar cleanser, scrubbed, rinsed, and then sanitized. Be sure to remove the o-rings during this process, and replace them if necessary. Note that the plug (post) o-rings are not easily removed, so should be kept on during cleaning and sanitizing unless the keg is being completely rebuilt.
Slide the o-ring on the long liquid dip tube and insert the dip tube into the keg through the "out" port. The end of the tube will usually angle in to the center of the bottom of the keg. Some home brewers will cut one half to three quarters of an inch off of the end of the liquid dip tube to prevent the uptake of yeast sediment when dispensing beer. To do this, cut off the desired length with a tubing cutter, a Dremel cutting wheel or hacksaw, then smooth the resulting jagged edge with sandpaper. The dip tube cannot be used right away, as the part that was cut will be prone to rust. Freshly cut stainless steel must go through a process called passivation before it is rust resistant. The rusting can be prevented by cleaning the tube with a cleanser such as Bar Keepers Friend. The easiest method involves cleaning the metal well, then allowing the metal to passivate over the course of a week or so by itself (this happens as it is exposed to the air).
Slide the o-ring on the short gas dip tube and insert the tube into the "in" port.
Install the o-rigs on the plugs (posts) if not already installed and place the small poppet valves in the liquid and gas plugs (posts).
Securely screw the plugs with the poppet valves onto the keg tank over the dip tubes so that a good seal is formed. Use a combination or box wrench or a socket wrench that can properly fit both plugs (posts). Keg plugs (posts) come in different sizes, but many will fit a 7/8" or 11/16" wrench. For common Cornelius kegs, the base of the liquid "out" plug (post) will be hexagonal. The liquid "out" and gas "in" plugs are not interchangeable. The keg plug (post) for the gas inlet will only connect properly to the keg connector (disconnect) that attaches the keg to the gas cylinder, and the liquid "out" post will only connect properly to the keg connector (disconnect) that attaches the keg to the beer faucet.
Fit the large o-ring on the lid, and screw in the pressure relief valve.
Fit the lid in the keg and secure by pushing down the bar that is attached to the lid.