A lock will keep kids’ hands out of games, and away from various high voltages.
A coin door lock is a really good idea, even in home use. Not only does the door look silly without them, they are a safety feature.
Any 7/8″ lock will work.
Coin door locks are a 7/8″ lock that installs easily with a 7/8″ deep-wall socket (a wrench isn’t important, just use the socket) or a 7/8″ wrench. Most locks also use a Phillips screwdriver to mount the cam on the inside of the door.
Pinball Resource sells their 1382 lock. That’s a great lock (when it’s in stock). Marco sells a different, good lock. Pinball Life has a nice double-bitted lock. The important thing about a “standard” lock, is that you can get a replacement key if (when) you lose it.
The other useful property is that the lock should work smoothly; it’s just less frustrating.
If you have multiple games, key them alike. It saves trouble of trying to find the right key. Conveniently, all of the links above are for key-alike locks. Pick a parts seller and get all your locks there. (I don’t re-key heads or back doors; just keep the extra keys in the game on the hook on or near the coin door intended for that purpose.)
If you have only one game, and you get a lock that comes with two keys, screw one key into the bottom of the pinball. For video games, consider taping it to the top of the cabinet in a discreet location.
If you lose all the keys, and you can’t pick a lock, you will have to drill the lock. The goal is to get a drill bit down the center of the lock and ruin the screw at the other end so the cam falls off and the door can be opened. Then vacuum up the mess, put a new $5 lock on the game, and screw a duplicate key into the cabinet for next time.
Other locks: 5/8″ and 1 1/8″ locks
Pinball head locks tend to be 5/8 (’90s Williams, Bally, and all Stern/Data East/Sega). These have a nut on the outside to hold the cam on. (Older games took other lock types; 7/8″ is most common.)
Video back doors tend to be 1 1/8″ or 7/8″ depending on the exact back door. As a rule, if the lock is recessed it’s 7/8″, and if it isn’t it’s 1 1/8″ (assuming a 3/4″ wood back door). Having said this, your cabinet is probably a special snowflake, and I apologize in advance.
I don’t recommend replacing these unless you don’t have a lock, or you don’t have a key for the lock.
Storing (Spare) Keys
If you have a spare key to the coin door lock, it’s a good idea to screw it into the underside of the pinball cabinet.
Back door keys (or head keys) live on a hook inside the game. Sometimes spares are screwed to the coin door. This hook is present on almost all pinball doors. On newer games (’90s) it’s usually a hook on the coin mechanisms. On older games (’80s) it is frequently a hook on the side of the cabinet, or near the leg bracket.
Bending the Cam
The “cam” is the flat bit that holds the door shut. These usually need a little bit of persuasion for a nice tight door fit. Remove the cam from the lock, get a couple big wrenches of whatever type is handy, and put a little bend in the cam.
Bob Roberts has a much more comprehensive writeup, in particular an absolute menagerie of the available cams for video games.