If you are going to fit a drain plug that can be wire locked you will need a 1/2" UNF (20 tpi) with a hex head that can be drilled for locking. Alternatively you can use some Torque Seal varnish with the standard flush fitting plug, look it up on eBay, very useful for checking that nuts and plugs etc. have not moved and helps stop them moving. Just nip the drain plug up, it has an O ring on it and doesn't require a lot of torque to seal.
