This brass plug serves many purposes on various Land Rover models. It's most commonly seen in North America as the radiator fill plug on NAS V8 Land Rovers. However, this plug also serves as the thermostat plug on the 300 Tdi engines, and the fill plug on the Disco2's front and rear differentials. Lots of different Land Rovers from the Series III 109 to the Disco2 used this plug or the plastic form of this plug. Common cross-reference part numbers include ERR4686, FTC5403, and KTP9401.
Unlike the cheesy plastic plug that came with your vehicle from the factory, this brass model will not deteriorate and fail at the worst possible time.
Beware of other brass plugs that do not have the skirt at the outer edge (many do not in order to minimize manufacturing costs).
The skirt is very important because it holds the O-Ring in place and prevents the O-Ring from squeezing out when the plug is tightened.
The hex head size is 13/16", the same size as on most spark plugs. Thus, a common spark plug socket will fit and no special tool is required.
John's Defender 110 takes two brass plugs.
John's 3.9 takes a second brass plug at the heater hose bleeder. Check your vehicle to see how many brass plugs you require.