This Monument Universal Radiator & Valve Key has tapered ends and are therefore self-restricting. The three profiles make the tool universal, catering for 3/8 in and 10 mm square section radiator vent plugs and bleed screws.
Bayonet or internal lug type radiator valves allow for variations in size, shape, and position. The 12 mm to 14 mm internal hexagon valves are tapered allowing for variations in size.