Our post top brackets fit on a 4.5 inch diameter post. The post top fitting is a sturdy cast aluminum with six stainless steel set screws. With a combination of extruded aluminum arms and elbows, you can mount from one to four signal heads. In configurations for mounting multiple signal heads, there will be a centre support arm that is not threaded at the top. You will have to specify the number and size of the signal head sections so that we can provide a centre arm of the correct length.