Abstract: A rain cap system includes a rain cap and an adapter to mount the rain cap to multiple types of gas flues or pipes. The adapter includes a first flange to mount to the rain cap and an adjustable mounting portion to mount to the gas flue. The adjustable mounting portion includes first and second moveable links having first and second surfaces with first and second edges and first and second interfacing portions, wherein the first and second edges define an opening of the adapter. The adapter includes an adjustment portion to move the first and second moveable links towards one another to decrease the size of the opening of the adapter. The adjustment portion is configured to move the first and second links away from one another to increase the size of the opening of the adapter.