the high flow balanced sleeve type of stats like the Robert Shaw 330-xxx have an air bleed in them and an arrow indicating which way it should be installed, usually towards the front rad
it helps burp the system and acts a very small by pass without causing excessive cooling or reduced heating for those that need it
http://static.summitracing.com/globa...0-195_w_ml.jpg
here is one with some holes drilled in it
http://static.summitracing.com/globa...p-300_cp_m.jpg