180 t-stat upgrade

i ordered my 180 tstat today and im just curious they gave me the option to order it drilled or regular i ordered it regular because i dont know what drilled is and id rather not buy something i dont know what it is. but what is the diff in the drilled vs non whats better. and should i have purchased the o ring gasket with thew t-stat. thanks guys

drilled means there are holes drilled so small ammounts of coolant are always flowing thru the system, they are very small holes. Yes you will need a new O ring gasket and a Tstat housing gasket.. it'll make reassembley easy as pie