I am doing an install on a stove requiring a 4 inch output. I have thimble similar to this one, http://www.dynamitebuys.com/store/cart.php?m=product_detail&p=5966 , that has a 2 and 1/2 diameter knockout to be used for an OAK. Can I make this hole larger so as to hookup the 3 inch OAK it calls for? Would I be cutting into the clearance on the thimble, piping? The chimney is PellentVentPro.