Bi-Metal Hole Saw with Locking Quick Change Arbor: 3.5 in.
The correct size hole saw enables the plastic sleeve to fit perfectly through the wall. Too big of a hole will allow more chances for outside air to enter the structure. Too small of a hole, the sleeve wont fit through the wall which will expose the lineset stub to come in contact with sharp wood which could cut the supplied insulation around the line set. Also the lineguard (MLG45) is designed to fit into the plastic wall sleeve.