This differential pressure tester has a .060" orifice and is intended for use with engines whose bore diameters exceed 5" per AC43.13 REV 1B. However, Lycoming and Continental service specifications call for the use of a standard tester (with a .040" orifice) even if their engine’s bore diameters exceed 5", and for those engines you must comply with the manufacturer's specifications and use a standard tester such as the 2EM instead of this unit.