Op, be advised, a new 144mm impeller measures exactly that. The housing, the impeller duct (yam's name) is one half mm larger. Translated into inch system: about 0.010" (ten thousands) clearance between imp. and housing. Once the impeller wears- and it will- and the clearance approaches 0.020" ( a half mm), you'll notice propulsion loss and acceleration similar to a slipping automobile clutch. Don't try to have impellers welded for zero clearance - that might be the theoretical ideal in fluid dynamics- but stick to the 0.010", there will always be some wobble there due to vibration.
