Air Plug Gauges being used to check bearing seats. Bearing fits in castings are critical as they determine the runout of the assembly on the whole. An undersize bore would mean a cramped bearing requiring higher force causing damage to the bearing. An oversize bore would mean a loose fitting bearing which runs the risk of service failure or unwarranted movement of the bearing outer ra e within the casting. An air plug gauge helps check the bore all around until the last few mm of bore depth. Some customers even go for a micron level grading or segregation.