6. Sonic Checking - Sonic checking—an ultrasound process—is employed to find thin areas of a block that are inaccessible to conventional measuring equipment. Cylinder walls are the most commonly inspected areas since core shift can render a block unusable. To find any hidden thin spots, a probe is placed at the area to be checked, and electronically generated sound waves are passed into the block. Those sound waves are reflected back to the probe and the amount of time required is converted to a dimension by the device. Different blocks have different minimum specs, but the key idea is that higher power levels will require thicker cylinder walls. Some types of cylinder blocks, for instance production 400ci small-block Chevys or small-block Fords, should be measured before receiving any appreciable increase in bore when horsepower is expected to rise significantly.