Final answer:
Filing the main bearing caps or simply replacing them will not correctly fix stretched main bearing bores. Professional machining, such as line boring or honing, along with potential cap replacement, is necessary to achieve the correct dimensions and tolerances required for engine longevity.
Step-by-step explanation:
When dealing with stretched main bearing bores in an engine block, neither simply filing the main bearing caps nor just replacing them is likely to correct the issue properly. The stretching or distortion typically occurs due to excessive stress or wear and requires precision machining to restore the correct dimensions and tolerances. The professional and appropriate method to repair stretched main bearing bores is often line boring or line honing. This process involves precisely machining the existing bores to ensure they are perfectly sized, aligned, and within the manufacturer's specifications. If the main bearing caps are damaged or excessively worn, they may require replacement, but they must also be machined in conjunction with the block to ensure a proper fit. Therefore, both statements by A and B are partially correct but miss the necessity for professional machining. It's important that this repair is handled by experienced technicians because improper fitting of main bearings can lead to severe engine damage.