How can main bearing bore misalignments be corrected?

• A. Line boring
• B. Replacing main bearings
• C. Replacing the main bearing caps
• D. Replacing the crankshaft

Answer: (A)

When main bearing bore misalignment occurs, the fix is to true the bores so they are straight and coaxial with the crankshaft axis. Line boring does this by machining all the main bearing bores in the block to the correct size and alignment in one setup. It removes material to correct any out-of-roundness or eccentricity and ensures the crankshaft spins true with even oil clearance across all mains.

Replacing the bearings or the main bearing caps won’t correct the bore geometry; they may fit better, but the misaligned bores will still cause uneven clearances and wear. Replacing the crankshaft doesn’t fix the bore alignment either unless the bores are first brought back into true alignment, which is accomplished by line boring.