Flexible Honing for Main Bearing Bores

Engine mechanics and automotive restoration specialists know that the Flex-Hone® tool from Brush Research Manufacturing (BRM) is the right choice for deglazing and surface finishing engine cylinders that meet tolerances for size, roundness, and taper. What some engine rebuilders may not realize, however, is that BRM’s flexible honing tool is also ideal for restoring the bores where the main bearings are seated.

Unless these “main bearing tunnels” meet all of the proper specifications, explain the experts at COME Racing, your engine rebuild “will never be optimized.” Line boring with a rigid hone is recommended for cylinders that are out-of-round or that require significant material removal. For a superior surface finish, however, BRM’s flexible honing tools are what do-it-yourself (DIY) mechanics and expert engine builders alike are using.

Plateau Honing for Cylinder Surface Finishing

Versatile and easy-to-use, the Flex-Hone® tool produces a plateau finish that promotes lubrication and reduces engine wear. In engine cylinders, this cross-hatch pattern of uniform peaks and valleys aids with piston ring seating and sealing. In main bearing bores, a cross-hatched surface finish allows the crank to turn with minimum friction while gripping the bearings effectively and dissipating heat efficiently.  

MSB Tuning, a high-performance engine builder in Queensland, Australia, describes plateau honing for main bearing tunnels in an on-line article called Flex-Hone® Applications in Engines.  As this experienced engine tuner writes, “Good contact between bearing shell and bore will not be possible with a surface that does not have a plateau finish”.  Line boring leaves a series of “high peaks” that, if not removed, “will bed the bearing against the surface” of the cylinder walls.    

Line Boring and Flexible Hones

Line boring is fine for fixing out-of-round conditions, MSB Tuning explains, but a “finer hone” is needed to improve the “contact surface” for the main bearings. With cast iron engine blocks, the Australian engine builder uses an 800-grit Flex-Hone® with silicon carbide (SC) abrasive. For alloy engine blocks, the tuner chooses 800-grit BRM cylinder hones with aluminum oxide (AO) abrasive.

Although BRM’s ball hones aren’t meant for heavy-duty material removal, MSB Tuning also uses coarser-grit Flex-Hone® tools for more aggressive honing. As the author explains, “I would initially use a 180-SC hone or perhaps even coarser for a minute.” Finally, and depending on the engine block type, this BRM engine hone user would “finish off” the bore with an 800-SC or 800-AO tool.  

Best Practices for Cylinder Honing

For best results, Brush Research Manufacturing (BRM) recommends starting with a coarser-grit Flex-Hone® and finishing with a finer-grit tool. Always use BRM’s flexible hone with a lubricant (preferably BRM’s specially-formulated Flex-Hone® oil), and never over-hone. As MSG Tuning says about material removal, “Once you remove it, you can’t put it back if you went too far.”