Flexible Honing for Aircraft Engine Cylinders

Aircraft engine cylinders keep fluids flowing and help create the power that’s needed to keep a pilot and passengers aloft. These engine components are machined from various metals and may be plated with chrome to enhance corrosion resistance. Air-cooled aircraft engine cylinders aren’t new, but the lessons learned about them are still valuable today.

For optimum performance, the inner diameter (ID) of any machined cylinder needs the right surface finish. If the surface is too rough, the rings or seals may rub against jagged metal peaks (burrs) and experience premature wear. If the bore’s surface is too smooth, oil retention and increased friction may become problematic. That’s why for cylinder surface finishing, mechanics and manufacturers use Flex-Hone® tools.

Quality As It Affects Performance

In Quality As It Affects Performance, a free technical book from Brush Research Manufacturing (BRM), the California company shares the results of a study on chromed cylinder walls. When a local rebuilder of aircraft engines want to improve piston ring seating and sealing, the BRM Flex-Hone® tool imparted an oil-retaining finish that improved performance while saving time and money.

As BRM’s technical book explains, the aircraft engine’s cylinders had been Van der Horst plated. This process of over-chroming and reverse chroming etches the cylinder’s surface with a series of worm-like channels that helps to keep the chrome from bubbling or flaking. The Van der Horst process also creates a series of worm-like channels that aid in oil retention, but there’s still room for surface improvement.

After surface finishing four engine cylinders with a rigid hone, the mechanic tried a 180-grit boron Flex-Hone® tool. Prior to using BRM’s flexible hone, the process of seating the rings could take from 12 to 20 hours. Moreover, approximately 25% of the cylinders wouldn’t support proper seating and required rework. With flexible honing, however, the rebuilder reported that the rings were seated in 6 to 8 hours with a 100% success rate.

Flexible Hones vs. Rigid Hones

Based on its own testing, BRM believes that using aluminum oxide (AO) instead of boron would have provided a better surface finish. Still, as the images from a scanning electron microscope (SEM) revealed, the use of a boron Flex-Hone® produced a long-lasting finish. BRM’s cylinder surface finishing tools are also easy-to-use and require no special instruction or training.

Unlike rigid hones, Flex-Hone® tools are self-centering, self-aligning to the bore, and self-compensating for wear. They also impart a uniform surface finish along the length of the entire cylinder. By contrast, honing stones can impart an uneven or unidirectional crosshatch, and require operators to reduce pressure on the tool even when surface finishing the top of the cylinder.

The Flex-Hone® puts little pressure on cylinder walls, which means the tool won’t gouge out metal and cause torn or folded-over edges. Plus, since pressure against the cylinder wall is equalized, operators don’t have to worry about making cuts in both directions. As the example of the aircraft engine builder in Quality As It Effects Performance shows, flexible honing can save time and money.