The Flex-Hone® for Valve Guides

Valve guides are tube-shaped metal parts that fit the cylinder heads of reciprocating engines. Typically made of aluminum, bronze, or steel, they support valve stem cooling and help position the engine valves to make proper contact with the valve seats. Over time, constant friction between the guides and stems causes valve components to wear. In car and truck engines, worn valve guides can cause the vehicle to burn too much oil. Loose valve guides and guides with too much clearance are also problematic because they reduce engine cooling and affect overall performance.
 
Valve Guide Replacement vs. Valve Guide Reconditioning
Some engine builders replace loose, worn, or improperly-sized valve guides with new ones.  Valve guide replacement can be expensive, however, and may change an engine valve’s concentricity. As automotive writer Larry Carly explains in Engine Builder Magazine, concerns over labor costs and replacement auto parts cause some engine builders to choose valve guide reconditioning instead. By installing valve guide liners, the engine builder doesn’t have to replace the valves and guides. A straightforward process, valve guide reconditioning has five steps: boring, installation, sizing, trimming, and flexible honing.

How to Install Valve Guide Liners
K-Line Industries, a specialty tools company from Michigan that distributes valve guide brushes from Brush Research Manufacturing (BRM), describes this five-step process in detail. Using a carbide boring tool with a high-rpm air drill, the engine builder begins by boring-out the original valve guide. After removing any stray metal chips and lubricating the guide hole, the engine specialist uses a short-stroke, heavy-duty air hammer to drive-in the new valve guide liner.    

To complete the sizing step, the engine mechanic then uses a ball broach, roller burnish, or carbide ball. Trimming is performed by inserting a pilot into the carbide guide cutter. Using a 950-rpm air drill, the engine rebuilder can remove excess liner material from the sides of the valve guides. Finally, by running the Flex-Hone® tool through the new valve guide liner with a 2,100-rpm drill, the desired finished is achieved with just “one pass up and one pass down”.    

Valve Guide Finishing and the Flex-Hone®
Although Larry Carly includes the Flex-Hone® procedure from K-Line Industries at the end of his Valve Guide Reconditioning article, he also adds a step – following the Flex-Hone® with a nylon brush – that BRM believes is unnecessary. As Carly correctly explains, however, flexible honing “removes any burrs left from trimming the liner to length and leaves a nice crosshatch finish that improves oil retention. One pass in and out is all that’s recommended to hone the liner”. For bronze and aluminum valve guides, brush Research Manufacturing also recommends using aluminum oxide hones.