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 a 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 Flexible Honing
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.
Author's Note: This CR4 blog entry appeared originally in BRM's Flex-Hone Blog.
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