Flexible Honing for Engine Valve Stems

Engine valves allow fuel and air to enter and exit from the combustion chambers in automotive engines. They consist of a cylindrical valve stem and a rounded valve head that are made of metal and machined to exacting specifications. Gasoline and diesel engines also have valve guides that contact valve seats, the surfaces against which intake and exhaust valves rest. For optimum engine performance, these valve train elements require proper lubrication. As Flex-Hone® users know, performance starts with the finish.  

Engine Performance – or Engine Failure?

The causes of engine valve failure are numerous, and the consequences can range from lower engine compression and reduced power to high operating temperatures that result in heat-related part failure. With engine valve stems and value guides, wear can cause problems such as excessive oil consumption, poor valve seating, and blow-by. A lack of engine lubrication increases friction, and worn valve stems that retain too little oil can become burnt and require replacement.

For engine mechanics, proper valve stem maintenance means using the right tool for the job. New valve stems from original equipment manufacturers (OEMs) and aftermarket auto parts can also benefit from the oil-retaining surface finish that Flex-Hone® tools from Brush Research Manufacturing (BRM) provide. Flexible hones are well-known for their use in deglazing the walls of engine cylinders, but ball hones can improve the inner diameter (ID) surface finish of any type or size cylinder.

Surface Quality Affects Performance

In Quality As It Affects Performance, a free technical book from BRM, the maker of the Flex-Hone® tool considers the consequences of valve stem and valve guide failure, and shows how its flexible cylinder hones improve surface finish. By imparting a cross-hatch pattern of oil-retaining grooves, flexible honing promotes lubrication and helps reduce engine friction and wear. This plateau finish of uniform peaks and valleys is also free from cut, torn, and folded metal.

Flex-Hone® tools are specified in an oversized condition (larger than the bore) and are available in diameters from 4-mm to 36-in. Their soft cutting action uses a low-temperature, low-pressure abrading process that doesn’t change the underlying structure of the base material. Available in 8 different abrasive types and 11 different grit options, Flex-Hone® tools feature abrasive globules or balls that are permanently bonded to flexible nylon filaments. BRM’s flexible cylinder hones are self-centering and self-aligning to the bore, self-compensating for wear, and easy-to-use with handheld power tools.       

Valve Stem Case Study

The valve stem example in Quality As It Affects Performance is a case study in flexible honing. The intake and exhaust valves that BRM purchased at an auto parts store had “no particular surface pattern to hold or retain or spread oil”, the technical book explains, and “the heat and friction can only be imagined.” The valve guide and the valve stem would probably “rub against each other at perhaps 50 times or more per second,” the technical guide continues, even after the parts “mated” by wearing down each other.

By using a 240-grit boron carbide Flex-Hone®, however, the valve stem’s “horizontal” surface pattern was improved to a plateaued finish. Images from a scanning electron microscope (SEM) provide before and after evidence of this improvement in magnifications of 100X and 1000X. To see these results for yourself, download Quality As It Affects Performance and turn to page 22. This free resource also shows how flexible honing improves the surface finish of auto parts such as shafts and master cylinders.