Valve guide

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Valve guides are cylindrical metal bushes, pressed or integrally cast into the

.

Guides also serve to conduct heat from the combustion process out from the exhaust valve and into the cylinder head where it may be taken up by the cooling system. Bronze is commonly used, as are various iron alloys; a balance between stiffness and wear on the valve is essential to achieve a useful service life.

The clearance between the inner diameter of the valve guide and the outer diameter of the poppet valve stem is critical for the proper performance of an engine. If there is too little clearance, the valve may stick as oil contaminants and thermal expansion become factors. If there is too much clearance, the valve may not seat properly and excessive oil consumption can occur.

Oil seal

The upper part of the valve stem, within the rocker box, is lubricated by oil. If this oil travels unchecked along the valve stem, engine HC emissions will become excessive. To control this, an elastomeric seal is fitted over the top of the valve guide. These may wear or stiffen with age, so are usually replaced whenever valves are removed for servicing.

Wear

Over time, the inner diameter of the valve guide and the outer diameter of the valve stem may become worn.

Reaming

In the 1980s, many U.S. production engine remanufacturers began

warranty issues are virtually eliminated.

Studies have been conducted which show that through the proper selection of the reamer and reaming process, valve guides can be quickly and efficiently reamed to a consistently repeatable size.^[2]

Replacement

Valve guides are typically shaped in a tube with a flare at one end. Their replacement involves removing the worn part by driving it out with a hammer and specifically-shaped punch. Installation may involve shrink-fitting, heating the cylinder head and cooling the valve guide so as to ease insertion, then driving the new guide in quickly with a press or a hammer. Once the parts return to room temperature the new valve guide will be solidly in place and ready to be reamed and honed to proper diameter.

References