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.
If this oil travels unchecked along the valve stem, engine HC emissions will become excessive.
Their replacement involves removing the worn part by driving it out with a hammer and specifically-shaped punch.
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.