The valve guide has the task to absorb the transverse forces which affect the valve stem.
The valve guide centres the valve on the valve seat insert and conducts a proportion of the heat from the valve head, via the valve stem to the cylinder head.
Due to this extreme strain, for the valve guides mainly the material and its properties are a decisive factor for the product quality.
Valve guides are made of materials with good gliding and thermal conductivity properties. Grey cast iron and brass materials with selected alloy components have been particularly proven.
Grey cast iron with pearlitic structure. This material is characterised by good wear resistance and is suited to guides under normal strains.
Grey cast iron with pearlitic basic structure and increased phosphorus proportion. The reticular formation of the phosphor provides higher wear resistance and better emergency run properties. For usage in engines under medium strain.
Grey cast iron with pearlitic basic structure and increased phosphorus proportion as well as increased chromium content. For usage in highly supercharged engines.
CuZnAl alloy. This material is characterised by good wear resistance with high sliding properties. The guide is suited to usage in engines under normal and medium strain.
Main dimensions of a valve guide
The valve guide in the cylinder head is kept in its position by a press fit. The valve guide is radially constricted when pressed into the housing bore of the cylinder head. The housing bore however is expanded. The extent of this deformation depends on the one hand on the ratio of housing bore diameter and external diameter of the guide and on the other hand on the rigidity of the two components. If there are strong diff erences in the rigidity of the housing wall, the radial distortion can differ quite significantly along its length.
When installing and removing the valve guides, ensure correct warming up of the cylinder head (specified by engine manufacturer). Use suitable mounting mandrels for installation and deinstallation. An additional cooling off of the valve guides facilitates assembly significantly.
Before installing a valve in the valve guide, it is measured whether the borehole of the valve guide is still cylindrical, i.e. it has the required diameter in all places. Motorservice generally recommends correcting the diameter and shape of the borehole by reaming it with a reamer.
Standard values for the clearance between valve guide and valve stem:
|Stem diameter||Clearance: Intake valves||Clearance: Exhaust valves|
|6 to 7 mm||10 - 40 μm||25 - 55 μm|
|8 to 9 mm||20 - 50 μm||35 - 65 μm|
|10 to 12 mm||40 - 70 μm||55 - 85 μm|