The function of the "trio" valve guide, valve seat and valve is to create a perfect, endlessly renewed and reliable air-tightness of the gaseous flux of the engine.

A metallic contact between two surfaces must constitute a reliable and airtight passage after hundreds of millions of consecutive opening and closing of the valves

The two surfaces in contact, i.e. the valve seat surface of the valve and that of the actual seat itself, must have similar qualities and must be close to perfection.

The shapes of the above mentioned surfaces must be perfectly identical and completely complementary.

The only shape with precision and in a repetitive manner to fulfil this function is the circle. Associated with other parameters, the circularity , i.e. the accuracy of the shape of the circles constituted by the valve seat and the valve itself, turns out to be the main and sine qua none condition to a good air0tightness between the valve and the valve seat. Circularity, cylindricity, surface finish, angles are all subjected to stricter and stricter tolerances.

Valve Guides

The valve guide is the reference of which depends the position of the repair machining operation of the valve seat, of the control of part of the conformity of the valve seat (concentricity) and, of course, it guides the valve in its movements. The quality of a valve guide is essentially defined by 4 parameters:

1. Circularity
2. Cylindricity
3. Surface Finish
4. H.. Diameter tolerance

To ensure a perfect guiding of the valve, cylindricity and the tolerance on the diameter are critical. Good geometric qualities will allow the valve guide to position the valve with precision over a long life time.

An important defect in the guiding of the valve, further to a poor machining of a valve guide — outside of tolerances, will lead to premature deformation and wear of the valve seat and a fast loss in engine output.

The tolerances commonly required by OEMs for current engines are:

•  Circularity: 0.005mm (5 microns)
•  Cylindricity: 0.006mm (6 microns)
•  Surface Finish: 0.4 micron
•  Diameter Tolerance: H7 (or H6 for the most advanced engines)

The above tolerances, difficult to obtain and respect by large series manufacturers, are even more complex to guarantee when repairing. Failure to meet these quality levels will make the machining of the valve seats even more delicate.