The valve guide should guide the valve in such a way that it returns concentrically and for sure to the complete valve seat. Furthermore, it should seal well, and lead, particularly the heat of the exhaust valve, either directly to the valve seat or via the shaft to the cylinder head.
How it works
There are very old engines with side valves in each case next to the cylinder. But the majority also of the (more than 30 year-old) old-timers features engines with overhead (hanging) valves in the cylinder head. For some of these engines the design is such that oil, stored above the valves, gets to the space between the valve and its guidance. However, important to notice is that a lubrication is not necessary if the valve guide is manufactured (like mostly) from copper alloy or copper/tin alloy.
The valve guide and its sealing are only in the centre of interest in case of problems. In particular blue smoke in the exhaust gas while lifting the accelerator pedal. This points first of all strongly towards the sealing in the intake valve. During the examination, the exhaust valve should also be taken into consideration of course.
It is possible just to renew the valve stem sealings, not having to dismantle the cylinder head. Yet, very often this only helps in the short run. If the valves have too much lateral play, the guidance must be exchanged. In earlier times this was done with a lot of force/impulse and a thorn. Today, however, particularly the mounting takes place by heating the cylinder head and freezing the valve guide, e.g., with liquid nitrogen. However, the latter method assumes a certain fastness.After such a repair the valve seat should be milled. This does not imply to accustom valve and seat to each other with the help of grinding paste, but the real treatment of the valve seat rings.
In today's light metal cylinder heads valve guides manufactured from Cu Ni or Cu Sn alloys are pressed in. 06/08