Cylinder head 10 - Processing
In earlier days the valve seats and valve guides were re-cut by hand. Here it is done by machine which is probably also sometimes used in the production process. It disposes of a great deal of latitude and can probably take up almost any angle to the cylinder head.
How it works
The picture on top displays the preparation for the milling of the valve seats. For this purpose the valve guide has, in addition, a guide shaft (pilot). A 45° rotating chisel is chucked which first produces the valve seat. The final width is adjusted by milling with 75°, 30° or 15° correction angles. Milling the correction angle causes a smaller valve seat, milling the valve seat a broader seat. Mostly, the exhaust valves receive the broader valve seat because of the better thermal absorption. Afterwards the new valve can be moved back and forth with the help of grinding paste opposite to the seat.
Since there are only light metal cylinder heads and the fuel is almost without any lead, there are pressed in valve seats which can be exchanged in case of too much wear. The installation of new ones is more difficult than to remove the old ones without damaging the combustion chamber. The new ones are mostly cooled down and shrink fit in the heated cylinder head. You should take care of a solid mounting.
The guide shaft makes sense only if there is no play. Therefore, the valve guide with the potentially new valve should be examined first of all for lateral play. Also this can be renewed by squeezing and pressing in. However, in this case the inside diameter changes which must be brought into the predefined measure with reamers. 06/08