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Thursday, March 26, 2009

Skimming The Cylinder Head

As per your advice before, true to your prediction, engine oil leaks from between the cylinder head and its gasket reappeared because nothing was done to rectify the cylinder head except changing its gasket twice. This time, the cylinder head was suspected to be faulty and the mechanic proposed that it be skimmed.

I am very wary of such a procedure and mindful of your earlier views about it because i would not want to suffer another round of workshop blues posed by different sets of problems. To use laymen, skimming seems a complicated and delicate process. What problems would arise if a hastily or not properly done skimmed head is fitted to the engine?

There are two ways to take material off the cylinder head:

1. If there is very slight uneveness, using valve grinding paste and a large flat glass, one cap lap the head until it is flat. This method takes off a very small amount of material and is usually not a problemat all.

2. The second method involves using either a shaping or milling machine to take off material from the surface of the head and this is done when there is a quite large amount of material that needs to be removed.

This procedure require quite a high degree of skill on the part of the machinist as he must know how to clamp the head in an absolutely level position in realation to the cutter and he must also know the limit of how much material can be removed.

Every cylinder head has a limit to how much can be removed because if too much is removed, the following can happen:
a) The compression ratio may get too high and you will get serve detonation resulting in piston failure.
b) The valves may hit the piston.
c) You may break into a water passage or the material may get too thin and when the head is torqued down, it may crack.
d) You may have difficulty keeping tension for the camshaft.

So my advice is to make sure that the guy doing the job knows that he is doing or else take the job to the franchisee holder who will have the right information as to what can and cannot be done.

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