The head gasket mating face of a cylinder head has to be absolutely flat and have the correct surface finish for the gasket to be fitted. The introduction of Multi Layer Steel Gaskets (MLS) have made this even more important.
At QEP we use the latest cutting materials to ensure the right finish every time. PCD (Poly Crystaline Diamond) is used on aluminium cylinder heads. It never needs sharpening and so gives the correct finish every time. As a bonus it leaves the head with a real shine that is hard to achieve with other materials.
What happens when I get my head skimmed?
First the head is asessed to see whether it can be skimmed. If it is too bent or below its minimum thickness then an alternative solution may have to be found.
Next the head is cleaned in a hot spray wash machine.
The head is placed on the machine and setup so that all the corners are within 0.001” of each other. Once secured and checked it is then fly cut.
Often when machining heads we get to see problems that are not obvious with the naked eye or a straight edge.
Re assembling an engine where the face of the head is not flat will result in early failure of the head gasket.
In the picture above the head (a 1.8 FORD CVH) is between cuts. (please note the head hasn't been cleaned so the detail can be seen better in the pictures).
The first pass, Usually 0.002", hasn't fully removed the distortion. You can see where the head has distorted, mainly on the hotter exhaust side of the head as well as between the chambers.
After the next pass with the fly cutter the head is almost flat apart from the two sections circled.
After the third and final pass the face is completely flat. The area circled in blue is due to there being a hole in the head gasket but no waterway in the head. This will not affect performance of the new gasket so is left to avoid removing too much material from the head face.
How Much Material Can Be Removed?
There are several points to consider when machining a cylinder head. If the head is only slightly bowed then there shouldn't be any problems. Unfortunatley some heads arrive very distorted and a decision has to be made whether to machine or replace the head.
Will the valves hit the pistons?
Not such a problem on older engines but modern 16v designs and diesel engines run the valves a lot closer to the pistons therefore the amount of material that can be removed is reduced. On diesel engines where there isn't a combustion chamber in the head the valve seats can be machined to lower the valves back into the head. We use 'Saver shims' for a wide range of vehicles. These are 0.5mm thick stainless steel shims that are fitted with the head gasket to allow an extra 0.5mm to be removed from the head.
What about the compression ratio?
As the face of the head is machined the compression ratio is increased. This leads to increased temperatures in the combustion chamber which can lead to 'Pinking' and 'detonation'.
OHC Engines. When the head bends the camshaft bearings also move in relation to each other. Every time the camshaft rotates it is bent which leads to fatigue and eventual failure. Vauxhall 8v OHC (1.3, 1.4, 1.6, 1.8, 2.0) engines are particulary prone but can easily be rectified by machining the top of the head flat as the camshaft carrier is separate from the head.
As the head bends the valve guides move in relation to the seats resulting in the valves not sealing properly. This can be overcome by having the valve seats machined true again.
| 
