The aluminium cylinder head(s) can be a real problem on these beautiful V8 engines. Damaged combustion chambers, porous castings, worn valve guides, burnt out valves, corrosion and cracks are all relatively common occurrences. Valve guides and seats can be repaired fairly easily, while corrosion porosity and cracks can be difficult and costly to repair.
In Fig.1 (below) you can see a burnt valve caused by poor valve seating. The seating issue can be down to worn valve guides, bent valves or incorrect valve clearances. Valve seat damage may also be apparent when the valves are removed.
In Fig.2 (below) you can see significant combustion chamber damage caused by foreign body entrapment within the cylinder. This can come from the carbs, spark plug failure or piston / ring break-up. Bent valves and cylinder wall damage will occur as a result of this. Extreme cases can result in a water jacket breach something best avoided !!
Corrosion
In Fig.3 (below) you can see a badly corroded combustion chamber has left the valve seat exposed around the edge. This has probably been caused by poor storage with a cylinder full of water or a leak or drip directly on to the chamber; the result is that the aluminium has been eroded.
Classic Daimler cylinder head corrosion can be seen around the water coolant passages in Fig.4 (below) and this can range from mild to severe and is often caused by dissimilar metal electrolysis. The cylinder block is cast iron and iron particles are circulated throughout the cooling system, notice the orange rust colour inside the alloy waterway passages. This effect can be combated successfully by ensuring your engine has a good quality corrosion inhibitor, which can be found in quality antifreezes.
For some it will be too late. These problems can be addressed by Tig welding the aluminium cylinder head around the damaged areas. This can be a difficult and time consuming process as impurities in the aluminium are plentiful. The extreme temperatures required to weld are also very stressful to these old cylinder heads and distortion will occur. Interference fits with valve guides and valve seats can also be compromised, hence care must be taken.
Frost
Fig.5 (below) shows a crack on the top cylinder head face underneath the rocker gear. These are caused by freezing coolant within the water jacket sometime in the past. Most classic Daimlers will not even see freezing temperatures these days as they have become well loved and much better taken care of, then they may have been in a previous life. When the coolant freezes due to a weak or non-existent antifreeze mixture, the coolant will expand greatly and because of the sealed system the extra volume has nowhere to go.
Hoses will expand with the freeze as they are made of rubber, which leaves only the thin wall casting of either the cylinder block or cylinder head. This is one of the more serious repairs and again, can only be repaired by aluminium welding in the case of the cylinder head.
Again, distortion and stress are the enemy and a fair amount of post machining may be required to reinstate the rocker shaft platform heights, valve spring seats and plug tube seating. Rocker shaft breakage will occur if all of the of the platforms are not parallel.
Porosity
As if all the above does not demonstrate enough of the problems that can occur with Edward Turner's V8 engine, casting porosity is without doubt the most difficult to identify. The rather innocent looking image Fig.6 (below) shows an area marked with black dirty looking spots. These are flaws in the alloy casting straight through into the coolant cavity. The only way to identify this problem is to have the cylinder heads pressure tested. This must be the first phase, before any work is carried out on the cylinder heads and it confirms exactly what is to be dealt with. There is no point in rebuilding cylinder heads that have a hidden fundamental flaw that will cause issues when put back into service.
Fig.7 (below) shows the same cylinder head during the pressure test procedure. The head is mounted on to the machine, then clamped using rubber blocks and a transparent plate which allows for combustion chamber and gasket face observation. Compressed air is then introduced inside the cylinder head, as the first stage air test. The second test involves filling the cylinder head with hot water and then pressurising the cooling cavities within the head. Any cracks, flaws or porosity are indicated by water/air leaking on the outside of the casting.
Fig.8(below) shows leakage around the core plug area, fortunately this is a straight forward repair and simply requires the core plug to be replaced. Corrosion is the most likely cause of this with the core plug rotting from the inside to out and is another indication of poor corrosion inhibitor within the coolant system.
In Fig.9(below) you can see where the head has become porous with the water / air mix, under pressure, bubbling through to the surface. You can fully understand the implications this would have if put back into service. The repair for this type of issue is much more complicated, compared to replacing a core plug or welding a crack. Yes, the area could be welded and replace the bad metal with good, BUT the problem is additional porosity in other areas of the cylinder head waiting to break through. This problem would not have been apparent when the head was new, it is highly likely that the dreaded corrosion has eaten its way through the aluminium.
With all the above information in mind and the evidence speaking for itself, it would be interesting to x-ray a saloon cylinder head in order that we could build a picture of what is occurring on the inside. From such an image we could ascertain how thick the alloy is in critical areas and more importantly the extent of any internal corrosion. This would be an ideal learning opportunity and hence, enabling a more informative view when replacing these cylinder heads; in essence it would be a cylinder head autopsy cut in to sections, see Fig.10(below).
Sadly, we do not have an x-ray machine.................... however, we do have a saw !!!!