Cylinder Head Autopsy
To initiate the autopsy, we will start with a scrap Daimler V8 saloon cylinder head. The head is cut into sections and with that done, those sections are painted to make the different areas identifiable:
Red = Cross Section,
Blue = Water / Coolant Cavity and
Yellow = Areas of Significant Corrosion see Fig.11(below).
(Some valve seats and valve guides have been removed to enable the head to be cut.)
To assist, cylinder head maps shown in Fig.12 – Top View & Fig.13 – Combustion Chamber (below) provide orientation, these show the sectional cuts of the cylinder head as debated and the direction that each section is viewed.
Section 1 & End Casting:
Fig.14(below) is the very familiar sight of one end of the cylinder head with the core plug removed. The cylinder heads are ambidextrous and only require the fitting of a water blanking plate at the rear. At the front, oil feed blanking plugs are fitted in order to make them fit either the left or right hand banks.
Fig.15(below) is a much rarer view and is the same end albeit viewed from the inside. The two vertical drilling's are where the cylinder head bolts would reside. The water cavity coloured blue, actually wraps around the end of the head allowing coolant to circulate from side-to-side. The oil feed drilling's can be seen, 2x top circular holes, these provide oil to both inlet and exhaust rocker assemblies. The diagonal tube on the right, is an oil drain back tube allowing spent oil to return to the sump.
Section 2:
Fig.16(below) shows the cross section of the famous hemispherical combustion chamber. The spark plug thread and the much larger spark plug tube thread and seat can also be seen. The large bore diagonal drilling on the left is a push rod tube. Note how the inlet manifold securing bolt actually breaches the tube !! This could be a possible cause of oil and / or air intake leaks. A large coolant cavity around the exhaust port helps heat transfer into the coolant.
Section 3:
Our first sighting of internal corrosion in two locations can be seen in this cross sectional view, Fig.17(below). Also visible is the middle casting seam where the two halves of the mould were fitted together. The two vertical drilling's are again for the cylinder head fixings. Note the larger machined upper & lower areas of the drillings to accommodate the location dowels of which there are many.
Section 4:
Here you can see the third cylinder combustion chamber and spark plug tube retaining thread, Fig.18(below). This section has been cut in two places across an area of concerning corrosion against the exhaust port. We will view this area in greater detail later in the autopsy. Again, you can see an inlet manifold fixing hole breaching the push rod tube. The threaded hole on the right hand side is an exhaust manifold fixing point.
Section 5:
In Fig.19(below) note the pushrod tube in this sectional view is at a different angle to the one in Fig.18. This one is for an exhaust rocker while the less inclined from the vertical is for an intake rocker arm. More corrosion can be seen, as well as a more defined casting seam line. The bulb like shapes on either side is where a threaded hole is located for the fixing of intake and exhaust manifolds.
Section 6:
This is an interesting section with the cut being made through the centre of the combustion chamber – Fig.20(below). The valve seats and guides had already been removed as they are a hard material and would have dulled the saw blade. You can clearly see the valve seat counter bores and the valve guide housings. You might also notice that the intake port tract is slightly longer than the exhaust; this is because it has a slight bend in it, in order to avoid a push rod tube. Also of note is the coolant cavity underneath each port; this is a critical area where most of the cooling is required. The valve uses the valve seat and the valve guide to disperse 75% of its heat which in turn passes to the coolant.
Fig.21(above) shows the valve seats being machined out, along with the combustion chamber without valve seats prior to cutting in Fig.22(below).
Contrary to popular belief, the valve seats fitted to these cylinder heads are not “dovetailed” as was the case with some of Edward Turner's motorcycle engines and the earlier SP250 cylinder heads. Here the valve seats are a parallel sided insert and only rely on a good interference fit to stop them dropping out, same as the Jaguar XK. The dovetailed seats as in the motorcycle heads have a 0.003” - 0.004” taper on both the seat and counter bore in the cylinder head in order to retain them.
End of Casting:
This concludes the first phase of the autopsy. At this end of the cylinder head Fig.23(below) you can see the core plug in place.
These core plugs are the old style dish type that rely on a small surface area to create a seal. Once placed in the hole the dish is flattened somewhat to increase the outer diameter and hold it tight. This type of plug is also used in the cylinder block. The fitting in the cylinder head is somewhat different, in order to combat thermal expansion problems when using aluminium as is the case of the cylinder head. Once the core plug had been fitted, the outer aluminium was then rolled over or burnished against the newly fitted core plug which stopped the plug popping out under pressure.
The trouble is, when the core plug is removed it unavoidably breaks off this small ring or rolled aluminium that used to retain it, as per Fig.24(below).
The image in Fig.25(below) shows a core plug, which was removed from the other end of this cylinder head. The reason it looks like the surface of the moon is due to internal corrosion. Core plugs will corrode from the inside out, so DO NOT be fooled by a good looking outer appearance. Using a Pin Micrometer, we can measure the different thickness to evaluate how much corrosion has occurred. The undamaged part of the core plug measures 0.082” (2.08mm) and the most corroded part measures 0.012” (0.30mm); this equates to 0.070” (1.78mm) of dissolved metal due to internal corrosion.
Remember the leaking core plug, in Part 1, found during the pressure testing - this is why you should always replace ALL core plugs when rebuilding your engine. Thankfully, a far better precision engineered option is available when replacing these old style core plugs !!
In the next instalment, Part 3, we will be travelling back through the cylinder head in the opposite direction, to look at the other side of the cross sectional views. It will focus significantly on areas of internal corrosion – all very enlightening !!