FredR

When the cylinder head is removed you should be able to see a clear impression of where the gasket was in contact with the head bounded by two perfectly concentric circles. In your case this has not happened as can be seen at the 9 O'clock and 3 O'clock positions where there is a clear narrowing of the sealing surface.

As I understand the earlier models like yours should have a 95mm bore that generates a swept volume of 4.5 litres whereas this was changed in later models to a bore of 97mm to generate 4.7 litres of capacity. As GB has alluded to, this discrepancy is probably answered by someone either changing the engine [is the engine number consistent with chassis number?] or boring out to 97mm and either way fitting the wrong gasket.

The obvious question being what is the current bore? One imagines that such discrepancy could not possibly have happened at the factory as they would not have had the 4.7 litre kit when it was built de-facto someone was dicking around with the engine at some stage.

Darklands

Why are the pistons on the bottom of the picture all so clean? Is this the norm or a bad spray pattern of the injectors?

Gage

BINGO!
A 97mm bore with a gasket designed for a 95mm bore. I guess that's one way to increase compression ratio

Interestingly, this engine ran well and for a long time, the fire ring does not appear to have suffered from combustion exposure. The heads are off to be cleaned and bead blasted in order to make an assessment of the deck corrosion. I anticipate my next questions will be whether a filler will be required and if anything, other than welding, has proven to give satisfactory fill results so long as the sealing surfaces are identified as being intact and supported.

The Forgotten On

^^ The heads are easy to weld as the aluminum is normal and not a high silicon alloy like the block which causes "dirty welds" from the silicon coming out of suspension.

Any good shop should be able to weld and surface the heads with the correct Ra.

Gage

Here are the findings after cleaning and bead blasting.
Head gaskets for the 97mm bore are on order to determine where the fire ring locates relative to the erosion. I expect some of this water jacket erosion will require welding filler to be serviceable.
Another concern is the pitting adjacent to the exhaust valve seats. Is this an erosion from lean combustion?

FredR

First of all many congrats on the photo's - best I have seen to date.
This is a clear examplw of the corrosion phenomena I have attempted to define.
Take a look in the transfer ports-perfect condition so that tells you there was nothing wrong with the coolant or it being exhausted.
The root cause of the problem is coolant getting between the gasket and the head this seems to be an unpredictable event that sooner or very later will happen.

Once this happens it sets a chain of events in motion. The thin layer of coolant will break down at temps over 55C once it loses the pH buffer that is alkaline. These acids seemingly can attack the binder in the gasket that is some form of nitrile rubber like buna N. The acids also etch the alloy of the head. The real estate impacted is invariably that where there is no counter support on the block side. I believe this process is very slow acting.

At some point the acids reach a critical point wherein the electrochemical potential between the acids and the coolant can induce a reaction known as crevice corrosion. Initially this is slow but once it takes hold the consequences can be both rapid and severe. The dead give away is that this is a classic problem that happens within gasketted surfaces and is typified by the pitting (small pinholes) clearly apparent in your photos.
The sound way to repair such is welding. I gwt the impression you may have skimmed these heads already- if so that may be a significant mistake unless you are somehow very fortunate.
The damage is significant and one of the worst cases I have looked at.
As for the damage around the valve seats I am thinking (guessing?) that may be heat related damage due to a leanish mix. All the exhaust valves look distinctive and that damage being close to the spark plug may also be a clue. As to whether that can be repaired is best answered by others with such experience-I suspect it will be possible to remove the valve seats, weld up then machine back.
Unfortunately I am away from my home base at the moment but will take a more detailed look whenI return home in a few days time.

RennHarry

Paging Doctor Brown. Code Blue.
Just my uneducated opinion.
Cheers

Gage

Thank you Fred for your efforts to help me / us understand this phenomenon.
As I study the patterns of corrosion on the head, with this new basic causation understanding, I wonder if a deterrent might be to irrigate between the transfer openings in the head. A small, machined trench, concentric to the bore, that would effectively connect each of the passageways and allow coolant to move freely and be exchanged with the greater coolant volume.
The deck surfaces have not yet been cut although they may have been previously as part of the last gasket service.

FredR

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If you look carefully at your heads you will see that despite some very aggresive corrosion sites, the walls of the transfer passage ways are spotless and yet immediately adjacent there is horrendous corrosion so I doubt that some kind of irrigation passage would solve the issue. The only modified solution I can think of would be a head gasket with additional seal beads around all those cast in passage ways- whether such would work remains to be seen.
Unfortunately we have no reliable data to tell us about when and to what extent this problem happens and I suspect the best approach would be to refresh the head gaskets at a fixed interval such as every 15 to 20 years knowing there is a small possibility such damage might in some cases occur earlier.