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Confirming I am thinking of heads. Corrosion repair on the deck is pretty routine, but repairing the fire rings maybe is not (or so I heard). Looks like your/Greg's guy had no trouble with that. Thanks for the response.
Confirming I am thinking of heads. Corrosion repair on the deck is pretty routine, but repairing the fire rings maybe is not (or so I heard). Looks like your/Greg's guy had no trouble with that. Thanks for the response.
As I am aware the fire rings are built into the gasket to form the combustion pressure seal.
Head repair is a pretty standard sort of thing with weld build up and machining. Corrosion damage similar to what happens to the heads can also occur on top of the cylinders and that is more problematical- I have seen a couple of engines with that problem over here and they were written off by the main dealers. Whereas alusil can be welded I doubt you would want welding close to the bores. I would think some metal can be shaved off the block and maybe compensated by thicker gaskets and with the right folks doing it some level of repair may be viable that or new cylinders inserted as has been achieved by some clever folks.
No idea if it is the case here but cars that spend long periods of time without being used appear to be the ones most at risk. What we regularly see on these cars is that the head gasket material appears to degrade with age and I suspect this problem is exacerbated on examples that are not used regularly. I also suspect that the gaskets have a finite life [20 years??] and once that timeline expires it then becomes something of a crapshoot.
Once crevice corrosion strikes [the likely corrosion mechanism I believe] it goes through the head or cylinder top material like a knife through butter. Leave the system stagnating and then it is game set and match. Change out of the head gaskets and regular coolant changes are probably the only mitigation- that and regular use. Systems with ionic fluids are particularly vulnerable to crevice corrosion as strong acids are formed within the crevice and then they nibble away at the metal. Regular use probably ensures the impact of this phenomena is reduced or possibly eliminated as coolant is circulated [localised flush] and operational temperatures compress the gasket as the alloy expands more than the bolting.
It's gotten pretty simple:
If you own a 928 that you want to keep the original serial number engine in, the heads need to be removed and the gaskets need to be replaced, along with whatever additional work is required. Anyone who owns a GTS, a GT, or any of the other rare models should retain the original serial number engine.
Here's a picture of a set of head gaskets that came off a 1994 GTS with 48,000 miles on it, last week. Check out the deterioration around cylinder #2! On top of that, the water circulation pattern was definitely severely altered by the holes eaten though the front of the head gaskets. This makes the rear cylinders run much hotter and accelerates piston/cylinder wear. Check out the head bolts. That's not "gold factory never seize"....that's rust, from the head gaskets leaking water around the perimeter of the head gasket where the silicone bead is that is supposed to keep the water from getting down into the head bolt passages.
As far as I'm concerned, I just "saved" this GTS from impending disaster.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
If you own a 928 that you want to keep the original serial number engine in, the heads need to be removed and the gaskets need to be replaced, along with whatever additional work is required. Anyone who owns a GTS, a GT, or any of the other rare models should retain the original serial number engine.
Here's a picture of a set of head gaskets that came off a 1994 GTS with 48,000 miles on it, last week. Check out the deterioration around cylinder #2! On top of that, the water circulation pattern was definitely severely altered by the holes eaten though the front of the head gaskets. This makes the rear cylinders run much hotter and accelerates piston/cylinder wear.
Check out the head bolts. That's not "gold factory never seize"....that's rust, from the head gaskets leaking water around the perimeter of the head gasket where the silicone bead is that is supposed to keep the water from getting down into the head bolt passages.
As far as I'm concerned, I just "saved" this GTS from impending disaster.
I take it you are very much aligned with my assertion that the head gaskets have a finite life and that 20 years is probably as good a number as any. Doubtless some will last longer than others but the fact that the example in question had a low mileage also supports my perception that low milers are likely more problematical than well maintained high milers and quite possibly, well maintained low milers.
I can well understand owners reluctance to pull heads pre-emptively but on the other hand if one leaves it too late there is the possibility they may end up beyond redemption. No idea what it costs to have a corroded head recovered but doubtless it is in the high hundreds per head and that before removal and installation are considered.
If you own a 928 that you want to keep the original serial number engine in, the heads need to be removed and the gaskets need to be replaced, along with whatever additional work is required. Anyone who owns a GTS, a GT, or any of the other rare models should retain the original serial number engine.
This ---^
Originally Posted by FredR
I take it you are very much aligned with my assertion that the head gaskets have a finite life and that 20 years is probably as good a number as any. Doubtless some will last longer than others but the fact that the example in question had a low mileage also supports my perception that low milers are likely more problematical than well maintained high milers and quite possibly, well maintained low milers.
The point I attempted to make is that, at this point, we do not know, and cannot know, for any specific 928 if the heads and gaskets have another 5 or 10 years or are 'about to blow.' Why? Because we do not know:
- how long 928 head gaskets should last (1, 2,or 3-sigma) under ideal conditions.
- what are the ideal conditions for head gasket life.
- the operating history from new for the vast majority of 928s still on the road and how that history might differ from the (unknown) ideal conditions for head gasket life.
When you sum these points together: for any specific 928, even if we know exactly the operating history from day 1, we can only guess as to the condition of the head gaskets.
We have some data. We have theories. We have at least one data point that disproves every simple theory that is independent of detailed knowledge of operating history (e.g. just low-mileage, just high-mileage, just stored, good coolant, bad coolant, etc.)
I've seen high-mileage head gaskets that were quite good. High-mileage head gaskets that were quite bad (Greg's above takes the prize though.) Low-mileage head gaskets that were great. Low-mileage head gaskets that were quite bad. Not-good, not-bad, mid-mileage head gaskets on a 'stored-on-a-stand-for-5-years' engine.
Thus, Greg is spot-on: It doesn't matter to what head gasket religion one subscribes; it's time, or past-time, to pull heads if they've not yet been off.
I, personally, have yet to see a 928 head gasket from a 928 with a fully-known operating history. I can acquire that data as soon as I see the head gaskets on my '91. I know exactly when the coolant was changed since new. I know what kind of coolant was used and I know mileage per year for each year. That's about a year or so away. Of course, the other variable is that my '91's been supercharged for the last 10 years and ~40k-miles.
The point I attempted to make is that, at this point, we do not know, and cannot know, for any specific 928 if the heads and gaskets have another 5 or 10 years or are 'about to blow.'
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Dave,
You appear to be trying to make a perfectly valid point as though I do not understand what is behind it and that is just not the case.
The very specific point I am trying to get across and trust you would understand and align with is that we should be able to form an intelligent consensus as to what is a reasonable service life for the head gaskets irrespective of the examples that live longer or die earlier. I know from personal experience that they can last longer than 15 years and that corrosion can eat the heads away in 10 years or less. Change the coolant on some reasonable basis and use the vehicle regularly they may last forever and a day. I am pretty sure that leaving these things sitting for months on end without running the motor and warming it up/circulating the fluids goes a long way towards destroying the heads and will stick to that unless and until someone comes up with a more credible theory.
For any wear item component on the car I try to get a feel for how long they can be expected to last and thus have a basis to make a decision as to what I want to do during my annual inspection and maintenance intervention. Pulling the heads is a major item and not something one wants to do prematurely but that expense and trouble has to be offset against the consequences of failure. My heads were removed 14 years ago after 15 years of service and both the cylinders and the heads were in perfect nick as was the gasket to the extent I could tell- they came off cleanly in one piece with no apparent degradation. Thus all being well, I reasoned that I can expect at least 20 years out of the current installation. The very last GTS's that rolled out of Stuttgart are now 25 years old so by my criteria any example that runs on the original gasket is well past its sell by date. Unfortunately there are no minimally invasive inspection techniques that I know of that could be used to determine remaining viable service life so the choice is simple- open up and replace or leave and pray for the best.
Bottom line- you chaps who work on these things day in and day out are in the best place to determine what is a reasonable service life expectancy and if your perception of such is different to mine I would appreciate knowing what it is and the reasoning behind such evaluation.
This is one of the main reason I’m taking my GTS to Sean later this year. I’m not taking any chances even though everything seems fine. I expect there to be some damage as there was a time of low use and unknown service.
Here is the engine ready for the test stand. It looks incredible.
The powder coating on the manifold and valve covers wasn't in great shape, so I had it redone, along with hardware plating.
I had done an intake refresh just a couple thousand miles ago, so we were able to reuse some pieces - Hall sensor, knock sensors,CPS, injectors, etc., but most wear items are new, including the 14-pin wiring harness, fuel pressure regulator and dampers, oil pressure sender and spark plug cables.
I really like how the powder coating came out on the manifold and valve covers
[QUOTE=PortlandTom;15987579]Here is the engine ready for the test stand. It looks incredible.
The powder coating on the manifold and valve covers wasn't in great shape, so I had it redone, along with hardware plating.
I had done an intake refresh just a couple thousand miles ago, so we were able to reuse some pieces - Hall sensor, knock sensors,CPS, injectors, etc., but most wear items are new, including the 14-pin wiring harness, fuel pressure regulator and dampers, oil pressure sender and spark plug cables.
I really like how the powder coating came out on the manifold and valve covers
/QUOTE]
Which hardware parts were replated and which ones are new? Who did the plating?
07-05-2019, 02:06 AM
