Michael Benno

Oh and the pistons are all out now, not cleaned yet but they all look decent and so do the bores. Pictures will follow this week.


Here is a view of one of the pistons. Should I be worried about the weird scoring above the upper ring? There is no damage on the respective cylinder bore.



The GTS rings are different than S4/GT. The first pic is the upper ring, then the lower ring, both in the upward-facing position



This video is for Fred and anyone else with original head gaskets!

Schocki

Thank you for the pictures and the video, guys!
@Michael Benno What year did you buy the car and with how many miles? The reputation for re-imported 928s from Japan in Germany is not the best. Most of them have very low miles and the interior as well as the exterior are in good condition, plus prices are low.
Many of them have mechanical issues, like TBF. Too bad that there is no way to see how many hours the engine has run and compare it with the mileage. I know that mechanics in Germany recommend avoiding them, because they think that they sat too for long periods in garages and when driven, they sat too many hours in traffic idling.

With this said, I would love to see the head gaskets of an engine with higher miles (about the same year) that was well maintained and driven like a GT. Common agreement here is not to pull the engine until there is an issue with it. Don't get me wrong Michael, if mine have used the same amount of oil like yours, I would have done the same already. I'm just trying to increase my 928 knowledge horizon and if I should do the same in the near future.

Found this from your MY 87 S4 engine teardown @ 144 kmiles... Any chance for head gasket pictures from back then?

FredR

Hi Michael,First of all many congrats on your progress to date- irrespective of the status of the rings and the cams, the head gaskets alone prove you were spot on to remove them.

I am alarmed at the state of the cams given you installed them 2017 and presumably not that many miles on them? Clearly you would not have installed them in any deleterious condition so I would guess the mileage since installation can only be 50k miles tops and possibly much less than that? Clearly some input from the team needed in this regard alone I suspect.

That scoring on the piston above the top ring land is also of concern . I tend to associate marks like that with a partial lubrication failure so again some dialogue on that one needed.

With regard to the head gaskets they show typical degradation- not as bad as some but not good either so they have been removed not a day too early and maybe a few years later than optimal but until the heads are cleaned up difficult to say. I have formed the opinion studying this issue over the last 5 plus years that the maximum time they should be left unattended is 20 years- after that it is a crap shoot. Back in 2005 was when I first took an interest in this issue. A colleague asked me to take a look at his stripped down engine in the local agents workshop. His early GTS heads had been condemned due to the astonishing amount of corrosion on them - car fully service as per Porsche requirements and carried out by the main agents who were also baffled. I took one look at the heads and immediately realised that they had experienced a crevice corrosion attack- this is a relatively rare type of corrosion and is typically found in saltwater service exclusively occurring in joints and characterised by a pitting type of corrosion. I was puzzled as to how and why this should occur. Porsche would not sanction weld repairs- if head skimming to the limits defined by Porsche was not adequate for repair then they defined the heads as scrap. I believe he got some "good will" support from Porsche but it seemed obvious he had been asked by Porsche to keep schtum on the matter. Whilst I was in the workshop the manager showed me two other motors in a similar state- an S4 and one GTS with similar characteristics. A few months later I lost my 90S4 and subsequently instructed the agents to remove the heads on my S4 motor prior to reinstalling into my by then acquired 92 registered GTS whose motor was TBF'd [I purchased it at a knock down price knowing this of course]. The S4 heads were in perfect condition and the gaskets looked more or less pristine being some 15 years old at that moment in time.

Fast forward to about 6 years ago. I noticed on Rennlist we were seeing more and more examples of such damage so I decided to use my professional skill set to investigate what was going on. Every time this problem appeared the same platitudes about how the coolant had not been changed religiously or non Porsche approved coolant was the cause of the issue. To anyone who knows the first thing about corrosion this was nonsense. How did I know this?- simple- in every case a look inside the cylinder head internal passageways showed no corrosion whatsoever- if the coolant state or type was the issue then every wetted surface would be corroded and that was just not the case- in fact I never seen a case where such has happened even though it is distinctly possible- clearly the "ransid coolant" syndrome was nothing more than systemic brain washing - folks being led to believe something that was clearly just not the case. Thus it was obvious to me that we were looking for something else that could explain this phenomena.

Answers quickly started to appear- ethylene glycol in theory should not dissolve in water but in reality it does so because it is a highly polar molecule. Unfortunately this particular construct means it is also thermally unstable and in its native state breaks down when exposed to temperatures in excess of about 55C. Coolant operates at circa 100C. To prevent thermal degradation the coolant needs buffering agents to keep the coolant in an alkali state thus why when new coolant typically exhibits pH9 and thus feels "soapy" to the touch.

The head gasket is basically a sandwich of two sheets of Klingerit gasket material [aramid fibre that uses NBR as a bonding agent or so I understand] and the sheets are pressed onto a galvanised steel barbed mesh. The cylinders are sealed by stainless fire rings that are pressed into position and the external services are sealed by the Klingerit material under compression. Thus two types of sealing in one composite structure as it were. These gaskets are now very old tech but still work just as well today as they did when first introduced with the 928 back in 1977 or whenever. If you look carefully at the gasket you will see beads of sealant around the secondary sealing surfaces. Your gaskets show no signs of distress on either the fire rings or the secondary sealing services so the gasket has clearly done its job and there are no signs of gasket failure - agreed?

Now try to visualise all the real estate of the gasket that is outboard of the fire rings and inboard of the secondary sealing surfaces- non of that surface is required for head gasket purposes rather it is nothing more than a structure that holds the gasket critical sealing surfaces together whilst it is being installed. All this real estate is characterised by the fact that on the block side of the gasket there is nothing to counter support it and therein lies the problem or so I believe. The cylinder head- presumably for reasons of casting during fabrication- has transfer ports all over the place and more so on pre-S4 models. Note how the gasket is used to blank off those transfer ports and in some instances has small holes to permit a controlled amount of coolant flow to help localised cooling at the top of the two middle cylinders that typically run a bit hotter even with this feature. Also note that at the fire ring location the head clamps over the fibrous material as well as the ring itself to form a secondary seal between the head and the top of the cylinder chimney. This is an important detail as it tells me that the designers knew that if coolant hit the fire rings that were in contact with the heads a galvanic corrosion attack would likely take place. Thus from this it can be intimated that the engineers expected coolant to be present in this location.

During operation the cylinder head is bouncing up and down as the engine rotates but the magnitude of deflection is of course microscopically small- the cylinder head is a dynamic joint and hence the most critical sealing surface on the engine. Thus not difficult to see why coolant can and eventually does work its way into the tiny gap between the gasket and the head in those areas that are not counter supported. Once in that gap the coolant is effectively stagnant and the chemical package including the buffering agent gets depleted. The coolant eventually is no longer coolant but transforms via thermal degradation into a mix of organic acids. These acids start to attack the binder in the gasket and then hit the galvanised steel mesh and start to attack that in a random localised manner. This process is very slow acting and usually [but not always] takes years to manifest itself. When the steel corrodes it forms ferric oxide and that is the orangy coloured substance we see leached into the body of the gasket material to differing degrees. The other point of note is that when steel corrodes like this the oxides formed occupy 5 times the volume of the corroded steel mass and hence the gasket delaminates. Your photos clearly show this phenomena to some extent. At some stage the acid build up appears to reach a point where it can develop sufficient electro-potential difference between the main body of the coolant and the acidic gunge duly trapped in the crevice and 'la voila' a circuit is established that can support a crevice corrosion attack and once it starts it cannot stop until such time as the coolant is drained. In your case this phase of the attack mechanism does not appear to have initiated. As and when you have cleaned the heads we will then be able to see better what has [or has not] happened.

If you have the time and or inclination perhaps you would kindly take photos of both sides of the complete gaskets- in daylight- preferably with a white background labelled to show the cylinder number at one end of the gasket. Also, again if so inclined, in one section of the gasket that is heavily stained with the orange colour- working from the head side of the gasket, maybe you can peel off a section of gasket material about 1 sq inch in size to reveal what is left [if anything] of the steel mesh underneath.

I suspect you will be aware of most of the above from my previous posts on the subject matter and trust you will find it a useful contribution and hopefully will be of interest/relevance to your current audience some of whom will probably not be aware of such.

Tom. M

Is it me or is the side of the piston coating a bit worn off, too smooth and shiny? Not speaking from experience, but I thought they were supposed more of a matte look and is this an issue?
I'd check with Gregg as he is the one to know for sure.

Jon B.

Well that's disheartening as that's exactly what I was planning. So many more to choose from outside of the US!

Rob Edwards

The issue of the wear on the skirts is a bit of a moot point, since (AFAIK) there aren't any more GTS pistons available, so you'll be going back in with those pistons. Obviously there's some coating wear but they look re-usable to me.

Is that the 6 o'clock position on the piston? Guessing some schmutz got into that cylinder at some point and took it out on the piston rather than the cylinder wall.

FredR

Schocki,

When I first came across this issue back around 2005 I saw 4 motors in pieces- three with major corrosion issues and my 90S4 motor with no corrosion whatsoever on the heads. The three examples that had corrosion had sat for weeks or months on end not being used whereas my 90S4 motor was regularly used covering about 12k km average per year. This made me wonder if regular starts with expansion and contraction cycles might just cause any coolant trapped in the crevice to get some small degree of replenishment and stop the additive package exhaustion - thus delay or eliminate acidification. Unfortunately I saw clear evidence of an example with plenty of year round use have an issue.

We have seen examples of long term significant useage where the gasket came out in one piece such as Jeff's [Speedtoys] motor. For sure when the gasket emerges undamaged then there is no head corrosion and there are no signs of coolant degradation- why this can happen is the mystery I have not been able to resolve to date but I do not rule out regular use as the underlying reason.

Michael Benno

This car was built in 04/13/1992 and was sold by Miswa Motors in June 1993. The car was imported from Japan to Canada in 2009 with 34,484 and was driven an average of 2300 miles per year. The dealer service history shows regular service increments, steadily increasing the mileage. There do not seem to be any long bouts of sitting in Japan. For the next 13 years the car lived in Alberta and accumilated similar annual mileage increments, the car was owned by a former RL member and was cared for very well with lots of parts receipts from the usual site sponsors. In 2012, the Automatic driveshaft failed and was replaced with a BlackSea driveshaft, super bearings, and superclamp. Driveshaft play was documented as Crank endplay inspected at 0.007" (New spec). When I purchased the car in 2021 the TB play was essentially the same 0.008". I counted six coolant changes throughout the life of this car, including the last one done one year ago. So there are some data points for you.

Michael Benno

These are the original cams to the car. They were not previously replaced.

Agreed

I will get some detailed photos as you requested.

Andrew Schauer

Michael, I've heard others sacrifice the rear washer by bending it enough to get a puller jaw behind, and use the washer to help pull the sprocket off. I'd try warming up the crank pulley with a propane torch, that might be all you need to get it to free up.

Jon B.

Rob, what is the overall consensus for having a set of pistons made? Obviously if the factory pistons are still usable that is one thing, but surely a company like CP/Carrillo can make a set for a "reasonable" amount of money.

EDIT: Looking at their site that's about a $3k ask! So, maybe not for a stock rebuild, but stroker or boost builds.

Tom. M

You have to work it off like Andrew noted (woodruff key isn't there to hold it in, just to keep it from turning if I recall correctly). Lightly pry from opposite sides and it should come off.

Rob Edwards

I think you could contact Carrillo and send them a GTS piston and say, I want a replicate of this piston made in aluminum. And then you'd have to send the piston with the block to a machine shop that can plate the bores with Nikasil. Making up numbers but it's probably $2500-3K for the pistons and 2-3K for the Nikasiling.

Rob Edwards

On the crank gear, soak it with Kroil, and then carefully heat the gear so as not to melt the belt cover. You can sacrifice the rear crank washer if you need to pry on it. The woodruff key is not coming out until the gear is off.

jschiller

Machinists use tapered wedges to remove tight fitting components like drill chucks from their arbors. They look like this:

If you can find a set with a wide enough opening, you may be able to place it behind the gear and tap the wedge until it loosens enough to remove it.

https://www.mscdirect.com/product/de...6&gad_source=1