The Solution can also be reused in another engine or rebuild of the same engine.

 

If "excessive heat events" were the sole case for cylinder bore scoring, then we would see cylinder scoring across all cylinders in the engine, but that's not the case. Some could make a strong argument that regular cold start enrichment in cold climates does negatively affect the engine during start ups and is one reason many owners install low temp thermostats. But what has been discovered is that cylinder scoring on the M96 affects cars in warm climates too, not just in the north. When these engines are rebuilt with nikasil cylinders, better pistons with more durable skirt coatings are used, and the piston pin offset is corrected, then scoring does not happen. The factory cooling design of the M96 engine stays the same aside from the thermostat. Therefore, if the engine could not control "excessive heat events", then we would see more cases of scored cylinders in M96 engines with Nikasil treated cylinders. Now, if you don't maintain the cooling system of these engines, like neglecting to keep the radiators free of debris or never changing out the water pump during proper service intervals, then bad things will happen and the engine will experience over-heating that could severely damage the engine. For me, cylinder scoring is a result of many factors that lead to the issue.



How can you tell from the ad that this car has the original engine? We don't really know it's service history. For all we know, the original engine could have been replaced or rebuilt in its 24 year lifespan. The seller is giving the impression that he put all the miles on it. I would get a CarFax and see what the history looks like. I bet you this car has seen a many owners. The seller is also wrong in stating, "dual row IMS bearing from the factory so it’s never needed to be changed". That's bad advice. While the dual row bearing has proven to handle more load than the 6204 single row, it still will fail and should be replaced proactively along with clutch service. Again, if he's not the original, how would he know conclusively that the bearing has or has not been changed?

As a side note, the '99 model used ferrostan skirt coating and has shown to be more durable than ferroprint and one reason why the '99 model 996 is a special car.

 

Again, I'm not an expert, so I'm happy to be corrected, but cold starting would only cause uniform cylinder wear (unless the piston pin offset you mention is a factor that creates uneven wear -- I'm not familiar with that). Failing that, the only way for the cylinders to get out of round is from excessive heat compromising the cylinder or, hypothetically at least, sag from aging metal (which I highly doubt, but again I don't know, so I'll toss everything out). I have definitely read that heat is the primary issue in what I felt was a pretty well supported article, but obviously that source could be wrong. I don't think the temperature is exactly equal across the cylinders in an overheating scenario, and the are probably manufacturing variations on how individual cylinders behave at the temperature limits. Also, individual cylinders running lean could could cause them to run hot -- not an issue for regular engines with a block. The fact that radiators get blocked is highly unusual and unique to these cars -- how many "normal" (non-RL) owners knew to clean them regularly? Many 996's, probably the majority, have gone a decade or more without cleaning because owners didn't know or didn't do it even if they knew. My car had extensive history and pristine maintenance from new, but the radiators had never been cleaned until I got it.

I think he would say if it had prior maintenance. I think the person he bought it from would have said it. The last thing you want to say in an ad is nothing has been replaced after 325k miles, so it seems likely it's legit (I do agree that we can't know for sure). I agree regarding the 99 comment -- I think the 3.4 in general is less prone to these issues and if there is a metallic difference in his 99, that would also help.

The air-cooled 911 engines were generally pretty tired and worn from normal wear by 100k miles in my experience (the 3.0 SC engine being the exception, and the 2.7 being the "poster child" for rapid wear being baked to death by pre-catalytic convert thermal reactors), the 996 engines with no traumatic events are just getting warmed up at 100k miles (normal wear is far less). Probably 3.6's are more prone to the issues than the 3.4's, and dual-row IMS bearing with smaller displacement is more robust than single-row IMS with larger displacement (still not sure why Porsche increased the displacement using an IMS that had been reduced from the dual-row IMS in the early 3.4 engines).

 

I found the article I referred to in my prior post on the cause of bore scoring => https://www.rpmspecialistcars.co.uk/...-bore-scoring/

In short, the cooling and oil spray design results in uneven cylinder temps and the 6th cylinder tends to run hottest. Excessive heat events deform the cylinder to a slightly oval shape. Once the cylinder has a slightly oval shape, the cylinder wears very rapidly for obvious reasons.

 

Thank you for sharing that article. The theory as spelled out by RPM has been circulating for many years. Over the years, some "experts" have said cylinders #6 suffers the most because of the cooling distance from the water pump. Again, I would argue that if cooling (or even lubrication) that leads to ovality were the sole reason for scoring in M96 engines, then we would see the same scoring on rebuilt M96 engines with Nikasil because factory cooling and lubrication design of the M9X engine is left unaltered. If these engines are rebuilt by reputable Porsche shops that know to correct certain core issues, then the scoring problem will not reoccur like the factory original. Many shops are increasing displacement beyond 4.0L with an open deck design and not suffering for scoring.

Now, a more plausible theory why bank#2 scores more than bank#1 is because Porsche chose to use the same piston pin offset for both banks of pistons unlike what was used in previous 911 engines. Why did they do it? Probably another financial compromise like putting a $20 sealed ball bearing in a $30K engine to support one side of the intermediate shaft only to change it three times before completely eliminating it. However, as mentioned before, I believe its many factors that lead to the cylinder scoring problem in M96 and M97 engines. Also, I wouldn't say closed deck blocks are the ultimate solution either because 9A1 engines with Alusil are showing problems with scoring too, but that's another topic for a different day.

 

Well, not going to debate the 325k miles on the car/engine/imsb..?, in the craigslist ad you posted, if it is true its most certainly the exception according to the expert engine builders right here on our forum. I find craigslist to once in a great while, to have a few scammers...

When you say "water cooled 911" I assume in this thread you are talking about 996 engines. Yes..?

We just had a thread where (996engine experts) Charles from LN and PT3 both said that by 100k miles all 996 blocks have excess out of spec out of round and taper. The consensus was, the only way to rebuild is with sleeves, and to remove that 1/4" of loco-sil so sleeve doesn't slip'. Sleeves will prevent more structure problems. Even if the block is correctly prepared for and direct plated with a quality nik-sil, was told the structure will still fail.
The 1/4" of loc-sil the direct plating would be applied to, will fail was the answer I got from that thread. Thread went on to say Hartec use to install a ring around the top of the cyl to prevent the structure problems, but have now found that to be unsatisfactory fix and stopped doing it.

This isn't an exact quote, just my recollection of the thread from a month or so ago. Thread went on to say that the 1/4" of loco-sil the piston rides on actually changes at a molecular level and particles of the silicone migrate out and cause the structural problems. I have no clue how true this may be, but he is one of a handful of the most experienced 996 engine builders in the world. Other builders from across the pond have also posted similar findings. Being an engine builder myself (not 996 engines) I have to defer to the experts and do find there opinion to be a valid explanation. Especially when an very credible independent Porsche tech with vast experience from decades of dealership work, says the same thing, and he has no nik-sil cylinder sleeves for sale...

Just my anecdotal experience from measuring a very well maintained 996.2 @60,000miles and finding the cylinders to look perfect to the eye, no bore score at all. But were just within specs @ .0025" out of round, with 1 at .0026-.0027" taper/out of round. Thats 3/4 of the way there to out of spec excessive run out taper for a meticulously maintained 60k engine... How long/many more miles you think it will take till those 3/4 of the way there cyls go out of spec to excess out of round/taper...? 10k more miles, 40k more...? I truly doubt it could go to 325k and still be in specs, but maybe....

jmnsho

 

Just to add to my above post, consensus was...
While low maintenance engines can certainly decrease the time it takes to get excess out of round, it doesn't matter what you do to meticulously maintain your 996 engine, by 100k they all have excess runout/taper....

 

There are many, many high mileage examples on the original motor and some even on IMSB (https://rennlist.com/forums/996-foru...e-996-a-8.html). My 3.2 Carrera felt tired by 100k miles, and my '74 2.7 911 was rebuilt (properly to fix all vulnerabilities) at 40k miles (which was the norm for those cars, I bought it just after rebuild). My 996 has 80k miles and runs like a scalded dog and feels like it just rolled off the assembly line. Many in that thread have double or more that mileage and feel the same. I do not believe high wear at 100k miles is thing absent causal events, but I definitely have not measured either of my 996 engines. I didn't need to measure my 3.2 to know that the engine was tired at 100k.

 

Like I said, this isn't my prognosis/diagnosis, its the 996 engine experts as mentioned above. I don't doubt your claim of "many high mileage examples on the original motor and some even on IMSB" and also don't doubt the expert 996 engine builders.

Keep in mind, just because the out of round/taper is not in specs doesn't mean the car is not drivable. From my experience excess taper/outofround, but still close to max limits might cause (maybe to a small degree, maybe more in some) a louder engine (piston slap), poor cylinder sealing, lower compression, excess leak down, excess blowby, as the OOR/taper increases with more miles all the above get worse and more noticeable,,, + more oil usage, oil contamination, bore score susceptibility/bore score, piston skirt fatigue and eventually piston failure, D-chunk, cyl cracks etc are all more possible with excess OOR...

Now, as the above out of round/taper gets farther out of spec, or bore score starts, then the observant owner probably will notice/hear something is up...

 

Basedon just fiddling with engines, of many flavors,,
Bore scoring is often directly tied to fueling and fuel residing in the cylinders too long.
1. Tired injectors their average efficient life is 80K miles.. When they die they leak and over fuel especially at cold start and when short tripped.
2. Modifications that don't play well with teh stock ECU.
3. Bad MAF, not dead, just tired and week, I consider them a maintenance item. about 40K and in goes a new one.

How many people have stuck a piston in a 2 stroke dirt bike? SAME problem just quicker and more violent..