The good news is that as far as engine wear goes, the M96 engine seems very good from what I have heard. The people who I have spoken to have all said that bearing, piston ring, and valve guide wear in high mileage engines that have been stripped down has been minimal compared with air-cooled flat sixes of similar mileage. Most 996 owners will tell you that the engine uses less oil as the mileage increases. My "˜youngster' of 79000 miles never needs any top-ups between oil changes. There was a chap on here who had something like 135000 trouble-free miles before he sold the car, so perhaps there are one or two 100000+ milers here who could post their experiences??
The above is my answer to your question, but I would hate to leave it at that if you were unaware of what follows.
There have been cases of severe failures in these engines, and they are well documented. A few early engines have suffered from intermediate shaft failure, which usually occurs within the first 25000 miles. The other concern is cylinder liner cracking (and slippage in a few cases). The extent of this problem is difficult to evaluate, and as far as I am aware there are no statistics available. My opinion is that although I don't deny there have been problems, people tend to get a distorted perspective by reading forum reports of these failures. A huge number of these cars have been built compared with previous Porsches, but the failures get so well publicised it makes the problem appear much larger than it actually is. Basically, you can be unlucky.
There are solutions if you do suffer one of these failures, but they tend to be expensive. Perfect Bore in Andover have developed a new cylinder liner to re-sleeve 996 cylinder blocks. Autofarm are the exclusive distributor of these units in the UK. Hartech in Bolton have some more cost-effective solutions where they can replace single liners, and fit special support rings at the top of the bores. Both of these companies make strengthened intermediate drive shafts. The other options are to fit a used engine from one of many dismantlers, or the most expensive way out is an exchange engine from Porsche, although this will come with a 2 year warranty.
www.perfectbore.com (Porsche Aftermarket > Water Cooled)
www.hartech.org (Buyers Guide > Buyers Guide Part 4)
The Achilles heel of the M96 engine is the Rear Main Oil Seal. There are numerous reports on this problem, with a few people having multiple failures although these are rare. The problem manifests itself with, at worst, the occasional drip of oil on the garage floor. The majority of people live with that until they need a new clutch, when it becomes 5 minutes additional work, and a £10 parts cost to fix. Porsche have made several design revisions to the seal, and the latest version seems to be less prone to failure. Tiptronics seem less susceptible, but not immune to the problem.
I don't believe any engine is truly "˜bullet-proof'. I've seen a 944 engine where the counterbalance shaft has tried to chew its way out the side of the engine. The early 911 engines had the occasional snapped timing chain. 968 engines are prone to jumping a tooth on the camshaft drive sprockets because the camshaft wears so badly. All can be very hard on the wallet.
If it makes you sleep better, or you don't want to take the risk of a catastrophe, the Porsche Extended Warranty (currently £895) covers all these issues, even the RMS. Bear in mind though that you can only take out the warranty until the car reaches its 9th birthday or 125000 miles. (10 years or 125000mls+12mths cover). As I have said elsewhere, the very reasonable cost of this cover, compared with the likes of MB and BMW, gives an indication of the extent of engine problems on these cars. Porsche are in the business of making a profit, and I'm sure this applies to their warranty too.
I hope I haven't totally put you off the idea of a 996. Just remember that for every bad report there are probably fifty unspoken good ones.
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