In looking for my 911 through eBay etc. I'm seeing wildly varying mileages before an engine rebuild. In some cases they were rebuilt at as little as 85K and then there are other examples with 150K or more, no rebuild, and stellar compression numbers. What conditions or use result in the earlier rebuilds?

 

What model 911 are you looking for? Reasons for 'earlier' rebuilds can vary by model.

 

'78-83 SC or the first year or two of the C2.

 

first years of the C2 were potentially troublesome. the SC was not - other than the possibility of headstud problems

 

On the SC's dilavar head studs can result in a teardown at any time, but at lower mileage it's generally not a full rebuild as many parts will not need to be replaced. If you are looking at an SC check for broken head studs, and even if there are none, if the originals are still there negotiate the price down in anticipation of a failure. I found out the hard way, with studs breaking 7 months after buying a low mileage SC for "full value". I'm now having to put many thousands additional $ into it to replace all of the lower studs. It will be great (also with a refreashed trans) when I'm done, but if I had to sell it now I'd take a bath.

 

Early C2s have been apart, early in life, mostly due to oil leaks. They can/could leak from the cylinder to cylinder head, from thru-bolt o-rings on the case, to cylinder base o-rings. Actual "rebuilds" would be rare on those cars, because other that the leaks mentioned they're a good engine. C2s and C4s were fitted with noise dampening panels under the engine & trans. Those panels retained a lot of heat, which exacerbated the oil leaks. One additional problem, also rare, a C2 could have had is the drive belt breaking inside the ignition distributor. The C2 engine is twin-plug, and the car is fitted with two ignition distributors (one for each set of 6 plugs), that are joined at the hip. The primary distributor is driven by the crankshaft, and it, in turn, drives the secondary distributor via a tiny rubber belt. When that belt brakes, if the secondary distributor stops with its rotor aimed at a firing point inside the cap, that cylinder will continue to fire, which can cause a failure inside that cylinder's combustion chamber. Usually drivers that are in tune to their car notice a drop-off in power, or other symptom, and the problem is found before damage occurs. If the second rotor stops between two firing pins the problem has no secondary consequence, and should be found during a routine maintenance.

'78-83 SCs are just about bullet proof. They do break head studs, I'm not aware of a percentage of the total cars sold but I'm pretty sure that it's small. They should also have their timing chain tensioners upgraded to the 3.2 liter Carrera type. On well maintained cars the first real failure can occur at about 200K miles - the camshaft lobes will start to pit. Yes, the cams can be replaced without doing a rebuild, or even a valve job. I've done a few total rebuilds, after 200K miles, and it's amazing how good the rod & main bearings still looked. High quality oil, Kendall, Valvoline, Quaker State, is the key, along with a change interval of 5K miles (one year for garage queens). About the head studs, a very strange fact is starting to emerge. Low mileage cars are more likely to suffer the problem than are cars that get driven frequently, and driven until they're at full temperature. I've replaced studs on a few SCs with less that 40K documented miles, but on cars with over 100K miles, only a couple. When it does come to rebuild time all 24 head studs should be replaced as part of the job. WARNING to all 911 buyers: There seems to be a very wide interpretation as to what "rebuild" means. I would bet that only 10% of cars represented to have a rebuilt engine actually have a "rebuilt" engine. Get those receipts and call the shop that did the work!

 

Peter - why would you replace the upper head studs on an SC, which are already steel? Most everyone told me the upper steel ones are fine and to just replace the lower 12 dilivar ones, which is what I was planning to do. They said they have never seen an upper steel one break.

 

JBO: There are going to be a number of opinions on this subject, and more than one might be right! On cases fitted with steel upper studs I was comfortable replacing just the lowerstuds, but ONLY WITH EARLY GENERATION DILAVARS. The latest generation stud w/full threads has a different coefficient of expansion, and Porsche technical literature calls for using them "in sets of 24". They might work OK with lowers only, but from the first set of them to show up at the shop I've replaced all 24, regardless if the case originally had 12 + 12, or 24 early Dilavars. On all of those engines, at the 1,000 mile follow-up valve adjustment & oil change, we found no change in cylinder head retaining nut torque. Compare that to earlier repairs using 12 lower studs only, those engines almost always changed, and needed the heads re-torqued. I hope this info helps, and it's not too late to do all 24 if you decide to go that route.
Pete

 

Thanks Peter. I will discuss this with my mechanic and will probably do all them all. I had never been told of the different threads, and your explanation makes sense.

 

Before doing any more you need to read Wyne Dempsey's book on rebuilding 911 engines - this gives chapter and verse on the various studs, where the weaknesses are in the various 911 engines and the most appropriate solutions.

PJC