Jim (chatas17),

I think you are getting some good perspective above.
Best is:
In its formative years, if your 911 had been “driven hard & put away wet”, it would shorten its life.

There are three important factors:
How it is (was) used (my above “…put away wet” comment).
and
How it was serviced.
A close third is your expectations – it takes regular service (top-ends &rebuilds) to maintain peak performance.
I have some race engines that I completely rebuild every 25 hours of green-flag on-track running.
My street 911 SC is a bit more extended.
My original ’68 911 doesn’t count as it had a fresh (test :rolleyes: ) engine too often to count.
Your question is “How close to ‘perfect’ do I want to stay?”
For a street 911, most of us accept something only slightly less than 100%.
I’ll leave the 100% to racing.

Bob S. (LakeCleElum)’s post shows you the ‘way’.
Maintain a history of cylinder leak, cranking compression and other.
This will guide you to when it is ‘necessary’ or simply prudent for a rebuild.

Best,
Grady

What is the average cost of a top end rebuild?

Mine has 118K miles on it. Never cracked open, burns about 3 quarts per 1000 miles. It's about time...................:(

124,000 on my 88 Carrera. No issues. No plans to do anything but drive and maintain. Runs excellent, even on top of the Continental Divide.

Karl
88 Targa

I bought a used 1986 3.2 engine with about 80,000 miles. It was obviously driven hard by the previous owner, noted by the fact it came from a wrecked car. It needed the guides done, everthing else was ok. I did the valve guides, exhaust valves, and rings. Bottom end was still good. It ran great and I ended up selling it to someone for his 914 conversion.

My 88 has 138k miles on it and burns no oil, still on first engine and runs really strong.
My neighbor has the same car but the 89 model and had to rebuild the engine at 80k miles (2 years go).
Even with all the miles my engine pulls stronger than his.

On my 87, was burning oil so did the top end at 190K. What I found surprising inside was the amount of build-up in the exhaust ports. They looked to be 25% blocked, been told thats from fuel. I'm glad its all done now and clean as a whistle inside. All the "misc" minor leaks are gone now also.

My compression and leak-down were great, but was consuming a quart of oil every 400 miles prior to a top end (guides, seals, intakes, rebush rockers, etc) at 90k. I ended up splitting the case to check everything out and replace the intermediate bearings. Re-ringed pistons too. Running great now.

Top-end done at 73,000 miles. I was burning a quart of oil every 300 miles. New valve guides, studs, rings, sensors, etc. - car runs great now.

110k and brought it in for a valve adjustment...two broken head studs fell out when the valve covers were removed...absolutely amazed at how much better the engine runs now 10k post-rebuild. The air-cooled sound is still there but it sounds so much tighter/better and the vibration level in the car is way down...of course it pulls much stronger now...best money I've spent on the car followed by the Griffith's A/C.

I was expecting mine to need it by 300k, but it still runs strong and pulls like a freight train.
Burns a quart of oil about every 6-700 miles these days, I'll wait until it loses power or starts burning more oil...

Chuck.H
'89 TurboLookTarga, 335k miles

196k and dry
 

I have an 84 with 196k. The bottom is dry and no smoke on deccel or accel. Quiet all through the rev range. Will need the chains replaced as I noticed stretch symptoms once on a really hot day. And love the Motronic over SC CIS especially with the addition of a Steve Wong chip.

135K on my 84 no leaks, smokes for a few seconds after it has sat for a few weeks. Uses about 1.5 quarts of oil on a 5K oil change.

My new to me '88 is smooth with plenty of power. Compression numbers are excellent -- BUT it burns oil. Get an occasional whiff of blue smoke at stop lights. I would estimate it burns a qt. every 500 miles or so. I knew when I bought it that the valve guides were showing wear, but I didn't know it would burn this much oil. It's got 113K and the (complete) records suggest that the previous three owners maintained it properly.

So what do you guys think: I can live with the oil consumption indefinitely, and I'd like to put off opening the motor until something forces me to (like a clutch replacement). I know that worn guides compromise the valves' ability to dissipate heat, and that in theory one could overheat and break. But that strikes me as an extreme case scenario -- or is it?

I was going to post my own topic, but it seemed relevant to this one.

Thanks!
Hugh

Bruce Anderson, "i have never rebuilt a 911 engine because it wore out"

Rebuilt the top end of my 87 at 100k miles, oil usage had climbed to 300 miles so it was time.
As noted on a previous post, my ports had very heavy deposits that likely choked the air flow 25%. Yours may run for years without issue, but if consumption is at 500 miles then it is just a matter of time. If you are motivated you can do most of the work yourself, will have to sub out the head rebuild to a shop but there is an abundance of documentation available on how to do remove and disassemble the engine. Start reading now and when you feel the time has come to make the leap you'll be prepared for the job.
Other good sources are Wayne's rebuild book, the Porsche service manual and the Bentley manual.
Cost to rebuild the heads will be around $1000, gaskets and other while in there items will add at least another $1000 to the parts total. Go whole hog and the while you're in there items can really add up so be mindful if on a budget.
Assistance from the forum is priceless.

My 85 911 has 140,000 on the clock. Never been opened up and runs great. Oil use is about 1 Qt per 900-1000 miles. Runs very well and passed Cal smog with ease. So that tells me it is in good shape. A well known San Diego Porsche mechanic told me that it should go 333,000 miles easy. Wow, these are super cars. Most cars our vintage have been crushed long ago. But when I see a vintage 911 it usually looks great! Inmside and out. Like mine!

110k on 89 3.2. It was smoking and using a quart per 350mi. I intended to do a valve job but that turned into a complete rebuild. I got curious about the bottom end and pulled it apart and I'm very glad I did. A couple of the main and rod bearings were worn and the intermediate shaft bearings were showing copper. This "valve job" has turned into a new set of Mahle pistons and cylinders, ARP rod bolts and head studs, new main and rod bearings, rebuilt rods, 964 cam regrind, rebushed and resurfaced rockers, etc, etc. I have a problem in that I can't seem to just do the minimum required with these car projects. :rolleyes:

When I swapped engines on my SC, I was told that the 84 3.2 engine I put in had 70k miles when it was rebuilt. I do not know why it was rebuilt so 'young'. I have since put another 5k miles of very happy motoring.

So why do these engines run so long? The geometry? The air cooled concept? Do the 996's last that long? Just curious. I'm amazed how much pull my car has with 115,000 miles.