3.0 rebuild question

[Tim Hancock]

I just purchased my first 911, a somewhat ratty 1979 SC coupe.
The engine had a occasional slight backfire while accelerating at low rpm. The car did not smoke. It had the pop valve and tensioner update. The purchase price was low and the car was solid so I bought it.

Well the backfire was due a problem with #5 cylinder. The spark plug was all crapped up with a dark carbon type substance. I checked compression and got the following results: all cylinders at between 143 and 148 psi except for #5 which read 50 psi.

Next I did a leak down on #5 and noticed air blowing out of the tail pipe. I talked to a local Porsche guy (Tab Tanner). He suggested that I might check the valve adjustment and check for broken studs. Upon doing so I noted that #5 had tight clearance so I adjusted and the car ran OK for about ten minutes then the backfire returned. I readjusted the valves and put in a new plug and tried again.
Once again, the car ran ok for a short time then the backfire returned. This time I checked the valves and the .004 gap was still ok the compression came up to 65 psi but the plug keeps getting crapped up.

While I know it is wishful thinking I hoped that the valves just had a layer of crud on them and due to running on a bad plug.
Now I have given up and resigned to pulling the engine.

I also noted that one lobe on the #4 cyl intake lobe had some corrosion induced pitting on it.

With compressions reading as they did and assuming #5 head needs work and at least the right cam needs attention, could I get away with just repairing the bad head and cam or do I have to bite the big one and go all the way with new pistons and cylinders? (money is an issue, my time is not so much)

Would I be wise to get hotter cams as opposed to stock (I will definitely keep my CIS and stock exhaust for now)?

Well if anyone wants to advise, I will listen.
thanks, Tim Hancock

[Damomma]

You didn't mention the miles. Could be a bent exhaust valve from an over rev situation an not seating ergo the air out the exhaust (I assume the exhaust valve was supposedly closed?) I'd say the backfiring is likely that exhaust valve not fully seating. Once you get it apart you will be able to more acurately assess but if there are no missing pieces, a new valve, valve guide and seat grind may be all you need. Good luck

[Tim Hancock]

Thanks Damomma for the reply.

The mileage is unknown, the PO said he that was what he was told by the guy he bought it from. The Odometer reads 66k.
I found a stamp in the owners manual that showed a oil change
dated in 1998 and the mileage was written down as 88k. Who knows? I realize I am a little premature in asking for advice without having even taken it apart, but sitting here at work at my desk my mind keeps wandering back to my project. No matter what, I will not put it back together with faulty parts.

I have a home business working on aircraft and it is standard acceptable practice to just send a faulty cylinder out for repair
when low compression is found (a complete engine overhaul costs upwards of $10,000), in other words just fix what is broke as long as compression and oil pressure are ok.

I have been told that just installing new rings (as opposed to new pistons and cylinders) is bad practice on these engines, so if I pull it apart and find no scoring in the cylinders should I leave the old rings installed?

thanks, Tim Hancock

[Damomma]

Wishfully thinking there may not be a problem with the cylinder and rings if parts of the valve didn't drop into the cylinder and it was just bent and not seating. But you need to skim this post about the pros and cons about reringing and honing etc. Do you know if you have Alusils or Nicasil cylinders?

Re-ring Alusils?

[Tim Hancock]

I do not know which cylinders I have yet. That alusil thread was
interesting.

I also am wondering if I should buy new lower steel studs? It seems as though I read somewhere that the SC's came with Divilars on the bottom. Is there any way to tell which ones I have?

I read somewhere on this site that 964 cam profiles are a good way to go if retaining the stock CIS system. Any thoughts?

[Damomma]

I say absolutely replace them if they are old Dilivars (you can tell if they are Dilivars as the metal is non magnetic). If they are original they are dilivars and would not be epoxy coated. As to what to replace them with I've probably gone against conventional wisdom and have used new dilivars after everything I read I could not get over the benefits of their original purpose (ie their expansion qualities). Wayne's book says and most of the people on this board would never use the dilivars again, only steel or raceware type ($$). The dilivars are now coated for what it's worth and threaded on their whole length. My 79 broke 3 of the dilivars due to corrosion caused by road debris sitting behind the engine cooling tins. They snapped right in the area where the surface was corroded. My 79 has nicasils and I am reringing.

Again not much experience here with the cams but everything I've read is that if you are maintaining the stock CIS a cam change isn't worth it. Now if you back date it with carbs I think there is some value.

[boyt911sc]

Tim,

Dilavar head studs are non-magnetic. Easy way to find out is to attach a magnet to these studs. The orginal bottom studs for SC were Dilavar and steel on top row.

And pulling these Dilavar studs will be an adventure. Some people had easy time removing them but others had difficulties loosening them.

There is plenty of help and advise you'll get from this board.

TD

[Groesbeck Hurricane]

No experience with cams and only limited knowledge base.

I re-ringed my pistons and have not regretted it. Check out on the breakin suggestions. I did the hard breakin.

I went with the steel studs available from our host. They were in-expensive and I have some peace of mind.

I would agree to fix what is wrong and not go overboard. The suggestion of an exhaust valve not seating is a very good one. Remember, this can easily become a "While I am in there" project.

I would take off the valve covers and see if I could identify any issues, see anything wrong as the motor was turned by hand. You can also investigate the headstud status at that time. If you do decide to tear into the motor, I would strongly suggest you plan to split the halves. Many of us (myself included) have found some extra wear on the bearings. It is just a suggestion and there is most likely no reason for you to go that far.

While you are in there, invest in some of the inexpensive replacement washers and senders. Replacing them while the motor is out will save you from an oil leak down the road and cost less money than one would think.

[Henry Schmidt]

Hi Tim
It has been my experience that you can rering Nicasil but not Alusil. Before you rering, check for roundness. I've seen cylinders .004 out of round. When buying new rings only use Goetze. Don't use a bottle brush or multiple stone hone. I've had the best luck using soap and water with Scotch Brite. It brakes the glaze without cutting into the Nicasil. Web 20/21 cams offer the most performance in a stock SC. They are very close to Porsche new Super Sport cam. They will make smog testing a challenge. As for head studs, I'm a little biased here. We make the best head stud kit on the market. It offers a new stud design no just a new material. Coarse thread in the case, fine thread at the nut. The kit come with 24 studs, 24 washers, 24-12 point flanged nuts, loctite, anti seize and instructions. The complete kit sells for $595. Did I mention a life time warranty. Now available at Pelican Parts

[Groesbeck Hurricane]

Henry,

Putting money up on the guarantee? I like it! Is the install guaranteed if we do it ourselves or only if done by a trained mechanic? Curiosity.

[Henry Schmidt]

The guarantee is that these studs won't fail. Even a 4 year old with a hammer can't break these studs. We will guarantee them against *failure no matter who installs them. Forever !! Remember, Dilavar studs break by themselves. We've been sitting in the shop eating lunch, heard a pop only to find 1/2 of a brand new Dilavar stud laying on the floor next to a rebuilt engine still on the stand.

[Rondinone]

You can certainly rering alusil. The cylinders need to be prepped properly, but it's not expensive and there are good resources available. It's not common knowledge, but there are shops that do this and a commercial product available in case you want to do it at home. PM me if you want more details.

One day I'll get around to writing a tech article.

[Tim Hancock]

Thank you all for your advice. With many hobbies and interests, I have searched many websites and forums for information. This site is by far the most active and useful I have had the pleasure to experience thus far.

In the past on my watercooled Porsches I have purchased most items thru Performance Products which was the only place I really knew about.
It looks like the host of this site will get my business from now on.

thanks again!

[Henry Schmidt]

I'll bite, what rings would you use?
Hastings doesn't make one, Deves says you can use the same rings as Nicasil (they're wrong), I have found no after market company that sells Goetze ring for Alusil cylinders ( if you have , please let me know) and that leaves the dealer. The dealer has them but for the cost of new Alusil rings you can by a good used set of Nicasil, new Goetze rings and be miles ahead. Replacing Ps & Cs is a big job, want to take a chance? GOOD LUCK

[Wayne 962]

In general, I don't recommend re-ringing Alusil cylinders. True, the Porsche dealer has a part number listed for replacement rings. I know plenty of people who have used the standard (Nikasil) rings to re-ring Alusils and they worked fine. I did this on one engine too, with excellent results (clean engine, passed smog, etc.). Who knows? I've always contended that reringing Alusil has been a gamble.

Tim, you should get a copy of the Engine Book: http://www.101projects.com - many of the questions you have asked are answered in extreme detail in the book...

-Wayne

[Tim Hancock]

Thanks Wayne, I will be purchasing your book shortly.

After further thought I have decided to forgo dropping the engine at the present time. I am clinging to my hope that the PO drove the car with a
fouled plug for a long enough time to crap up the #5 cylinder and that I now have carbon/crud deposits on the valves in that cylinder. My first couple of cycles of adjusting the valves/changing the plug showed on one occasion that the valves went tight after running OK on a new plug.
Then the plug fouled again and backfire returned. Last night I bought a copper plug 2 heat ranges hotter (instead of the platinum plugs that I was replacing the fouled plug with), the car runs good now with no backfire after 1 hr of hard driving. Today I will readjust the valves and check compression again to see if it is coming up some more. The fact that my compression has been coming up and that the car does not smoke, leads me to believe I may be on the right path.

My goal is to slowly, as spare time allows, start going thru this car and bring it back up to snuff bit by bit. I would like to drive the car a little this summer before dropping the engine. I will probably drop it this winter to replace cams and fix oil leaks and cleanup and detail the engine bay.

Once again thanks for the replies.