When should the SC 3.0 engine be rebuilt?
 

The thread on the gentleman's 3.0-to-3.2 has me thinking; is there a definitive set of circumstances that dictate a rebuild, either top- or bottom-end? Your expertise could really help me as a new owner.

My situation is this: the night before I bought my '78 SC Targa (w 122k) it was being buyer-checked by someone I trust. I had thought I was about to buy an SC with one blown cylinder. The owner believed the same due to a shop's leakdown of 95% in that cylinder. The car had had a topend rebuild at 60K (missed shift), but that was it.

My buyer's check showed a leakdown in the suspect cylinder of 15%, with others around 5 to 7%. The mechanic told me "...don't worry about it, Carreras left the factory with more leakdown than your SC."

So I purchased the car at a firesale price and haven't looked back. My thought was that a valve job/Techron runs might free up a ring or alleviate a valve problem somehow. So my question is this: do I just go ahead and do a cylinder head overhaul on that "bad" cylinder? Does 15% indicate problems? If not, what leakdown number would have you rebuilding? Also, BruceA tells us to never put new rings into old piston lands due to "catching" and fresh ring breaking. Does this mean that a topend rebuild must be done within a certain number of use-hours or miles, and that, after a certain amount of time, a topend rebuild is frivilous and that the entire engine ought to be rebuilt?

I know these questions have many answers, but I would REALLY appreciate anyone's expertise on this one. Now that I've adjusted the valves and run mucho Techron through, I will have a fresh comp and leakdown done, this time having my mech. check for where the hissing is coming from (oil tank/airbox/exhaust pipe).

Thanks in advance for any help you can provide,
Jw



[This message has been edited by Jdub (edited 01-18-2001).]

There are other folks on this board with greater technical and practical knowledge than me , but I certainly believe that 15% is well outside of the box especially in relation to the other cylinders.
I am sure you will hear more from the others!

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Bob
1982 911SC Targa
PCA - Founders Region

How does the spark plug look on this cylinder in question? Also what is the oil consumption? Have you done a compression test?

I am of the general opinion to leave it alone. Why rebuild when you do not have to... Unless you like parting with lots of money?

If you feel you have to fix this, I recommend just doing rings and head work... You may get crazy and put the S-cams on too http://www.pelicanparts.com/ultimate/smile.gif But that may require more detail on the heads like port, polish, stiffer springs, and so forth. Once the jugs are off you can check the rod play and decide if the bottom end needs attention. And since the engine is appart anyway you may want to put on a SSI stainless steel heat exchanger conversion.

Run it till it frags and chokes on the pieces! It is an aluminum block it can be welded back together, plus you will rework the whole thing anyway on a rebuild so what is the difference? Nothing.

.... I can feel the flames coming....

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Adrian Pillow
1979 911 SC
1966 VW Microbus
PCA - Peachstate Region

Plug for that cylinder is a little bit oily (very slightly glossy black, not flat black as per a sooty rich mix.). All others the standard tannish, given whatever gas vendors are adding to the fuel.

Oil consumptions is nil, nada, nothing. I lose only what comes out of my cam tower...very stable readings between changes.

Compression test is about 150 in that cylinder where the others range around 167.

I know this is a very open-ended question I've asked. The main question is am I harming the engine? Is a bigend being adversly affected by a weak cylinder?

Thanks,
Jw



[This message has been edited by Jdub (edited 01-18-2001).]

JW,

It sounds to me as though you have a piston ring problem in that cylinder, but it is not an absolutely critical problem. Whether it is wear or a broken compression ring is hard to say and maybe impossible to ascertain from outside. That is definitely oil fouling on the plug, but the critical question to me is, what is the valve guide condition? It is possible to test for valve deflection at max. cam lift while doing a valve adjustment time, so I would recommend doing that, ASAP!

I'm not familiar with the 'paraphrase' you attributed to Bruce A, and the simple fact is, you ALWAYS are putting new rings into used piston ring lands ... any time an engine is re-ringed or do a top-end job! The ONE IMPORTANT CONDITION he warns against is putting rings into, and consequently, reusing pistons with the top ring land worn out ... with excessive 'side gap!'

The simple fact is that SC engines do have a finite lifetime limited by the Dilivar top row of head studs, and your engine has conditions that indicate a top-end rebuild sometime in the near future, but it is not an impending disaster, just a pressing, continuing problem that will have to be dealt with ... whether six months or two years from now, I can't say. If the valve guides are worn to the extent that they can be felt at the valve stem tip, then I would suggest attention toward the six month end of that interval.

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Warren Hall
1973 911S Targa

The engine should be good for another 50,000 miles http://www.pelicanparts.com/ultimate/smile.gif And by that time you may want to do something more adventurous in the rebuild. Unless it's blowing clouds of smoke, just leave it.