What causes a valve spring to break?

[KobaltBlau]

Yes Grady! I should have mentioned that - you get full 'I told you' credit on this one. You can imagine I was very happy to have that tool at hand and not to have begged or borrowed it last time!

I really like the theory that the funky CIS dome is just the right shape to close the intake valve gently I will do a proper leak test when I get a chance, but during the repair the 5 or 7psi that I applied to the cylinder to keep the valve shut seemed to be sealed in quite well. As an aside, John Walker says he doesn't even bother with that - the valve pretty much just stays in place. I had already set it up the way I had done it before, however.

I also agree that the rust is the most likely candidate, and it makes sense that the top part of the cam box would see more condensation. Also since this is such a low mileage motor, it has had fewer oil changes and therefore probably a bit more water in the cam boxes. The 4 parts of the outer spring had very polished and rounded broken ends, nothing like the sharp fracture I saw in the other motor, though I believe they were sharp fractures at some point. I believe this valve spring had been broken for thousands of miles, perhaps even before my purchase of the car at 28k miles (47k now).

It was great to see you last month, too. I wished I had more time, but I was glad we could fit it in. Next time we'll head out to the track or similar.

Best Regards,

Andy

[Steve@Rennsport]

Originally Posted by KobaltBlau

Still curious why these break on the intake side, as John Walker mentions.

JMHO, but I think its due to the weight of the intake valves. Porsche told us at that time that the root cause was improper heat treating.

I'm really surprised these cars can be run to redline with a broken outer spring without valve to piston contact, but hey, I'm not complaining!

Damned risky business, that,.... While these have a lot of piston-to-valve clearance, running to redline with just the inner spring (basically a damper) to control the big, heavy intake valve is the Road to Perdition. The financial consequences are quite steep.

[Peter Zimmermann]

If your SC was one of the first 1,000 '78s built, replace all of the springs when possible. I documented this in my book a few editions ago, because I believe Porsche's spring supplier sold them a bad batch. What's happened in the last 10 years (since I sold my shop to Marc), is another story, but apart from the very rare spring failure in other cars, even well-maintained ones, nothing more can be said other than sooner or later everything will break.

John's comment in Post #15 above is right on the money. The vast majority of broken springs that we found were intakes. And Steve's recommendation, to push on every valve before adjustment, was also correct. My guys all were sensitive to a spring that no longer developed the correct tension, and occasionally we found a broken one.

[sc_rufctr]

Peter

What replacement springs would you recommend for a non modified SC?

I suspect one of my springs has broken. (no 6 intake ) I would rather replace the lot than just one.

[fumanchu]

Point of data.....

Just replaced #2 intake spring on my 4/82 build date SC. One clean break right in the middle of the spring. Doesnt run any differently with the new one, and interestingly has probably been broken for 1500 miles( had a valve tick that would'nt adjust out). That cylinder had one of the highest compression numbers of the engine at 165psi. Pretty robust motor.

[Peter Zimmermann]

Originally Posted by sc_rufctr

Peter

What replacement springs would you recommend for a non modified SC?

I suspect one of my springs has broken. (no 6 intake ) I would rather replace the lot than just one.

We always bought o.e. replacement springs from one of our wholesale suppliers, and they came wrapped as a set, with both a new inner and new outer.

[DUK]

Originally Posted by Steve@Rennsport

JMHO, but I think its due to the weight of the intake valves. Porsche told us at that time that the root cause was improper heat treating.

ding ding ding.... We have a winner. We always see Int. spring fatigue and failure before exhaust.

[KobaltBlau]

Interesting responses this resurrected thread has generated.

Originally Posted by Steve@Rennsport

Porsche told us at that time that the root cause was improper heat treating.

Interesting - my experience really feels consistent with faulty parts rather than abused ones, so the bad heat treating rings true. Non-scientific on my part of course.

Originally Posted by Steve@Rennsport

Damned risky business, that,.... While these have a lot of piston-to-valve clearance, running to redline with just the inner spring (basically a damper) to control the big, heavy intake valve is the Road to Perdition. The financial consequences are quite steep.

No kidding. I am very happy that I have avoided those financial consequences, thoughI can say with confidence that both engines were run to 7k while they had one broken intake outer spring. Of course I did not know that at the time or I wouldn't have done so!

Originally Posted by Peter Zimmermann

If your SC was one of the first 1,000 '78s built, replace all of the springs when possible. I documented this in my book a few editions ago, because I believe Porsche's spring supplier sold them a bad batch.

This is extremely interesting. My current SC is apparently a very early one, the 250th serial in US Coupes, with an 8/77 build date. This car only has 47k miles and the failure does not look recent, so it occurred at a much lower mileage than in the 81 I had. However, I am confident that no other springs are currently broken, and other than the mileage difference the symptom is very similar to the 81.

Bottom line, Peter: do you have information that suggests the bad batch applies especially to the first 1000 78s? Unfortunately I don't have a copy of your book at hand. Experience of myself and others with other years seems to indicate the problem extends to other SC years?

Thanks in advance,