1979 SC Chewed a Valve - Total Rebuild

[snbush67]

Originally Posted by OsoMoore

I was in first gear during the initial bang-bang-bang. I did try re-engaging when I thought it was still running, to see if I could get speed to clear the tunnel and construction zone. I think that was when I confirmed the engine had stopped.

It was an awful spot to have something go wrong - construction in a tunnel. I have a lot of what-ifs looking back at it. What if I'd cut power as soon as it sounded off? I did end up having to push the car out the last bit, but I made it most of the way out before then. I'm glad I didn't end up run over in the dark.

That was a bad spot to break down.

So you heard/ felt the bang, you must have pushed the clutch in, then thought it was still running and let the clutch out. Did you let the clutch all the way out? Or maybe just until you felt resistance? I guess that could have spun the engine which stripped the cam gear and as Danobee wrote, loosened the nut.

The rocker shaft misalignment theory is looking like a top contender.

[917_Langheck]

Originally Posted by David Inc.

There's a reducing gear between the crank and cams that changes rotation.

Edit: Ask a 996 owner, it's their favorite part!

For whatever reason I misremembered that change of direction. More sleep, must get more sleep...

[OsoMoore]

Originally Posted by snbush67

That was a bad spot to break down.

So you heard/ felt the bang, you must have pushed the clutch in, then thought it was still running and let the clutch out. Did you let the clutch all the way out? Or maybe just until you felt resistance? I guess that could have spun the engine which stripped the cam gear and as Danobee wrote, loosened the nut.

The rocker shaft misalignment theory is looking like a top contender.

Its hard to piece together exactly what was happening with trying to find a spot, analyze the noise in a noisy tunnel, and watch traffic. I think I tried to let up on the clutch to see if I could get more power to make it out of the tunnel, and found that it was slowing me down instead, so that would have attempted to spin up the engine.

Time to pull it a part a bit more before bed, will post back soon.

[OsoMoore]

Analysis later, here are the photos from finishing bank 1. Video later.

[OsoMoore]

[OsoMoore]

Sooty but no major visible damage on bank 2.

Picture order is 4-5-6 left to right. Note the bit of metal caught in 6, holding the exhaust valve open a fraction.





Cylinders in order 4-5-6 right to left. They are also quite sooty, a little bit of metal scraps in there. Potentially fell in during the many engine flips on the stand. Didn't spot any scarring on the cylinder walls with a cursory look.





Next disassembly steps are to get the chain ramp off so I can remove the chain housing on bank 2. They wiggle but do not come free.

I'll consult the book for the next step after that!

[David Inc.]

Originally Posted by snbush67

That was a bad spot to break down.

So you heard/ felt the bang, you must have pushed the clutch in, then thought it was still running and let the clutch out. Did you let the clutch all the way out? Or maybe just until you felt resistance? I guess that could have spun the engine which stripped the cam gear and as Danobee wrote, loosened the nut.

The rocker shaft misalignment theory is looking like a top contender.

I know it's my theory but now I don't love it--I think I'd expect to see more wear on the cam lobe if the rocker had been sticking.

[OsoMoore]

Originally Posted by David Inc.

I know it's my theory but now I don't love it--I think I'd expect to see more wear on the cam lobe if the rocker had been sticking.

I'll take a look at the cam lobe for #3 intake during lunch break. Also I'll check the position of the rocker shaft and see if it was far enough over to potentially expose the seal and jam the rocker action.

[OsoMoore]

Number 3 cam lobes intake and exhaust are both scarred. All the junk flying around, plus the jam, contributed to that. I think the jam was on the exhaust (leftmost in the photos).

Interestingly, it appears the barrel/shaft (not sure of the name) on which the rocker arm rides must have been very far out of center. Here is the barrel re-inserted without the rocker arm, so you can see the position of the seal.


And here is an earlier photo from disassembly showing how far it was before I touched it. It is significantly farther than even my re-setup photo.



So the barrel was far-far-far out of center, possibly far enough that the barrel was actually only seated on one end. Could this possibly lead to the rocker misbehavior, valve collision, and cascading failure?

[kmcteer]

Wow, do the cambox bore or inner faces have markings? I would expect the rocker arm to bind if the shaft went out of alignment when the end fell out of the cambox bore. Was that shaft difficult/easy to remove?

-Kelly

[OsoMoore]

Originally Posted by kmcteer

Wow, do the cambox bore or inner faces have markings? I would expect the rocker arm to bind if the shaft went out of alignment when the end fell out of the cambox bore. Was that shaft difficult/easy to remove?

-Kelly

All the shafts took a little bit of convincing to come out, but not a remarkable amount. I can take high detail photos later of each of the components of the #3 intake valve, and check for markings on the rocker and shaft and where it mounts in the case. I re-assembled #3 rocker and shaft and it moved smoothly at that point.

Is that what you mean I should check?

[Flat6pac]

When you blow up, inside the intake broken parts float from side to side. Things don’t stay in the area of the mess.
Bruce

[OsoMoore]

Work proceeds - bank 1 cylinders and pistons out. Tomorrow morning we flip and finish the pistons and cylinders.
The rod for #3 has a serious scar on it, probably will need to be replaced.

[snbush67]

Originally Posted by OsoMoore

So the barrel was far-far-far out of center, possibly far enough that the barrel was actually only seated on one end. Could this possibly lead to the rocker misbehavior, valve collision, and cascading failure?

That’s the first time seeing that picture, Yes, ground zero, that shaft walked out and seized.

[OsoMoore]

Originally Posted by snbush67

That’s the first time seeing that picture, Yes, ground zero, that shaft walked out and seized.

Now I'm kicking myself. If I'd done a valve adjustment recently, I'd have had that lower valve cover off and noticed the out of place shaft.

I wonder if I should buy those locking shaft nuts.

[pmax]

The valve didn't get chewed up, looks more like it punched a hole in the piston !

[OsoMoore]

Originally Posted by pmax

The valve didn't get chewed up, looks more like it punched a hole in the cylinder !

Wow, that's wild. The design of these things is that the rocker breaks when things get bad, saving the cylinder, heads, etc. But in this case

[ahh911]

Crrrap! you saw the shaft out of position, that stings. I'd read a few of your earlier posts, was that tinking noise from the same side? Might that have been the earliest warning?

Sorry about this, but it's kind of fun (the rebuild process) and you're moving right along...

Phil

[OsoMoore]

Originally Posted by ahh911

Crrrap! you saw the shaft out of position, that stings. I'd read a few of your earlier posts, was that tinking noise from the same side? Might that have been the earliest warning?

Sorry about this, but it's kind of fun (the rebuild process) and you're moving right along...

Phil

Do you mean the brief noise right before the engine failure? With that, I didn't notice any specific direction other than "behind me" from the driver's seat.

[ahh911]

Sorry, it was another poster's thread from the past about engine ticking sounds you'd responded to.