1979 SC Chewed a Valve - Total Rebuild

[OsoMoore]

Upon closer inspection, it appears to be gasket/sealant compound of some kind. I think the marks are fingerprints from it being molded. Maybe it was supposed to be on a suspected oil leak scratched joint area? Not ceramic.

[OsoMoore]

Morning of surprises here! I decided to try to take off the bank 4-6 tensioner, seeing as 1-3 had been tight when I tried it yesterday. Bank 4-6 came off easily, so I bagged it and went back to 1-3 with renewed optimism.

Back on bank 1-3's tensioner, I was reaching in trying to get some wiggle in the tensioner seating. And I brushed against the giant 46 mm nut... and it was LOOSE! The nut was loose and easily spun off with my fingers. The cam itself is locked into position, held tight by the bolt jammed under it at cylinder 3. I pulled off the giant nut, pushed out the pin in the sprocket (forgot to rotate the engine, it fell somewhere deeper within). The washer also warped a good bit after removal.




I then removed the locking washer-thing behind the sprocket, and it was quite scraped up.



The chain was still very taught, so I went back to work on the tensioner. I found a good angle to pry and protect my fulcrum from damage, and eventually got the tensioner off. Then I noticed a small cap-and-spring from the tensioner were loose, and a small ball-bearing type ball fell out when I picked it up. There was also some scoring on the tensioner pivot shaft.




Is this what a failed chain tensioner looks like? Did we find our culprit? John Walker suspected tensioner failure earlier in the thread.

Also, how can I get this camshaft out seeing as it has a bolt jammed under it, digging into the housing down by cylinder 1?

[draw]

The big washer is a “spring washer” and is supposed to be like that.

I think you can remove the cam housing without removing the camshaft. Remove the three bolts that go through the camshaft thrust plate, then remove the nuts that secure the camshaft housing to the heads. That should give you enough room to pop it up and slide it out of the chain housing. You may also need to remove the chain housing.

[David Inc.]

That looks more like the camshaft stopped dead and everything between it and the chain was stripped to hell as the chain kept spinning. I don't think those tensioners failed, I think they did their duty to the bitter end.

The locking washer thing that's chewed to hell is what actually holds the sprocket to the cam, and the threads on the big cam nut also look pretty shiny...

Edit: As for getting it out. If it were me (and I can only judge by the pictures) I think I'd try rocking the camshaft back and forth a bit while alternating with light taps at that jammed nut with a hammer and punch. It got jammed in there in one direction--maybe in the other direction it'll loosen up.

[OsoMoore]

Originally Posted by David Inc.

That looks more like the camshaft stopped dead and everything between it and the chain was stripped to hell as the chain kept spinning. I don't think those tensioners failed, I think they did their duty to the bitter end.

The locking washer thing that's chewed to hell is what actually holds the sprocket to the cam, and the threads on the big cam nut also look pretty shiny...

Maybe the rocker arm broke first, and jammed the camshaft thus stopping it dead? The tip of spark plug 3 was hammered down, so something slammed it hard.

Tonight I can take some pictures of the sprocket in more detail.

Update: Maybe the shaft on which the intake rocker #3 rides slipped and led to the jam/break? Or maybe resulted from it?

[David Inc.]

I can only speculate, but I had highly accelerated camshaft lobe wear from misaligned rocker shafts in the past. When the shaft is out of alignment the rocker starts to ride on the ends which are wider due to the through-bolt's compression, and the rocker stiffens up on the shaft. Maybe at a certain point it locked in place and the valve couldn't, or took too long to, close, leading to interference?

[OsoMoore]

The loose nut on the end of the camshaft also seems pretty wild to me. Maybe it got loose first, leading to the damage to the hub teeth, eventual timing slip, and catastrophic results? But wouldn't a chain slip result in damage to all the valves on that side?

Or could the nut be knocked/shaken loose by the cam getting jammed at high RPM?

[David Inc.]

Originally Posted by OsoMoore

The loose nut on the end of the camshaft also seems pretty wild to me. Maybe it got loose first, leading to the damage to the hub teeth, eventual timing slip, and catastrophic results? But wouldn't a chain slip result in damage to all the valves on that side?

Or could the nut be knocked/shaken loose by the cam getting jammed at high RPM?

I'd look at the threads on that nut, they look very shiny from the photo. If the sprocket were spinning while the cam was frozen it might have stripped the nut. Same with the keyed sprocket on the cam being chewed up.

This is spitballing, I bet someone can chime in that's seen this same thing before.

[rwest]

Stomski tool in route for Wednesday delivery. Sent you a PM with tracking.

Good luck on the rebuild; there’s some serious damage there.

Best,
Rutager

[OsoMoore]

Originally Posted by rwest

Stomski tool in route for Wednesday delivery. Sent you a PM with tracking.

Good luck on the rebuild; there’s some serious damage there.

Best,
Rutager

Thanks, I'm really enjoying the challenge and hopeful to solve the mystery and get it back on the road.

[snbush67]

Originally Posted by pmax

What's the deal with the texture on the yellow thing ?

That is probably epoxy that was applied during a rebuild to plug a potential leak/thin spot inside the case. Doesn’t look like it was applied correctly.

Looks like your tensioner collapsed and timing skipped causing the valve to contact the piston.

[David Inc.]

Wouldn't a collapsed tensioner be more... well... collapsed? He mentioned having to take a prybar to it.

[dannobee]

The big nut backed off because the cam stopped. (Remember, the cams spin CCW on these engines) Once the cam stopped, the timing chain kept moving and the big nut backed off.

[OsoMoore]

Originally Posted by David Inc.

Wouldn't a collapsed tensioner be more... well... collapsed? He mentioned having to take a prybar to it.

After some thought, I have what I think is a reasonable theory.
1) Camshaft giant nut starts to loosen. Maybe not tightened to spec, or some other reason.
2) Nut loosens enough that the sprocket hub behind it gains some play and begins to wobble, resulting in some of the damage we see to the hub.
3) Running near redline, the sprocket's wobble became enough that the cam is just out of sync enough for #3 intake valve to collide with the piston, slamming it back and breaking the end of the rocker and freeing the valve adjustment bolt.
4) That's when I start hearing the banging, as the timing for the entire bank 1-3 is slightly off and the chunks of rocker are bouncing around. Cylinders 4-6 are still fine and keep things moving for a few seconds.
5) The loose valve adjust bolt lands in between the cam and case and slams bank 1 camshaft to a stop. The force is enough to jump the sprocket a few more nubs on the camshaft, resulting in further damage to the hub.

So the precipitating failure would be the loosening of the camshaft nut, leading to a loss of timing, collision of intake valve 3, and catastrophic effects of loose bits.

I'm no expert, but this seems more likely than the rocker breaking on its own, and I don't see how a tensioner failure could have led to the loose nut.

I'll pull off the cam housing tonight and we should be able to see more. Maybe I can then get the camshaft out of its housing, once the housing is off the engine.

[OsoMoore]

Originally Posted by dannobee

The big nut backed off because the cam stopped. (Remember, the cams spin CCW on these engines) Once the cam stopped, the timing chain kept moving and the big nut backed off.

Missed your response while I was typing mine.

I didn't realize the spin direction was that way, so I guess my theory is off.
The cam jamming was caused by the bolt, which was freed by the wrecked rocker, which was impacted by the valve collision...

So then the first damage was a valve collision caused by the tensioner allowing a slip of the chain? And the hub damage was from the slam to a stop?

[snbush67]

Originally Posted by David Inc.

Wouldn't a collapsed tensioner be more... well... collapsed? He mentioned having to take a prybar to it.

I’m not sure. The tensioner ball and cage were damaged, but that could’ve happened while trying to remove it.

It is hard to wrap my head around a cam nut loosening up on its own. Have you ever heard of that happening?

I don’t think the cam nut just loosened up on its own. I think the tensioner collapsed , the engine lost time, then the valve hit the piston, the valve and spring broke, the rocker arm broke, the cam was then obstructed by the broken pieces and then I don’t know what the hell loosened the cam nut or stripped the keyed gear because the engine shouldn't the engine have stopped turning at that point? Maybe the car was still in gear.

[David Inc.]

Originally Posted by snbush67

I’m not sure. The tensioner ball and cage were damaged, but that could’ve happened while trying to remove it.

It is hard to wrap my head around a cam nut loosening up on its own. Have you ever heard of that happening?

I don’t think the cam nut just loosened up on its own. I think the tensioner collapsed , the engine lost time, then the valve hit the piston, the valve and spring broke, the rocker arm broke, the cam was then obstructed by the broken pieces and then I don’t know what the hell loosened the cam nut or stripped the keyed gear because the engine shouldn't the engine have stopped turning at that point? Maybe the car was still in gear.

Really not sure. Hard to say without touching the parts myself, but my gut is eyeing that misaligned and broken rocker very suspiciously. If that froze up and wouldn't let the valve back up everything else could result.

[OsoMoore]

Originally Posted by snbush67

I’m not sure. The tensioner ball and cage were damaged, but that could’ve happened while trying to remove it.

It is hard to wrap my head around a cam nut loosening up on its own. Have you ever heard of that happening?

I don’t think the cam nut just loosened up on its own. I think the tensioner collapsed , the engine lost time, then the valve hit the piston, the valve and spring broke, the rocker arm broke, the cam was then obstructed by the broken pieces and then I don’t know what the hell loosened the cam nut or stripped the keyed gear because the engine shouldn't the engine have stopped turning at that point? Maybe the car was still in gear.

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.

[917_Langheck]

Originally Posted by dannobee

The big nut backed off because the cam stopped. (Remember, the cams spin CCW on these engines) Once the cam stopped, the timing chain kept moving and the big nut backed off.

What? How do the cams turn counter clockwise when they are chained to the crank, which rotates clockwise?

Edit: I'm going to leave this idiotic comment to remind myself to reanalyze the reply before hitting submit.... Duh.

[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!