Primary Sprocket Failure

09/18/2009

I recently bought a (cheap) 1997 parts SHO with a failed engine. I planned to use the transmission (minus the valve body) in my 1999 welded SHO and part out the rest. However, before I junked the car, I decided to look at the carnage in the engine. Last night, I took off the front valve cover and did not find what I was expecting. When the cams fail on these cars, it is usually the secondary sprockets which slip (right?). Look at the pictures I have attached. I found the PRIMARY drive sprocket broken into at least 8 pieces(!!) with the timing chain still appearing to be intact. Also, the secondary sprocket's chain was broken. The secondary sprockets do not appear to be slipped at my first glance, and I can't move them.

How often has the primary sprocket failed? Do you think there is another reason for its failure? I would think it would need a LOT of torque to fail like that. It looks like the broken chain on the secondary sprockets really dug into the wall. Does this chain fail very often? If it binds, that could cause a bit torque spike. The previous owner said it ran fine (and without any noise) until one day the motor wouldn't budge when she tried to start. I can now see why the motor didn't want to move!

Any ideas on how easy this would be to fix? This is the first cam failure I have seen in person. I imagine there are sprocket pieces that have fallen down deeper to near the crank (ugh!). I really need to also pull off the rear valve cover to make sure it is ok. I'm trying to evaluate whether I should fix it or just continue with my original plan. I hate to part out this car at only 115k miles if I don't need to. The car is in decent shape, but not great either.

Thanks,
Peter

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I see some secondary chain wedged beneath the exhaust secondary sprocket. I'm believing this locked the exhaust cam with a lot of inward radial pressure that would have forced the sprocket bore tighter against the tube and not let it break loose. The larger primary diameter transmits a lot of torque and with those weight lightening holes I guess the strength wasn't enough. Pull those cam bearing caps and see if that cam rolls out without much difficulty. It would definitely be neat if the initial point of failure could be identified.
Lucien

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