Here is a good illustration on how a over head cam engine works.
http://auto.howstuffworks.com/engine1.htm
http://auto.howstuffworks.com/engine2.htm

Does that help guys?

 

Thanks Spike, that does help explain why failure on an OHC engine is more common than on a pushrod v-8. As for interference, though, it seems that if a pushrod engine has valve/piston interference, it is still like to suffer in the (even though rare) event of a camshaft drive failure.

However, it seems that most if not all pushrod v8's are NOT interference engines, including the Rover V8.

As you pointed out in another post, this is due to the lower compression ratios relative to what is found in smaller OHC engines that (generally) have higher compression ratios and thus lower clearances. Also a concern for diesel engines where the compression ratio is higher to produce combustion.

That a pretty close layman explanation? As I have said before I'm a papermaker not a car designer. Thanks again.

 

Yes Bob, I would call that close enough to be called a "leaner".

 

Now that we've established that my camshaft is broken in the middle between the front 4 and back 4 cylinders, I have to ask this next question.

How does a camshaft experience such an unlikely event?

The conditions are as follows:

Engine still ran, but only the front 4 cylinders.
Good oil pressure.
Oil changes at every 3500 miles.
I never hotrod my cars (I like not spending money on motors).
No apparent damage to any other parts of the engine.
No bent pushrods.
No oil starving at any of the bearings.
No fragmentation of the shaft at the breaking point.
The shaft was not frozen into the block, it moved freely.
35,000 miles on the engine with no other mechanical failures.

Plausible hypotheses:

Could it have been dropped by the assembler and bent or otherwise damaged?
Could it have been a bad cast?
Could my bearing housings in the block be out of alignment?
Could a stuck hydraulic lifter cause the problem without bending a pushrod?

I want to avoid this problem in the future, and want to investigate every possibility while it's apart. I've opted to try to fix rather than replace the engine since there seems to be no damage beyond the camshaft.

Has anybody else ever seen this kind of thing? I feel a bit singled out, and am starting to look for the black helicopters.

 

Drop an new cam in and put her back together man.
I would call this a freak occurrence.

 

Have you gotten far enough into the engine to determine if the back (non-rotating) end of the camshaft has or has not seized?

If nothing else is obvious (ie no seized bearing and all rotates just fine), one possible explanation is metal fatigue. Once you get the shaft out of the engine, take a close look at the fractured ends. What you're looking for is an area in the fractured zone that looks like a new break, typically shinier that the rest of the fractured cross section, which in turn looks like it may have been fractured a while. Typically the shiny zone will be in the center of the shaft, but not always.

Not anybody's fault, really, could have been a defect so deep in the shaft that it was not detectable, and it propogated its way through the shaft. Unless it was magnafluxed you'd never know.

Saw this happen once on a 4" dia. shaft - the shiny area in the center was down to about 1" when it finally failed. Made a real muckin fess, too.

Any way you cut it, this is a rare occurrence. Good luck with the rebuild!

 

Nothing is seized. It's the weirdest thing I've ever seen. I'll post some pics of the shaft when I get it all out.

 

I finally got back to my project and took the broken camshaft out, and replaced it with a non-broken one. I am now in the process of putting the truck back together.

In case you didn't believe my story, here's a pic of the busted shaft.

 

jrmnw1 - just a thought, that cam broke right behind a journal, the weakest point, but something had to be trying to bend the cam along its length to cause something like that,,,,,IMHO.

 

So now I'm putting this thing back together, and I noticed that the timing chain seems a bit floppy. How much slack is too much? Could this whole adventure have been caused by a slack timing chain that jumped a tooth and then broke the camshaft?