Hey 90…

I’ve never heard of this failure.

I assume the first cam shaft what was sheered off was replaced with a new camshaft.

Was the replacement cam a different cam or the same part?

After installing the camshaft into the head, it should spin freely with almost zero resistance before installing the rocker rack.

In order to sheer that dowel pin it would take incredibly unusual instantaneous rotational force by the timing chain and sprocket caused by the engine.

The dowel pin in the camshaft could be defective, not properly pressed in, or bad metallurgy.

There would have to be a very high resistance in the camshaft bearings and journal saddles in the head.

What is the timing sprocket being torqued to?

The bolt for distributor gear to the sprocket is factory spec’d at 58 ft. llbs.

What does the mating slot in the sprocket look like?

I would mic the dowel pin and the mating slot on the sprocket and replace the sprocket if it is too large for tight fit with the dowel.

Gnarls.

The dowel is not designed to see any load under operation. It is used for alignment of the cam and cam sprocket during assembly. During operation, all the torque is transferred from the cam sprocket to the cam through the friction caused by the clamping force of the cam bolt.

If that bolt isn't tightened properly, the dowel is the only thing left to transfer all the torque required to drive the cam and it's getting hammered back and forth the whole time.

I'd guess the cam sprocket bolt wasn't tightened properly. It might have backed off, but I've never seen that happen.

+2 on this.

Hey 90...

As it was mentioned the dowel is used for locating the sprocket to the cam… cam timing and ignition timing.

That pin fitting into the slot on the sprocket is a VERY tight as I remember.  So.. although it is designed to locate the sprocket to camshaft, it will prevent any slippage and the sprocket to rotate, even if the bolt is torqued to 58 lbs. – like other “pins”.

How did he “just replaced the dowel pin”?? 

One thing I did not mentioned is where are the sheared off down pins? 

Did they drop into oil pan?  Is there any evidence of metal wear and possible tiny metal pieces getting into the crankcase oil?   

And.. if the pin has been sheared because the bolt has become loose, did the sprocket rotate out of clock with the camshaft?  What happened to the ignition timing and the cam timing? 

Gnarls.

If the camshaft bolt is too long, you would reach the torque reading on the torque wrench before you had sufficient clamping load.

Or a low grade replacement bolt was used and it stretches so you don't get the clamping load.

Or if the washer was left off?

Will do, I was just getting ready to start my exo cage so now the plate is really full. Thank you all. Will report back with photos as we are disassembling if anyone is interested.