OT: Broken parts, comments on theory?

[CRZN427]

Recently I was driving a friends LAE (428, Auto) and noticed a little valve (tappet) noise when pulling back into my garage.
I am storing it, and occationally I have to drive it , for him.

I removed the valve cover, passenger side, and found the rocker arm shaft broken off in front of the front support (this is the exhaust valve on #1). Further inspection revealed a bent, and broken, pushrod on the next valve back from the front (this is the intake valve on #1).

I removed the #1 spark plug and did a compression check. Neither valve could open and I never removed any other spark plugs. After four "compression strokes" the gague read 65psi.

The rocker shaft broke through the hole that the front support bolt goes through. (We don't need any comaprisons to 5.0 or GM tappets here either.)

There was another small crack starting at the next mounting bolt hole back from the front of the shaft.

We got new rocker shafts and a push rod, installed them and the car runs fine, about 140 miles this week end going to a car show.

Here is our theory, where we do want comments:

The hole in the rocker shaft at, the front support, developed a crack, most likely from age, and the shaft broke.

The #1 exhaust rocker arm could no longer open that valve.

The intake valve allowed a fresh charge in, and it was ignited.

The burned charge was still contained in the combustion chaimber when the piston came up on the exhaust stroke.

Near the end of the exhaust stroke the intake valve opens.

The added pressure, from the exhaust valve staying closed, gave the intake valve a significantly higher load to over come, resulting in the pushrod being bent.

In a very few strokes of the camshaft lobe, the bent pushrod became fatigued and broke.

It makes no sence to us to consider the intake pushrod bending first and causing the rocker shaft to be over stressed and break.

OK, Comments..?? anyone...???

[Gaz64]

The rocker shaft broke from poor design in the first place.
Do they ever break in the middle?
No, because there is support of any rocker on both sides. The end rockers only have support on one side.

Once the shaft broke, the flexing of the shaft allowed the intake pushrod to dislodge and it gets bent up in the process.

Compression testing with no valve operation is not a fair test, since the cylinder is never exposed to atmospheric pressure during the intake stroke, a leakdown test would be more accurate.

The cylinder pressure is NOT the reason for the intake valve not opening, the shaft failure is.

Gary