Loose valve seat caused bent pushrod, not AFM failure

I posted a couple of weeks ago how a trans rebuild resulted in a bent pushrod. I assumed it was an AFM failure: lifter lockup and bent pushrod.
Turned out to be the intake seat coming loose. Seat holds valve off, piston hits valve and bends, valve is stuck and pushrod bends.
So not caused by AFM? What caused seat to come loose?

John H Daniel said:

I posted a couple of weeks ago how a trans rebuild resulted in a bent pushrod. I assumed it was an AFM failure: lifter lockup and bent pushrod.
Turned out to be the intake seat coming loose. Seat holds valve off, piston hits valve and bends, valve is stuck and pushrod bends.
So not caused by AFM? What caused seat to come loose?
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Oof. 6.2? Apparently, at least with the GMT900s, this is an issue with the 6.2. Maybe @mikeyss can enlighten us more.

Ah, yes I've been down this road. My L9H had 823 heads and I kept hearing what sounded like a bad lifter. Long story short, I bought a new cam, lifters, etc... And on the day I was going to do the install, the valve seat broke in half. One part wedged under the valve, and the other half took a trip down to the piston and shattered it.

I thought the AFM was enough to worry about. Now I'll be towing down the road dreading another valve seat to depart?

mikeyss said:

Ah, yes I've been down this road. My L9H had 823 heads and I kept hearing what sounded like a bad lifter. Long story short, I bought a new cam, lifters, etc... And on the day I was going to do the install, the valve seat broke in half. One part wedged under the valve, and the other half took a trip down to the piston and shattered it.

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@mikeyss or @John H Daniel, I'm curious, had your heads ever been overheated even just a little? Aluminum expands faster than steel, and so I would expect excessive heat to increase the risk of a valve seat coming loose. The only other common cause I can think of is having either a slightly undersized (defective) seat installed at the factory, or the spot where the seat goes machined slightly too large. Either way, I would expect it to be an isolated incident unless the head(s) were overheated at some point.

Sorry for the late reply, but to answer your question... My engine never got hot, I never saw it budge over the normal mark on the instrument panel. I even have the K5L cooling system from a 2012 cop hoe, but a better radiator.

Is this your 2011 Denali or a 2015+Denali? You posted in the 2015+section.
Putting what you drive in the first post would help!
Moving it over here for now.

My mistake; I started in wrong section...