Originally Posted by SuperiorStyles
Well known fact is that it does drop with a very noticeable knock also you would know when it does a good lack of performance, MPG and so on. Mine started with a light tap and goes away which the valve adjuster bore was cracked clicking back to back my MPG was good and my performance as well, My compression was right on the dot and my Dyno run proved that there was no loss. As the Time passed the valve adjuster failed causing the problem on the weak OEM design of the Cylinder Head.
You might be completely right, but here's why I'm not convinced. If the hydraulic adjuster bore actually failed FIRST as you suggest, then when did the valve seat drop out? Because if the follower bore broke, the the valve didn't ever open again to LET the seat drop out of the head. And I think its pretty unlikely that both events happened on exactly the same turn of the cam. The only way to sort out what happened in those last few milliseconds was that the valve seat dropped, propped the valve open, the valve not returning to its full-seated position let the roller follower move sideways off the lash adjuster, then the cam lobe came around again and smashed the follower while it was sitting half-off the adjuster, breaking out the bore in the head casting with an unnatural side-load on the adjuster.
How would that explain the light tapping you heard other times? I don't know for sure- could be typical 4.7 slow pump-up of the lash adjusters. Could be that the valve seat was able to drop down a millimeter or two but not come all the way out, and then stick firmly back in the head and everything would run fine until the next cool-down cycle.
The other problem I have with the theory that the light tap was the bore beginning to crack: why would it ever STOP the light tapping once that adjuster bore began to crack?
Just food for thought- none of it really matters now. And I agree that persistent tapping probably should be watched for other problems.
One question for you- how did the cylinder walls in your original engine look? I know 150,000 miles is around the "might as well do it all" point, but since people report running these 4.7s out to 300,000 miles or more without even burning oil, I'd expect the bottom end to still look very good.