In general, if a newly assembled engine with a hydraulic lifter cam is adjusted properly (preload), and then later develops valve lash (tapping), it was either not adjusted correctly from the beginning, or something is changing dimensionally in the valve train (possibly bad)?

I have seen the rocker arm studs break, I have seen then cap screws break..I have seen the springs break...could be many different issues...but if all the parts were new..(rebuild) when assembled, then most likely it was incorrect adjustment.

Raymond,
If the engine starts and runs for even a few minutes without making noise, then it should be adjusted properly. If it starts to make noise after that, then something is failing or coming loose.

First possibility is a camshaft lobe starting to wipe, if it is not a hydraulic roller cam. Getting to be a pretty common occurance unfortunately.

Next on the list would be an adjustment backing off for some reason. You did not mention the type of engine, usually you can look at the rocker nuts and see if they all are at about the same heigth or not.

Last thing I would expect would be a lifter failure.

Bottom line is, pull the valve covers and take a look before you have a catastrophic failure of some type

Bill Koustenis
Advanced Automotive Machine
Waldorf Md