Originally Posted by Guy
Somebody name just 1 piston engine in this whole wide world with poppet valves that gets more valve clearance when the valve faces and seats wear.
I can only come up with a couple of theories that might explain a "loosening" valve train.
1. Carbon build up between the valve face and it's seat.
2. camshaft, rockers, valve rotators or tappets wearing excessively. Lycoming aircraft engines are bad about losing cams, but when they start wearing and wear goes though the hardening, you lose a cam quick, and often a crankshaft too from excessive metal in the oil.
I've never experienced valves getting loose myself, I have adjusted valves on engines that were loose, but feel real sure they were that way because someone didn't know what they were doing when they adjusted them, like maybe adjusting to cold specs on a hot engine.
On edit, It was common on GM engines back in the day if you installed high performance springs and a cam that the studs in the head
would loosen increasing valve clearance, fix was to "pin" the studs by drilling through them and inserting a roll pin.
I have never seen the carbon build up either, maybe possible I guess?