mEl, I know you probably already checked this but sometimes when we are so convinced that one thing is happening it turns out to be another.
Since this is happening over and over, did you think of a mechanical displacement of the lip rather that pressure?
If a part, the rockshaft, got through inspection at the plant (or has been damaged over the years) a surface finish that is not quite up to par and exhibits a slight helical surface could actually screw the lip out. After 35 years around oilfield manufacturing shops I have seen a lot of starge things happen during the machining processes and not be caught in inspection as the problems are so small or thought to be insignificant. As we all __it happens.
Might want to check with the owner and ask if the shaft had ever been damaged, weld repaired and remachined. Just a thought.