Sorry, I was not precise, thank you for your attention.
Roto cap (valve rotator) is the bottom seat of the valve springs. It makes the valve rotate in its seat. Sometimes it gets stuck.
Other questions to the forum are:
1. Is it possible to make it unstuck? How?
2. Is it necessary to dismantle the valve assembly?
3. How to do it best without the head removal? I heard about compressed air or sticking some support through plug opening. My suspect valve is the exhoust valve, so simple support with the piston may not be enough...