Hello i own fd 15 years and now i dissasembly rx8 engine.
i take out ecentric shaft bolt to inspect the thermal pellet and i see this pin ,fall down when i take out the bolt .
Any one know what this pin do; it is a mod in order to increase the oil pressure;

 

That piece is part of the stock thermal pellet. It is not a mod. I'd replace it with a new one since you have it out.