Only the 6cyl engines have the thrust pin and it's to keep the cam from walking forward.
The 2.5 has a chain tensioner and the chain stays tight so the cam can't move forward but the 6cyl doesn't have a tensioner so if the chain gets slack in it the cam can move forward and the pin and spring keep tension on the front of the cam to avoid this.
That's about the best I can explain it.
Originally Posted by Zazoos
My Haynes manual shows a picture of a thrust pin and spring placed in the end of the cam gear bolt head. However, they only talk about the thrust pin in the 6 cylinder section. Does the 4 cylinder have a thrust pin and spring? What is the purpose of the thrust pin?