Dorman Coolant reservoir tanks

I bought one back in february and already it's giving me problems. the car is a 96 Mercury Sable GS with the vulcan engine.

The cap will not stay tight. I'm thinking either it's cracked or the neck is stripped. although from visual inspection the neck is fine.

the car hasn't overheated yet, but I don't feel the cap should be so loose-fitting. it screws on but will not tighten.

Has anyone else had problems with these tanks? It's the type with the black top.

I always bought used at a scrap yard. Its possible Dorman makes a cap for their own tanks. This is what I would check first.

Dorman does make caps for their own tanks, it comes included with the tank.