Drain the pineapple, reserving 1/4 cup of the juice. Place the chops in a large shallow dish. Combine the reserved pineapple juice, the soy sauce, oil, onion, garlic and sugar. Mix well. Pour the mixture over the pork chops. Cover and marinate in the refrigerator for 2 hours. Remove the chops, reserving the marinade.
Grill over medium coals for 40-45 minutes, turning frequently and basting with the marinade. Place a pineapple ring on each chop during the last few minutes of cooking time.