During the landing roll the pilot felt that the left wing appeared to drop, so immediately initiated a go around. He noticed that the three green landing gear down lights were still illuminated before he cycled the gear to check for a malfunction. The left gear light then failed to illuminate again. The pilot decided to divert to Parafield where emergency services were available, and advised Adelaide Flight Service. On arrival, a ground inspection confirmed that the left landing gear leg was partially extended, canted to the rear, and the gear door was missing. After the emergency services were in place the pilot shut down the left engine, set the feathered propeller in the horizontal position, and carried out a landing. During the landing roll the left gear collapsed, the wheel became lodged between the flap and ground, stopping the tyre rotation and turning the aircraft left through 90 degrees to the runway centre line. The left gear trunnion forward pivot pin bracket/bearing assembly, attached to the wing spar, was found to have failed, and the rear pin then failed in overload as a secondary failure. This allowed the left gear to separate, but remain attached to the aircraft by its retract arm. The failure of the bracket/bearing assembly appeared to have been the result of excessive wear, possibly due to insufficient lubrication. Further examination revealed that the trunnion was an incorrect part, being manufactured from cast aluminium for fitment to PA28R type aircraft. The correct trunnion is manufactured from forged aluminium, and is of a stronger design for the heavier PA34 type aircraft. The aircraft log book and other records did not indicate when this component was installed on the aircraft. The incorrect trunnion type did not contribute to the failure, or the accident.