I just had my engine rebuilt by somebody last year and they forgot to install the rear galley screw in plug. I had no oil pressure in the pax side rockers. I had oil psi in the rest of the motor. To remedy this problem I did not have to pull the motor. I had to pull the tranny and remove the small pax side freeze plug(non threaded) to get to the threaded plug which could be seen with a mirror if the dist was pulled or can be felt with a finger if you know where to feel. Be very careful not to drop the plug into the motor. After replacing it everything was normal. Two machine shops said everything would probably be ok in the motor if it wasn't driven too much because that aft plug is at the end of the oil routing except to the pax rocker arms.