I am trying to get the size oil change valve I need for my JD 320. It has the FD590V engine. I just put in full synthetic and really don't want to drain it back out. I backed it out a few turns and put my micrometer on it and it came in at 18.5MM but I couldn't get the thread part. Not even sure that the 18.5 is accurate because of the limited space. Any help would be greatly appreciated!