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Well, I decided to change the plugs in my GT40 this weekend (just bought boat last month) and apparently encountered a common problem with various plugs that cross reference to the Motorcraft AWSF22 that is listed in the manual. I stopped in the local auto parts store and as the Motorcrafts were no longer available, was given several choices including the Autolite 103, 104 and 764. The girl at the counter was nice enough to pull all three and let me see them side by side. Since I made the mistake of not pulling a plug before I went to the store, I took a chance and blindly grabbed a set of the 103's. As I drove away the light bulb came on and I realized that I was in the 21st century and could do a forum search on PN with my iphone 
Now, after doing a search I found many threads addressing plug choice and most leaned toward the 764 over the 103 but no concrete answer as to why. The one thing that I didn't see mentioned in all the posts I reviewed was any mention of the differences between the 103 and 764. It was immediately apparent when I saw all three plugs side by side that the length of the threaded body on the 103/104's was almost double that of the 764! Some posts mentioned "a friend" who had valve/engine damage because of improper heat range?? But no one said anything about that extra plug length protruding into the head. At the time, I didn't think to take a pic but I can say for sure that the threaded body on the 764 is MUCH shorter than a 103/104. After looking at the actual threaded portion of the GT40 head, it's obvious that the shorter threaded 764 is the right plug for the engine.
I hope this helps someone, it took me about a half hour of reading posts in the parking lot before I felt comfortable with the 764's (of course, when I got home and pulled a plug, that's what was in there from previous owner lol).
Now, after doing a search I found many threads addressing plug choice and most leaned toward the 764 over the 103 but no concrete answer as to why. The one thing that I didn't see mentioned in all the posts I reviewed was any mention of the differences between the 103 and 764. It was immediately apparent when I saw all three plugs side by side that the length of the threaded body on the 103/104's was almost double that of the 764! Some posts mentioned "a friend" who had valve/engine damage because of improper heat range?? But no one said anything about that extra plug length protruding into the head. At the time, I didn't think to take a pic but I can say for sure that the threaded body on the 764 is MUCH shorter than a 103/104. After looking at the actual threaded portion of the GT40 head, it's obvious that the shorter threaded 764 is the right plug for the engine.
I hope this helps someone, it took me about a half hour of reading posts in the parking lot before I felt comfortable with the 764's (of course, when I got home and pulled a plug, that's what was in there from previous owner lol).