I don't think there will be many people here who have ever seen a Lawnboy D400 engine - I certainly haven't - so we are going to have to rely on you posting high-quality pictures, plus pages from the Lawnboy technical manual (which fortunately is very good). At this point I haven't even found the right page in the manual, and have no idea what this hair pin is. Can you post a picture, and the page reference in the manual please?
Meanwhile, I hope you haven't had to remove the hair pin from the other mower, so we can see what it is supposed to look like.
