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Are they 5/16
And now I ask why I went out there anyway. I knew darned well they are 1/4-20 and they are. 3/4 inch long on a 7.5hp, 7/8 inch long on a 20hp. And they are not hardware store screws. They have a special flat-bottom on the head so they seal if you sock them in tight as you are supposed to do.
EDIT: OK, so I don't have a 35hp. But look up the part numbers and see if they are the same as the 20. I say they are, but I did my work for today.
They are 1/4-20. That's my story and I'm sticking to it. And now I am going out to the shop and checking it.