To add a bit to this, below is an older YF bonnet. It looked flat visually but it wasn't. Actually it was out by .006" on one side and .008" on another side causing it to sit slightly crooked and to leak between the corner screws. The sitting slightly crooked part wasn't a problem nearly as much as the fact the diaphragm didn't seal on 2 sides. Once that was fixed, all was Jake. There's also a cutaway view showing the metering rod & jet plus the diaphragm scheme.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
