Heat expansion in the length of the manifold. Expansion is X thousandths per inch, so on say 3" the expansion will be very little over #s 3 and 5, and 4 and 6, while #s 1, 7, 2 and 8 will be considerably more length change. I believe the conical washers let the manifold move a bit under them so it does not break the manifold.
