Re: Nylon Balls
When the thermostat is closed, the water exits through the check ball outlets, then flows to the risers, skipping the exhaust manifolds. When the thermostat opens, the check balls provide some back pressure to push water through out the block and out the non-check ball outlets. These are typically plumbed to the exhaust manifold. You NEED the check balls to get good flow through the block.
In later years, Mercruiser redesigned the thermostat housing to eliminate the need for the check balls...
OOPS I didnt see you answered Don.