The Mercruiser OEM ring is the stock ring used by GM in that motor.
Mercruiser comes up with a specification, xxx horsepower in a four, six or eight cylinder engine. Any engine manufacturer (man.), who feels like one of their engines can fill the bill will submit a price per unit (for xxx,xxx units) to Mercruiser. The cheapest bid is usually the one Mercruiser takes (a horsepower is a horsepower is a...). Now, the engine man. doesn't setup a different engine assembly line to produce engines for Mercruiser. The engine man. takes the engines off the existing line, paints them black, then will add the marine accessories (water pump, dist, starter...) to the engine. The block, heads an internals are EXACTLY the same as the truck/car engines already being made.
