But, like the tires on your car, they do not last indefinitely. Mercruiser, for example, has a wonderful reputation for risers that sometimes don't last a year. On average, in saltwater use, good risers last about 4-5 years. At 4 years, you should start worrying about them. Mercruiser 4.2 V6 riser.
Arrow shows where corrosion hole occurred.
It doesn't take much corrosion to block these small water passages. Note that the passages top and bottom only have a tiny hole where it met the larger passages on the manifold. Merc 350 riser. Notice the difference, especially the width of the gasket surfaces between the openings. This one had been cleaned of scale and put back in service.