Increase riser height for clearance or put exhaust outlets further above the waterline 4" can make a big difference in combatting back-flow issues on engines installed close to the waterline or transom. These Sierra spacers provide the extra height to prevent water from traveling back into the riser when throttling down and they can also provide added clearance for exhaust hose routing. Engineered to be compatible with GM based Crusader marine engines, these riser extensions feature machined mating surfaces to eliminate additional preparation during installation. Each riser is leak tested prior to paint to ensure quality. Boaters and technicians can be assured a Sierra part will fit right the first time and restore function to OEM standards. Key Features Engineered to meet or exceed OEM specifications Adds 4" of height to exhaust riser to prevent back-flow issues or add exhaust hose clearance Fits: Small Block V8 305-350ci(Models 220 & 270) GM Big Block CH350 & 454XL) Requires two Sierra p/n 18-0305-1 gaskets Backed by SeaStar Solutions Limited Two-Year Warranty The Sierra Engine Parts Selector In addition to riser extensions, we offer a wide variety of other engine parts manufactured by Sierra. To see if we have the part you need, see ourEngine Parts Selector, where you can search by engine make, serial number and model or year of manufacture.ApplicationOEM Part NumberBarrCR 20 97772Crusader97772Mallory9-40401Osco7772