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True, but they come in a wide range of diameters- like 7.6 inches, 7.25 inches. I tried a 7.25, and it rubs the upper pulleys on the circulating pump.See SEQ # 23 here.
These are nothing more than a GM double shiv V belt pulley.
However, in order to index your crankshaft style seawater pump, the pulley must have the 2 raised dowel portions stamped into it.
http://www.marinepartseurope.com/en/volvo-penta-explodedview-7745660-21-19956.aspx