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The Yanmar Engine 2GM20 utilizes two specific Yanmar pump models: #128397-42501 (previously 128397-42500) and #128377-42501 (previously 128377-42500). Both pumps, available through a Yanmar dealer, feature an o-ring sealed end cover, integrated cam, and hose connections. The main difference between these models is the integrated cam height, which generates different flow rates. If your pump has a loose cam or threaded inlets and outlets, it indicates that you have an older generation engine with a pump from another manufacturer. Otherwise, both Yanmar pumps use the impeller 128990-42200, also available in the Yanmar impeller kit 128990-42570. This impeller is made of MC97 rubber and manufactured by Johnson Pump, whose equivalent version is #09-810B-1 (IMPELLER KIT F4 MC 97). The Johnson Pump impeller 09-810B-1 comes with an impeller, o-ring, and gaskets. For these Yanmar pumps, you should use the impeller and o-ring, while the gaskets are meant for other Johnson Pumps that require a gasket for sealing between the pump body and end cover.
For the leisure boat market, Johnson Pump offers its proprietary MC97 EPDM rubber, designed specifically to resist swelling when in contact with water. This innovation minimizes the risk of hub separation and reduces lifespan caused by excessive friction during intermittent pump operation. For the commercial market, Johnson Pump provides a custom-developed neoprene compound, where water swelling is acceptable. Continuous operation naturally wears down the axial swelling, maintaining optimal performance over time.