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The R designation on the model numbers began in 1928 with the introduction of the 40 series engines. In 1928, the racing engines did not have significant external differences from the service models (ex P40 vs PR40). The changes on the racing models for 1928 included bearings in place of bushings on some late production motors and other internal changes.
The racers changed significantly in 1929 when the 45 series was introduced. The mid sections got shorter, the gear case was more streamlined, carbs had velocity stacks rather than shielded intakes and the exhaust was above water. The 1929-32 models were similar. There were more refinements after 1933 when the 65 series was introduced.
Although the 1928, 40 series motors are not what most people would think of as a racer, they are part of the evolution of boat racing and are an important part of my collection when it comes to showing the evolution of Johnson motors.