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Beginning with single-cylinder engines, Yamaha gravitated to an air-cooled parallel twin two-stroke engine beginning with the YR3. The R5 was preceded by the YR3 and was succeeded by the RD350, RD400, and RD400 Daytona. The R5 had port induction for the intake and the RD350 had reed-valves.
Though different in appearance the R5 basic architecture lived on in the RZ350 (American market) and RD350LC (Euro market). The main difference being the cylinders became water-cooled. The engine cases are similar enough, that with modifications, they can be interchanged.
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The Two-stroke era
In the early 70s, a 350cc engine was considered large for a two-stroke engine. Two-stroke street motorcycles from Yamaha, Suzuki and Kawasaki collectively developed a reputation as "giant-killers". Even though four-stroke motorcycles engines (not chassis) were being developed rapidly, during the 70s, two-strokes were able to best them in straight-line performance at times. Because of the lighter weight of the engine and chassis, two-strokes were typically dominant on curved roads.
During the seventies, the two stroke developments were between Kawasaki, Suzuki, and Yamaha. At this time, Soichiro Honda was alive and active in his company. He did not personally like two strokes, so Honda stayed focused on four-strokes. As the decade went by, Suzuki added displacement, cylinders, and water cooling, culminating in the GT750, a touring bike. Kawasaki added cylinders and displacement, ending with the infamous H2 750 mark IV. By default, Yamaha became the bantamweight, maxing out with a 400cc twin, still air-cooled.
In the early days of the Yamaha racing team, factory race bikes were not as specialized as they are now. In fact, they were hand-built versions of the production street bikes. Beginning with the basic parts of an R5, the racing TR3 model was built.