The Wisconsin Robin WI-390 was a one-cylinder engine used in lawn mowers and small carts. The engines were actually made by Subaru, which has since discontinued the Wisconsin Robin line. Subaru now manufacturers its small engine line under the Subaru brand name.
The WI-390 had a bore of 23.7 inches or 388 mm, which is the diameter of the cylinder in the engine.
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The engine's stroke was 5.1 inches or 3.8 mm. Stoke is the length the cylinder moves in the engine.
The displacement of the WI-390 was 6 cubic inches or 4.5 cc. Displacement is the total volume of air swept by the engine's pistons in a cycle.
At 1,500 rpm the WI-390 had 6.8 horsepower. At 1,800 rpm it had 7.5 hp.