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 Last Update: 03 October 2014

- Reversing the Rotation of the Crankshaft -

Sometimes it's worth the effort

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In some cases of engines with reduction units to drive the propeller, it can be an advantage to have the crankshaft rotate in the opposite direction as the propeller. (For a complete discussion of that subject, see Prop Rotation.)

For example, the GTSIO-520 crankshaft turns in the opposite direction of a standard, direct-drive IO-520, because the gear reduction unit on that engine is a single-mesh type, in which the output shaft rotates in the opposite direction of the input shaft. By reversing the crankshaft rotation, the GTSIO-520 can use the wide range of available propellers designed for clockwise (from the cockpit) rotation.

Reversing the direction in which the crankshaft of a given engine rotates is not particularly difficult. But reversing the engine can add considerable extra expense because of the nonstandard pieces needed, including:

  1. a special crankshaft with the rod-journal oiling passages drilled to the opposite side of the rod throws in order to support the reversed bearing load phasing,
  2. a special camshaft because reversing the crankshaft reverses the firing order (no, you can’t just use a gear-drive to run the cam in its normal direction, and running it backwards opens the intake during the exhaust cycle and the exhaust during the intake cycle),
  3. special accessory drive provisions,
  4. reverse rotation crankshaft seals,
  5. specially-engineered oil pan (especially critical in dry sump systems);
  6. reverse-rotation starter and alternator,
  7. and a host of other details depending on the ignition and oiling systems used.

Given those factors, keeping the standard crankshaft rotation (clockwise viewed facing the waterpump-end of the engine) and using a gearbox with an idler to maintain that direction of prop rotation will appeal to some builders.

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