GM Users Group: Replacing the gearbox on a 2GM

A 2GM user asked the following question...
Is it possible and if so, how does one use a gear box KM2P (ratio 2.62) in place of a KM2A (ratio 2.21) on a GM2 1st generation engine.

Huynh (who sails a Beneteau Kerlouan in Binic, France)

On the surface, it would seem that it is not possible to swap the KM2P for the KM2A. Consider Yanmar’s advice in their GM service manual.

Some online suppliers, however, appear to suggest that the KM2P will in fact fit the 2GM.

So, aside from the differing gearbox ratios meaning that Huynh’s propeller will need to be recalculated and replaced, some owners also report that the propeller shaft itself may also need replaced as the distance between the engine and the coupling flange is different.

Transmission Flange
Output Flange

Oil filler

The following excerpts are from the service manual.

The KM2 and KM3 series transmissions are inline transmissions that provide forward and reverse direction. All gears are constant mesh. A cone-type clutch engages internally tapered gears to transmit power to the output shaft. Oil contained in the transmission case lubricates the internal transmission components.

Basic design is the same for all the transmissions with the exception of the shifting mechanism. Transmissions with a P suffix are equipped with a shifting device that engages detent notches in the shifter housing. All other models use spring-loaded pins and a spring-loaded actuator that engage detents and ramps on the shift shaft.

So, the questions for the owners group are;

  • Will the transmission flange of the replacement gearbox be able to be bolted on to the original engine?
  • Will the input shaft of the replacement gearbox align / fit the new engine?
  • Will the coupling flange align with the existing propeller shaft?
  • Are there any other issues?