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Discussion Starter #1
I'm in the process or rebuilding the transmission on my Yanmar 2QM20. I burned up a clutch. Meanwhile, I am looking at a transmission from a Yanmar 3QM30 which I may get cheap and use as an interim.

From what I understand the two engines are physically identical but for the number of cylinders but they use different transmissions. I assume the 3QM's is beefier to accommodate the greater HP?

Also, the interim transmission is 2.1 : 1 ration. I am not sure what my current gear box is. It may be 3:1 - of so will the higher gear ratio work?

Yes, i realize the original design choices were made for a reason but given that this boat was built for conditions far worse than I will ever expose it to I figure I have some latitude. The engine never works hard to move the boat at 5 knots. There's a little power to spare at 6 knts. I sail in the Albemarle Sound, always in sight of land, no currents or tide to speak of. Just a big lake.

thanks
 

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IF your are going from 3:1 to 2:1 the prop will be way to big and the motor will not be able to reach the proper RPM resulting in motor dammage
 

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"IF" the ratio is different, you would need to do a prop change too. Not sure if you would need to go to a smaller or larger diam, would assume smaller, along with how the pitch would need to be changed. It could work, but probably not like you would hope!
 

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Sea Dweeb
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You may want to double check the gear ratio of each gear box. I think that most Yanmar engines in this range had 2.62:1 gears.
 

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Discussion Starter #5
THanks, that was quick. OK, a radical gear change is a deal breaker.

I had a feeling but I posted the question because I can't determine the gear ration at the moment. There is no ID plate on the trans, I can't find it in the literature and I'll have to get to the boat and spin it by hand to know for sure. But I can only be certain of the reverse ratio because the forward clutch is smoked.

Does anyone know whether or not the gear boxes are physically interchangeable?

thanks
 

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Sea Dweeb
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The 2QM20 most likely has a 2.03:1 Gear and the 3GM a 2.62. If you were to change gear ratio your current propeller will not have nearly enough load.

2.03 gear = 14x9 if 2 blade +-
2.62 gear = 17x10 if 2 blade +-

It would be an expensive blender
 
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