C&C 27 Association Forum

Hugh Morrin

Member

Yanmar 2GM Transmission Slippage; De-glazing KM2-A Clutch Cones

Most C&C 27s are equipped with the Atomic 4 gasoline engine, but some later Mk 3s and Mk 4s were shipped with Yanmar 2GM diesel engines, particularly those that were being delivered to the U.S. The Yanmar engine is paired with a Kanzaki KM2-A transmission bolted on to the back of the engine. My 1981 Mk 3, #894, is so equipped.

There is a well-documented condition (see references below) which results in the transmission slipping and clunking in forward gear. Typical symptoms are that upon shifting into forward, some thrust is realized, but increasing engine RPM does not result in an increase in thrust. Sometimes there is a clunking sound, and then, sometimes, the transmission fully engages and normal thrust is achieved. The problem is not seen in reverse and it is not a cable-adjustment issue. The fix (more on that below) involves removing the transmission from the engine.

My first question for this group is, has anyone disconnected and removed the KM2A transmission from their Yanmar 2GM on a C&C 27, without moving the engine? It appears that there would be enough space, especially if one has a plastic coupling installed, as we do. One could also slide the prop shaft aft. I'm really hoping that we can remove the transmission, without having to move the engine. If anyone has done this, insight and suggestions from your experience would be much appreciated.

For those interested, these posts discuss this issue, commonly referred to as de-glazing the clutch cones:
https://www.cruisersforum.com/forums/f5 … 07948.html
https://www.cruisersforum.com/forums/f5 … Engine.htm:

For anyone undertaking this, the following two YouTube videos created by an S2 85 owner provide invaluable information on the process of disassembling and re-assembling the transmission:
https://youtu.be/UNSW5S8EXus
https://youtu.be/p3eMGM8w0PE

According to the above, the Yanmar 2GMF, 2GM20, 2GM20F, and 1GM series engines all use the same transmission.

The only discussion I found on this forum on this subject is this very dated post, http://www.cc27association.com/f4/viewtopic.php?id=162, but the two replies did not lead the original poster in the right direction.

See photos below.

Last edited by Hugh Morrin (2024-07-24 07:59:10)

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Hugh Morrin
Blue Zulu, C&C 27 Mk III, #894
Nepean Sailing Club, www.nsc.ca/cc27
President and back-up webmaster, C&C 27 Association

Hugh Morrin

Member

Re: Yanmar 2GM Transmission Slippage; De-glazing KM2-A Clutch Cones

Sincere apologies to all for not providing an update on this earlier.

After haul-out in late October, 2024, we did indeed remove the transmission from the back of our Yanmar 2GM, and we were able to do so without moving the engine. Another C&C 27-3, hull number 902, had been experiencing the same problems as we were, and they also had no issues with the removal of their transmission.

In the subsequent weeks, we teamed with the crew of Escape and disassembled the transmission, methodically following the guidance in the above references, and in the Yanmar Service Manual. We lapped the cones per the videos, replaced all the bearings, seals and o-rings with new ones purchased from General Bearing Service in Ottawa, and re-assembled. Installation back in the boat went smoothly, though not quickly, on November 20, and then we sealed the boat up for the winter.

In early May, after launch, we started the engine, and the transmission shifted smoothly into forward, grabbing nicely with no slippage, providing full torque. Unfortunately, however, it did not want to shift out of forward -- the exact opposite of the issue we'd had before lapping the cones! With a crew member pushing the shift lever on the transmission to assist in changing gears, after shifting in and out of forward and reverse multiple times, it did get easier to shift. Thereafter, the transmission performed reliably all summer long, during 60+ outings over the summer, with no slipping. Interestingly, the crew of Escape experienced the exact same behaviour; after an initial break-in period during which it was difficult to get out of forward, and to a lesser extent, out of reverse, the transmission worked very smoothly.

A huge thanks to Keith, Mike, Chris and Peter for their leadership on this project!

Last edited by Hugh Morrin (2026-04-08 21:01:43)

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Hugh Morrin
Blue Zulu, C&C 27 Mk III, #894
Nepean Sailing Club, www.nsc.ca/cc27
President and back-up webmaster, C&C 27 Association