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Old 09-04-2020, 11:33   #16
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Re: Yanmar 1GM (cone-type trans) needs revving to CLUNK into gear both FWD and REVERS

Had the same problem going into forward.
A Marine transmission shop told me to drain the old fluid and run mineral spirits for a SHORT time then use 30 weight oil.
Shortly afterward problem solved.
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Old 09-04-2020, 12:00   #17
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Re: Yanmar 1GM (cone-type trans) needs revving to CLUNK into gear both FWD and REVERS

Side note, can anyone tell me whether the dipstick is read by resting it on the end of the threads or threading it completely into the housing?)

I understand perfectly well .It Was my problem too .Solution .I Use a wooden dipstick graduated according the marks of the original dipstick .The main reason is I'm incapable to see the level and alway remain the doubt as you mention .
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Old 09-04-2020, 12:21   #18
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Re: Yanmar 1GM (cone-type trans) needs revving to CLUNK into gear both FWD and REVERS

I had a yanmar 2 Gm with the same clung. It appeared 1000 rpm was too high. Once back to 750 the clung vannished. That was all. The motor did not idle without trembles but it solved the problem. And take time to switch reverse and back. Slowly is another trick.
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Old 09-04-2020, 12:38   #19
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Re: Yanmar 1GM (cone-type trans) needs revving to CLUNK into gear both FWD and REVERS

You are getting some good comments here. I especially like the running it for a very short time in mineral spirits to remove any glazing. Then it is SAE 30 weight detergent oil, this is definitely worth a try. If it helps but does not go away completely try again.


On my 2GM in 2016 it started doing the same thing. I just pulled the transmission and took it to a local shop who put in a new cone and it has been fine since. Painless but expensive, wish I had tried the above.

It is defiantly not ATF fluid and it is important to NOT fill above the fill line, with it resting on top of the threads.


It is hard to read the level on the shiny dip rod so dip it and then lay it on a paper towel and see how far up the oil shows.



Good Luck!
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Old 09-04-2020, 14:00   #20
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Re: Yanmar 1GM (cone-type trans) needs revving to CLUNK into gear both FWD and REVERS

Had this EXACT issue on my 2gmf13 with the KM2a transmission. The original manual had a misprint and told people to run SAE30 NON-detergent oil. Non detergent will leave varnish in your cone clutches that adds up over years until you get slippage and intermittent failures to engage.

Easy fix. Get the proper SAE30 HD (high detergent oil) and change it every few hours of run time. I did, one about every 4hrs of engine and each time and it came out cleaner and cleaner until a nice gold. The transmission shifts PERFECTLY ever since. That was about 1000hrs ago. Hell I've made it down the Erie canal, icw, finished my 3mo in the Bahamas and am sitting here in eleuthera for a weather window to WPB after the weekend. Zero fails engagements zero slips.

Anyone who isn't sure go into your engine bay and find the metal stamped tag on your trÓnny. Hope that fixes it for you too! I was pricing repowers when i fixed mine for about 30$ worth of common Quaker state 30HD.
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Old 09-04-2020, 16:55   #21
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Re: Yanmar 1GM (cone-type trans) needs revving to CLUNK into gear both FWD and REVERS

I put 2300 hours on a 1GM10 powering a Freedom 25 sailboat before the carbon fiber mast broke at the deck and the boat was totaled by the insurance company. I never had any problems with the trany but did have to take out the thermostat at around the 1500 hour mark to keep it running cool. Scale had built up in the block. One question that I haven't seen anyone answer is the dip stick. Drop it on to the opening in the top of the trany and read the oil level at that point. At least that is true as regards checking the oil level on my 3YM20 for my Dana 24{and I now have put 2100 hours on it in 14 years since new without any problems}. You may have scale build up eventually in your raw water cooled engine. Some on this web site have advocated procedures to get rid of it. I'm a medical doctor, not a diesel mechanic, but my diesel mechanic{also a Yanmar dealer}says that's all baloney.
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Old 09-04-2020, 18:55   #22
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Re: Yanmar 1GM (cone-type trans) needs revving to CLUNK into gear both FWD and REVERS

I don't doubt the success others have had in using some kind of flushing fluid, but it's not always the solution, as I experienced. Regarding shaft float, this is controlled by shim washers fitted beneath the tapered bearings, and pretty easy to set up, and should be done as it will affect cone engagement and longevity.

To diagnose problems caused by incorrectly set cables and end-stops etc, you can disconnect the cables (while tied up to a berth) and shift the transmission lever by hand with the motor idling. Do it gently so you can feel the built-in detentes and aren't adding excess travel.
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Old 09-04-2020, 21:09   #23
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Re: Yanmar 1GM (cone-type trans) needs revving to CLUNK into gear both FWD and REVERS

Thanks everyone! Lots of good info here. I think that I will try the mineral spirits idea. Wouldn't want to try it in my pickup but it makes a lot of sense to me on such a mechanically simple transmission. I already have a 13hp westerbeke waiting for when I have time to do the repower so I don't mind being a little rough on it if it saves me having to pull it and tear it down. Any thoughts on how long to run it? My gut says somewhere in the neighborhood of 2-5 minutes?
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