KH18A Transmission Fluid

[mitchk]

My KH18A gear box says to use ATF fluid, I have heard that soem of these were being run using standard 30 or 40 weight oil.

What is everyone using in these transmissions?

[bruadair]

Hi Mitch,

When we bought Bruadair she had ATF in the gear box, then after a year we developed a leak. With the ATF in the gear box it leaked at a pretty substantial rate. At the time I did some internet research and found a lot of references that stated it was okay to use the same oil that was used in the engine. Then I called Yanmar and spoke to a couple guys and they agreed that in this particular gear box, the KH18, it was okay to use engine oil. So we flushed the ATF out and put in 40w, the same that is used in our 3QM30F. In the past six years of running it this way we have not had any problems.

In regards to the substantial leak while using the ATF it has subsided considerably since we switched to 40w. ATF is much thinner in viscosity and will find every possible leak. The 40w is much thicker in viscosity so it doesn't leak near as much. By the way, our leak is at the rear seal which requires removing the coupling, and to do this one will need a special tool to remove that odd shaped nut. Fortunately we just had a tool fabricated and expect to change the seal at our haul out in October.

In any event, I would suggest that you talk to Yanmar about using the engine oil just to be sure you're getting the info right from the source. Our experience doesn't necessarily mean it's the right thing for you (but for us it was).

[mitchk]

Hi Damon, Thanks, I thought that you had told me once before you had switched to oil. I have had my transmission apart and replaced both seals, I have been in the power transmission business for 30 years and after re-working the transmission myself I could not find any reason standard oil would not work in this gear box.

Be very careful when replacing the input shaft seal, it is very easy for the sharp edges on the spline to make a very small cut in the rubber lip of the seal, this happens when you slide the new seal over the shaft. It is best to wrap the spline with some plasic before inserting it through the seal. I believe this is what has happened to mine and now I have a very slow leak, it does not apear to be coming from the rear seal so I can only assume it from the input which I cannot get to.

[bruadair]

We had the input seal done when we were in St. Petersburg in early 2005. Fortunately that seal hasn't leaked since replacing. In our particular case it was easy to see that it was leaking because you could look into the bell housing and see oil splattered all around (inside the bell housing). Does your bell housing have oil on the inside, you call look through the small grate.

I've also read that a common place for an oil leak is through the seal of the shifter. And I think I read a while back that if your transmission gear box dip stick isn't the right one, and if you over fill with oil, then it'll lead through the vent on the dip stick. Fortunately there really isn't a lot of places that would allow leaks on the KH18.

Your input seal should be fairly easy to fix, I think I would rather do that one that the rear seal.

How hard was it to take off the nut that holds the coupling on? Did it need a lot of force to get it broken free? Were you able to do it in the boat or did you have to remove the gear box? I'm hoping I can do it in the boat, we're only going to be hauled out for four days. Any tips when doing this seal or is it pretty straight forward once the coupling is off?

Say hi to Deb and Gizmo for us

[mitchk]

I do not remember the details of replacing the rear seal? I already had my transmission out of the boat and on a work bench. I do remember it took a tremendous amount of force and holding the box without damaging it with a vise was a problem. The recommended torque to retighten is more than I could produce with my home made tool.