This is an old thread, but I thought I would post to it because I just rebuilt my Kansaki Hurth KBW10 transmission. The issue with the transmission was the forward gear was barely engaging, but the reverse gear seemed to work fine. The cost for a new transmissions today is about $4,200 which was a little steep for my old boat. So repairing it myself seemed to be worth a try. There obviously can be many issues with transmissions, but the KBW10 has a good reputation for being very durable. At the end of the day - I replaced 6 of the steel plates and 4 of the friction plates and got a whole new world of boat education. All of the the work I did was on the output shaft. The other two shafts were fine. If you are going to try this yourself, follow the manuals and keep the work place clean. I had two or three reference manuals and surprisingly some parts were referred to by different names - which was annoying.
Special tools needed -
Spanner nut wrench - Yanmar sells one, but it will be necessary to remove and tighten the output shaft nut
Gear Puller (local automotive store will loan you one)
1.5 foot piece of pipe that fits over the shaft, used to press the assembly back together.
Micrometer - measuring part for wear
Pulling out the transmission - A chore in itself depending on your access to your engine compartment.
The transmission itself is small and weighs about 25 lbs. It's bolted up to the rear motor mounts, so when you tear it out you still have to support the back of the engine. But plan on your boat being down for at least a month. Everything takes time - Tear down, identifying parts, ordering parts, reassembly and installation.
Starting with the professionals -
First, I didn't open the transmission, I let the Yanmar repair shop in Costa Mesa open it. They told me the threaded end of output shaft was damaged and they didn't recommend repairing it. They disassembled the side plate and main housing and visually inspected the transmission. The shaft coupler, and the output shaft end nut were still attached and the nut appeared damaged (previous damage). Step one in the instruction manual is to remove the nut. I tried for about two hours to remove the nut, it was really not even recognizable as a nut it was so corroded and distorted. So I thought I would bring it to the professionals to remove the nut. They did not try to remove the nut, but removed the other covers. They charged $250 to open it up and inspect it, but they don't put it back together, so I brought down a five gallon bucket and collected the parts.
Damage on the threaded end of the output shaft -
I went down to a machine shop and they removed the nut from the end of the shaft. They had some special tools and actually cut the side of the nut and it popped off. They said they could fix the threads but they wanted the new nut for the end of the shaft. The shaft looked great except for the end threads. The new end nut was $50, but they traced the threads and the new nut fit perfectly Later, I discovered after torquing the shaft nut you stake it. Staking the nut means distorting the end threads so that the nut will not loosen. Some of the damage at the end of the shaft was actually when it was staked and then 35 years of marine environment.
Pulling the gears and components off of the Output Shaft -
Go to the local auto parts store and use their loaner program to access a free puller bring the assembled output shaft with you so that you know the puller will fit.
Best tip - Immediately before pulling the gears with the puller in place - take the heat gun and heat the gears and the parts to about 220 degrees farenhiet. It will be much easier to get the gears off if they are heated.
Place the parts in order in a clean spot in your work area and get out the micrometer and start measuring your steel plates and friction plates. Write down the measurements on a piece of paper for the forward gear and reversing gear for each of the steel plates and friction plates. Order new plates for any that are out of spec or close to being out of spec. You don't want to have to tear down the transmission again because the marginal parts have worn down in a couple of years.
Ran out of time - if you have questions you can send me an email - firstname.lastname@example.org
Bottom line - The replacement parts and the machine shop costs totaled about $850, but the transmission is still working great after a full year of service.