Lapping Yanmar Kansaki Gearbox KM4A cone

[Talulah]

When boat speed does not match engine speed there are a number of possible causes and one such cause is a slipping cone in the gearbox.
What happens is after a high number of engine hours wear on the cone coupled with polishing of the contact service on the adjacent gear causes the clutch to slip.
Instead of replacing the cone a common technique is to relap the surface where the cone and gear meet.
Service manual here: http://sagaforumet.com/yanmar-manual/4JHE/kapittel_7b.pdf
Youtube link here: https://www.youtube.com/watch?v=kCo16Os09vU

So having experienced the lack of speed I decided it was time to relap the cone/gears.
The service manual talks about dismantling the gearbox. With the gearbox removed drain the oil (25mm socket) and remove the shifter mechanism. Be careful not to lose the spring in the end of the mechanism. In our case the shifter itself dropped inside but was easily extracted once the gearbox was opened up.
According to the manual the next stage is to undo the output shaft nut that holds the gearbox coupling on. In our case (Yanmar KM4A) this requires a special tool. One I don't have.
Number one tip: 'Don't remove the coupling'. Instead, undo all the bolts holding the end cover of the gearbox on and remove the cover with the output gear and coupling still attached. The clutch shaft easily lifts out. Retrieve the shifter if you haven't done so already.
To break down the clutch shaft we used an impact drill to undo the nut (clockwise). 30mm socket required. Definately needed the puller to remove the gear and bearings.
Valve grinder and paste bought from Halfords. (Comes with smooth and course paste.)
Reassemble straight forward. No difficulties. All back together and tested ok.
If you have reasonable access to the rear of the gearbox you can do the complete job without removing the gearbox from the engine. In our case it is easy to remove the gearbox vertically through the access panel in the cockpit sole. To start off with I tried to do the repair with the gearbox in situ but couldn't get the end cover off. It was only when I removed the gearbox and raised it up that I discovered one end cover bolt still in place.

 

[dgibs]

"If you have reasonable access to the rear of the gearbox you can do the complete job without removing the gearbox from the engine ..." How is this possible? It looks like the access to the gears is from the side of the transmission that faces the engine?

 

[Talulah]

"If you have reasonable access to the rear of the gearbox you can do the complete job without removing the gearbox from the engine ..." How is this possible? It looks like the access to the gears is from the side of the transmission that faces the engine?

It’s fours years ago since I did this but I seem to remember it all coming out via the end cover. Not through the bell housing end.