Hints and Tips



Yanmar SD50 cone clutch & shaft seal repair

Replacing the pinion shaft seal and ‘lapping’ the cone clutch on a Yanmar SD50 sail drive fitted to a 2012 Sun Odyssey 409


An upgrade kit that should eliminate future problems has been developed by Gideon Coetzee and I plan to fit this during our 2019/2020 lay-up.The video below explains the modification and demonstrates how it may be fitted.



Gideon developed this initially for his own use, but has subsequently supplied over 200 units worldwide, he can be contacted by email: saildrives@yahoo.com


The boat is ‘Topaz’, a 2012 SO 409. The pinion shaft oil seal on the Yanmar SD50 sail drive has been leaking slightly for the last few seasons, this year the leak got worse so I decided I had to do something about it. Also this year we started to get the dreaded slipping of the cone clutch, a well documented problem on the Yanmar SD50 sail drives. With both these issued now occurring it was time to do the repair job. As I am reasonably competent at DIY maintenance, I decided to do the job myself.

If you do a bit of research on Yanmar SD50 problems there are one or two videos on how to repair both the pinion shaft oil seal and removal of the cone clutch. These however relate primarily to multihulls so the engine and gear box installation is not the same. As part of this write up I have not gone into a great amount of detail on removal of the clutch and lapping as this is covered in the yanmar service document. What I did focus on was the work required to separate the engine and reinstalling the engine as in a SO409 there is not a lot of space to work in. The biggest issue with the pinion shaft repair is that the engine has to be moved forward around 15-20cm. When I was doing the work the boat was out of the water so sail drive oil drainage was easy however this job can be done with the boat in the water if you pump the oil out.


Preparatory Work

A few steps before the hard work starts

1. Ensure all batteries are isolated.

2. Power: Unclip engine harness connectors, disconnect wiring from alternator, disconnect power lead to starter, disconnect earth strap from rear of engine. All these are situated on the port side of the engine and can be accessed through the hatch in the heads. The earth strap can be undone from the rear of the engine. Make sure you mark the two sockets and plugs on the engine harness. This is required because on my set up there is a spare plug for adding a second set of engine controls. You need to get the right plug when reconnecting.

3. Diesel: Disconnect fuel lead from bottom of fuel pump. There is sufficient slack on the return pipe so it does not need to be disconnected.

4. Cooling: Disconnect bottom pipe from the water pump and the input pipe to the exhaust elbow.

5. Exhaust: I initially tried to remove the flex exhaust pipe from the mixer elbow but it would not move. I found it easier to unbolt the mixer elbow from the engine and leave attached to the exhaust pipe.

6. Engine mounting: Unscrew all four engine mounting bolts, they have captured nuts glassed into the support tray so no problems of losing the nuts. I marked the exact position of the engine mounts with black marker to ensure when refitting the engine was in the exact same position.

7. Bellhousing: Loosen the 8 bolts holding the bell housing of the SD50 to the engine but do not remove yet

8. Disconnect the throttle linkage. There is a small split pin you need to remove to disconnect the front of the linkage from the engine. The rear of the linkage is attached to a bracket of the rear of the engine. Loosen the bolt and the linkage will slide off this bracket (note this bracket is used when attaching the lifting chain in the last stage of refitting the engine).

At this point the engine is now almost disconnected from the SD50 and the boat, the only thing connecting them is the splined pinion shaft and the loose bell housing bolts.


Moving the engine.

The next steps are to move the engine forward the required 15-20 cm to allow the pinion shaft assembly to be removed. In order to support the engine weight to make the move easier I used a small chain hoist on a wooden beam across the companion way.

1. Remove companionway steps: Remove small pins on the top of each gas strut to disconnect the strut from the steps. Once the struts are disconnected lower the steps then remove the split rings from the hinge pins and remove the pins. You can now lift the steps clear.

2. Supporting the engine: There is lifting point on the top of the engine to which I attached the hook from the chain hoist.

3. Mounting the Lifting Block: I attached the lifting block to a wooden beam mounted across the companion way. As there is not a straight lift, the engine lifting point is further aft than the edge of the step, you need to protect the edge of the step. I used bits of wood to hold the chain away from step edge.

4. Slowly take tension on the chain, remove the 8 bolts between the bell housing and the engine that you loosened in the earlier stage. Once these are removed you will find the engine moves quite easily. Keep an eye on the wood that you put in place to protect the step as it will move as you lift. As there is an angle on the chain the load will tend to pull the engine forward anyway. It will take a bit of wiggling but should come forward easily. I released the tension and repositioned the wood a couple of times. I moved the engine about 20cm forward.

5. Before taking the tension off the hoist put a block of wood under the bottom edge of the rear of the engine below the fly wheel. This is required because there is nothing holding the rear of the engine up now that it is disconnected from the sail drive. This will keep the engine roughly level.

6. You can now lower the chain pulley and disconnect from the engine.


Removing the pinion shaft assembly

Next stage is to remove the pinion shaft assembly from the sail drive.

1. Drain the oil from the SD50, Topaz was on the hard so I completely drained from the bottom drain point on the leg. If you are in the water then use a suction pump to drain the leg.

2. Loosen the four nuts which hold the pinion shaft assembly into the saildrive. Back the nuts off until the end of the threaded element is just inside the nut. The threaded portion is about 40mm long

3. Slowly tap each of the four nuts in turn to push the pinion shaft assembly out. I used a piece of wood so as not to damage the nuts. Keep taping until the nuts are against the saildrive housing. You can now remove the nuts.

4. Putting your hand between the engine bell housing and sail drive, grab the pinion shaft and with a bit of gentle pulling the assembly will come free. Make sure you note the orientation of the assembly for reassembling. On my assembly there were a couple of metal shims fitted between the assembly and the saildrive.

5. You should now be able to remove complete assembly through the gap between the sail drive and bell housing.

6. The item can now be taken to your workshop for repair (see repair section)


Removing Cone Clutch Assemble (There are a number of documented procedures for this in the Yanmar manual)

1. First remove the gear selector assembly by loosening the two bolts keeping a note of the position

2. Unscrew the 4 bolts on the top casing of the saildrive then remove the top casing

3. Once removed you can screw one of the bolts back into the centre of the clutch so that it can be removed. I made a 15cm x 3cm flat plate with a hole in the center to act as a handle to pull the clutch out.

4. Pull the whole assemble straight up ensuring you keep it as straight as possible.

5. The clutch can now be taken to the workshop for inspection and lapping


Repairing the pinion shaft oil seal

1. Firstly mount the assembly in a powerful vice, be very careful not to damage the spline on the shaft. My vice had ‘soft’ jaws so was softer than the hardened metal shaft.

2. Undo the bolt which holds the shaft in. It is very tight and you may have to add some extra leverage on the allen to key remove it.

3. Withdraw the shaft from the assembly and inspect

4. In my case I found that the oil leak was due to wear on the shaft. The oil seal had actually worn a grove on the shaft. On investigation I found a Yanmar Service Bulletin issue in North America (YASA2015-004-M) which described this exact problem and offered a solution by fitting a hardened stainless steel ‘speedi-sleeve’ (SKF CR99177) over the damaged part of the shaft. The alternative is to fit a new shaft at a rather high price.

5. I had to modify the supplied speedi sleeve fitting tool slightly by drilling a hole in the centre of it so that the spline shaft would pass through.

6. Before fitting the sleeve it is recommended to use some metal epoxy filler to fill the grove on the shaft as it was probably 0.5mm deep. I then put some thread sealant on the shaft before tapping the speedi-sleeve in place. I fitted the speedi sleeve from the pinion gear end of the shaft.

7. Follow the instruction that come with the speedi sleeve and once fitted carefully remove the fitting flange. I used a small dremel cutting disc to cut through the flange. Once that is done you can then slowly pull the flange off with a pair of pliers.

8. The shaft now has I nice new surface for the oil seal to sit on.

9. Reclamp the rest of the assembly in a position where you can remove the 4 bolts holding the flange on so you can get access to the oil seal. On my unit there was one shim fitted between the flange and the pinion gear.

10. Once removed you can get to the oil seal, prise it out being careful not to score the housing and discard. Fit the new seal being careful not to damage the seal as you tap it into position.

11. I replace both O rings even though they both looked in good condition.

12. Refit the flange back on to the pinion gear assembly, fit the shaft and then fit the bolt. I added some Loctite thread lock on the bolt.

13. Once again mount on the vice to tighten the bolt. The assembly is now ready to fit back into the sail drive.


Lapping the cone clutch (Yanmar Service Manual shows a detailed breakdown of the clutch components)

1. In a similar process to the pinion gear you need to grip the bottom of the shaft in the vice.

2. The top nut has a small lock tab which need to be knocked back out of the grove. The nut is a left hand threaded nut so opens by turning it clockwise. I needed to put a piece of pipe on the wrench to give enough purchase to remove the nut.

3. Make sure you have a clean area to lay out the components when you take the clutch apart.

4. When disassembling make sure you lay the components out in order and correct orientation so that the all go back on the shaft correctly

5. Once you have removed every thing inspect the upper side and lower side gear where they make contact with the drive cone to check for any abnormal wear or glazing. If there is any significant damage then they will need replaced

6. Inspect the drive cone for any damage or scoring. Again if there is significant scoring or damage this will need replace.

7. Assuming all is good then you can start the lapping process. In my case the surface of both cones was very smooth but no apparent damage.

8. Using gear paste, coat the inside of the cave with paste, set the gear back on the shaft and then set the cone on the shaft. By hand push the cone against the cave on the gear and using a little pressure rotate the gear about 5 times in both directions. Remove both and clean thoroughly with petrol to ensure all traces of the paste has been removed. Repeat for the lower gear. The inside surface of the cave now has a dull finish created from the grinding paste.

9. Once all the components have been cleaned then reassemble the complete clutch

10. When tightening the nut I added some Loctite on the threads plus managed to use the existing tab,

11. The assembly is now ready to refit to the saildrive.


Reassembling components to saildrive

1. The Clutch is first item to go back in, drop it in ensuring the opening for the pinion and the gear shift are in the correct place. It may need a bit of rotation to line the spline up with the shaft coming up from the leg.

2. Fit the pinion assembly back in, this is a bit more awkward as it needs to be dead straight to slide into the housing. Once located I tapped it gently on each side until the bolts appeared on the outside of the bell housing. Refit the nuts and then work round them alternatively until it is fully tightened. You will need to keep checking that the pinion shaft keeps rotating freely as you tighten up the nuts.

3. Next refit the selector lever ensuring it is in the same orientation as you took it out.

4. Refit the top plate covering the clutch.

5. You are now ready to replace the oil in the saildrive and move the engine back in place.


Refitting engine

1. This required making a simple wooden support in the rear cabin above the engine box to mount the chain hoist above the sail drive. This position gives a slight angle in order that the engine can be pulled back towards the sail drive. The wooden support needs to take the vertical load plus a short horizontal support against the bulkhead in the cabin to stop the support being pulled forward.

2. I bolted the two lifting chains to the rear engine lifting points. Once the tension was on and the engine weight was on the hoist it was relatively simple process to ease the engine back towards the saildrive.

3. The fiddly bit is now lining up the pinion shaft with the flywheel on the engine. At first I aligned the engine and bell housing by sight until it was close and the shaft was touching the flywheel. I then used a few spare long M8 bolts which could go through the bell housing, bridge the gap and reach the engine. I then slowly moved the engine about until all the bolts lined up. Once this was aligned I then took these bolts out and with a bit of shaking of the sail drive and the engine it slid back into position. Using the hoist allows for small movements and does make the lining up of the pinion shaft and the flywheel on the engine easier. I had been told that this process could be very frustrating however I found this method lined it all up first time.

4. Refit the bell housing bolts. While the hoist still holds the weight refit the main engine mounting bolts. To get them exactly lined up with position I had marked before removal required a bit of leverage with a piece of wood against the front of the engine. Once the bolts were in and tightened the hoist can be removed.

5. All other hoses, exhaust, water, electric and diesel can now all be reconnected in the reverse of the process in the very first section.

6. Refit the companion way steps.

7. As the boat was out the water I could not try the engine however I did check the ignition and gave the starter motor a quick blip to ensure all the electrics were working.


I did the work on my own and over two weekends but having done it once I suspect it could be done in a long day. If two people were involved some tasks may be a little quicker. Other than my time the cost of the repair is relatively low including the new tools I bought. It is certainly a cheaper option than replacing the damaged shaft which I understand could be in the region of £700!!


* Pinion Shaft Oil Seal £10

* Various O rings £8

* Speedi Sleeve £15

* Loctite £6

* Coarse Grinding Paste £4

* Chain Hoist and Chains £40


I guess I will need to give the repair some time to ensure that it has worked properly but initial indications are that everything is working as it should and there are no leaks and I have full drive.


Rick James - Yacht Topaz