Disappointing rate of wear

My 2012 MFS20 I thought had a gear change issue but turned out to be a mis-adjusted gear change cable. However having removed the engine from my boat I decided to check it over before selling the engine as I had then bought a bigger engine. On disasembling the lower leg just to have a look I found that despite the gear oil being pristrine, all the gears immaculate and gear oil having been changed every season there was noticable wear in the prop shaft outer needle and ball bearings. I have always treated the engine gently.
That is with only 400 hours of use (auxillary engine on a sailing cat) over 6 seasons ( I had a couple out). So new bearings and seals to replace before selling it on. Already previously replaced the engine power tilt switch and carburettor.

What gear oil did you use? How did you measure wear? Obviously this did not magically happen overnight, so if it was occurring, why was it not addressed when you were in your 5 year warranty? Is there a chance that the prop shaft is not straight?

Could it be that prop was spinning in neutral while sailing? Was the outer lip of seal allowed to dry out?

All good points but:- I used the recommended gear oil in the correct (measured) quantity each season. The 5 year warranty expired 4 years ago. I cannot say I conciously felt for play in the prop shaft when changing the gearbox oil and the impeller each season, although I would norrmally spin the prop and generally feel that everything was ok. The prop has never had a knock, I always raise the outboard leg when sailing, the oil seal has not leaked water into the gearbox oil and the water seal O ring has not released oil into the water. The seal lip is still working ok but I imagine that with play in the shaft bearing that would not go on for too long. I have not measured the play but it is quite noticable just rocking the shaft by hand. I am not a mechanical engineer by training but I have worked in engineering and built and rebuilt many engines, some gearboxes and a couple of back axles so I am reasonably familiar with fits and tolerances. Most of the bearings and seal parts are not off the shelf in the UK so I guess a 6 week lead time from Japan.

So you have a nearly 10 year old motor which may or may not have excessive play at the needle bearing of the prop shaft, and there is no seal leak... and no noises or other symptoms... and you don't know whether the bearing is in tolerance. It may have been that way from new... The C gear has a ball bearing, so odds of a lot of wobble are remote. Myself, I would probably just run it as is.

Can you check the movement with a dial indicator?

timguy said:

Can you check the movement with a dial indicator?

Click to expand...

I could if I had one. I could set it up firmly held in a vice and use feeler gauges to check the play.

pvanv said:

So you have a nearly 10 year old motor which may or may not have excessive play at the needle bearing of the prop shaft, and there is no seal leak... and no noises or other symptoms... and you don't know whether the bearing is in tolerance. It may have been that way from new... The C gear has a ball bearing, so odds of a lot of wobble are remote. Myself, I would probably just run it as is.

View attachment 24959

Click to expand...

I do not know what the spec tolerances are but I am fairly sure that an oil seal by its very nature of its construction would not be designed to cope with much wobble or eccentricity in the shaft rotation. Do you know what the spec is on the bearing tolerances?
I had already bought another engine as I wanted a larger HP engine and I also thought the Yamaha High thrust 25hp engine with a 13" prop versus 9.25" and a lower gear ratio box was more suited to the displacement hull of a 3.5Tonne catamaran.
I plan to sell the engine and would like to do that with a clear concience so with the total cost of the bearings and seals at only £83 GBP it seems the right thing to do. Also the problem with running it as it is, is that if there is oil loss or water ingress the first you may know about it is that the gearbox runs dry or with water has accelarated wear.
Thanks for your comments I appreciate you taking the time.

Taking out the oil seal it was then apparent that the rim of the housing was badly corroded and crumbling so a new bearing housing is now also required. Odd that the corrosion was so localized. Anodes all only half worn. Perhaps the rim is where the exhaust water flows so causing accelerted corrosion but still only 6 seasons usage and <400 hours.

Having fitted the new bearing housing complete with ball race, needle roller and water and oil seals there is no play at all in the bearings and al is re-assembled ok. However it was disapponting to see that the new housing was unpainted, particularly as where the old one had corroded was where the hot salt water exited behind the prop. A little "built in obsolescence " perhaps.