Transmission leaking

[colinhow]

We recently acquired a 1977 27' Marben Trawler with a 70Hp Hercules Diesel and Twin Disc MG-502. We ran the boat down from Nanaimo a week
ago from where it had probably been sitting for a couple years.

At the end of our maiden trip about 100' from our moorage the transmission stopped working. Luckily we got the boat into
safe moorage with the kicker.

The low gear oil level combined with the oil in the bilge under the transmission is pretty clear that transmission is leaking and reached
a point where the transmission shut down. Distaster averted and we are now in trouble shooting mode.

It turns out the engine mounts have collapsed and so the shaft alignment is not perfect. But lifting the motor is not in the cards anytime soon.

Short term:
My local Diesel guy is suggesting I used a combo gear oil and sealant to slow the leak. Anyone agree or disagree strongly?

Mid term:

I will need to make sure the shaft angle isn't causing damage. The shaft alignment may require motor mounts?

Long Term:

I may want to pull the motor and transmission and repower? I love the boat hull and layout but the motor and trans are old and corroded. I have been wondering about going electric!

Any thoughts on how to keep the boat reliable and on the water without big expenses would be very welcome!

 

[rgano]

That looks pretty ugly. It is not necessarily required to pull an engine to replace motor mounts - been there done that. Once changed and with the tranny shaft seal replaced (often doable with the shaft slid aft a few inches - been there done that too), make sure the shaft coupler's face is perpendicular to the shaft and realign the tranny and shaft couplers to within a couple of thousandths on an inch. That should see you through. Aligning on the hard comes with the need to realign once back in the water, which is why I prefer one and done while waterborne.

 

[colinhow]

Thanks Rich. Super helpful. So you think I could potentially replace the mounts and align the shaft while on the water?

 

[Mac2]

Definitely replace the motor mounts or you will damage other parts-shaft log, cutlass bearing. Just jack up each corner of the engine to replace one at a time. YouTube videos will show you how to align coupler with feeler gauges. Preferable to align in the water.

 

[rgano]

Thanks Rich. Super helpful. So you think I could potentially replace the mounts and align the shaft while on the water?

Yup, just like Mac2 says.

 

[kapnd]

You need to find a better mechanic.
There’s no way that sealing the transmission leak is a smart solution, even in the short term, and unlikely that it would seal anyway.
Replace the motor mounts, check the packing gland and stern tube, and cutless bearings for damage and wear, then realign.
This will require a haulout, not a cheap out!

 

[Mac2]

You need to find a better mechanic.
There’s no way that sealing the transmission leak is a smart solution, even in the short term, and unlikely that it would seal anyway.
Replace the motor mounts, check the packing gland and stern tube, and cutless bearings for damage and wear, then realign.
This will require a haulout, not a cheap out!

Im no expert, but don’t you think the race(or whatever the seal is called) got partially crushed when the motor mounts gave out? I agree pouring magical goo in the transmission is not the answer. Twin Disc transmissions are not cheap.

 

[Diep]

I am also no expert but I will agree, do NOT just dump some sealer in the transmission and try to keep running.

If you are out of alignment and have bad mounts you will make things worse if you keep running.

I haven't done it and would defer to others who have, but if you are somewhat handy I think you can get a pretty long way DIY. You don't lift the whole motor, just do 1 at a time. This should be doable in the water on your own with minimal equipment. Then you could try to align it yourself or get it close. Even if you can only get it that far you will save a lot having someone come in just check/finish the alignment and shaft stuff.

You didn't actually say where the leak is. We are assuming it is related to the alignment issue but it could be something else.

 

[Lepke]

Running a misaligned shaft usually leads to damage to the transmission rear bearing, seal and stern bearing.
If the rear bearing is damaged and not replaced, the new seal won't last long.
The motor mounts can usually be changed by using a long bar to lift the trans and engine rear. Two people can do it easy. One side at a time. I can do it myself and I'm approaching 80. Then check alignment.
To check, slacken the bolts on the coupling so you can just get a feeler gauge between the flanges. Check all the way around, adding or subtracting individual gauges to obtain the difference between top, bottom, and side to side. I do mine to .002". Most yards to .005" or more so they have something to fix later. If you're going to troll with the main engine, the closer the alignment the better. Small misalignment makes a noise that effects fishing. I learned that the hard way when I was commercial fishing salmon and tuna.

 

[Comodave]

I would agree on a new mechanic. I would not put in a sealer because I would be afraid that it may seal up a small passage in the transmission and cause further damage. You need to fix the root problem, start with the bad motor mount. Then progress to the other issues as necessary.

 

[SteveK]

I will echo what Lepke said. Before anything else, uncouple the shaft enough to determine alignment. Don't go changing mounts until you have a baseline on the alignment. Then confirm where it is leaking. Now plan the repair.

 

[luna]

I have changed my current boat's engine mounts twice, over about 17 years.
Just did them in September 2025 and it makes a difference. Smoother and quieter.

When you do this work, you uncouple everything and lift one side or end (your choice) of the engine up, re & re the mounts and then do the other side or end.
Once all the new mounts are in place, you then set about equalizing the load on each forward (P & S) mount, then the same for the aft mounts and only once that is done do you start the alignment shuffle using whatever movement inducing tools (no hammers please) and measurement tools that you see fit.
The goal is to have the transmission's output flange centred on and parallel with the shaft coupler's flange, within an error of less than 0.003" measured at the outer edge of the 2 flanges.
I then wait for 48 hours and adjust things again as the rubber in the new mounts has now settled in enough to make a difference.

I have never done this:

Before anything else, uncouple the shaft enough to determine alignment. Don't go changing mounts until you have a baseline on the alignment.

What is a baseline on the alignment with the old (possibly collapsed ) mounts still in place?
How is this important when the new mounts change almost everything?

 

[Delta Riverat]

I think you should fix the mounts first. Bad engine mounts are a good reason to stay at the dock. Once it's all solid, proceed from there. Your transmission went out because the clutches had no pressure as there was no oil. Probably caused by the mount failure that beat the seal to death. You were lucky to be close to the dock.

You need to have a solid engine-tranny mount first before you do anything next. Don't even think about running until the mounts are solid and tranny leak is fixed. Then the alignment can be done.

 

[SteveK]

I have changed my current boat's engine mounts twice, over about 17 years.
Just did them in September 2025 and it makes a difference. Smoother and quieter.

When you do this work, you uncouple everything and lift one side or end (your choice) of the engine up, re & re the mounts and then do the other side or end.
Once all the new mounts are in place, you then set about equalizing the load on each forward (P & S) mount, then the same for the aft mounts and only once that is done do you start the alignment shuffle using whatever movement inducing tools (no hammers please) and measurement tools that you see fit.
The goal is to have the transmission's output flange centred on and parallel with the shaft coupler's flange, within an error of less than 0.003" measured at the outer edge of the 2 flanges.
I then wait for 48 hours and adjust things again as the rubber in the new mounts has now settled in enough to make a difference.

I have never done this:

What is a baseline on the alignment with the old (possibly collapsed ) mounts still in place?
How is this important when the new mounts change almost everything?

new mounts and alignment will of course fix a problem that so far you have assumed exists.

 

[luna]

The OP said the mounts collapsed. That, if it exists is more than likely significant. The OP also hypothesized that this collapse caused the gearbox to leak it's oil, causing a propulsion failure.
Sounds reasonable as if the engine mounts did collapse, the gearbox's rear seal may be subjected to forces that could tend to cause a leak. But let's not assume anything.

Regardless whether the Op is right or wrong, what is getting "a baseline on the alignment" and how is that important? When replacing engine mounts, I've never done this. What am I missing?

 

[Comodave]

What has not been mentioned so far is that the prop shaft needs to be centered in the shaft log before you try to align the engine. You want to calculate the hanging shaft weight, or the weight of the shaft and coupler ahead of the shaft log. Then lift half that weight so the shaft is centered. Then after replacing the engine mounts align the engine/transmission coupler with the prop shaft coupler side. As to the clearances on the coupler you take the diameter of the coupler face in inches, say it is a 6” coupler then you are allowed .006” difference around the coupler. One thousandth inch per inch of the coupler face. Measure this clearance around the coupler at 12, 3, 6, and 9 o'clock. Although I would try for less since it will help everything run smoother. Remember that you are aligning the engine/transmission to the prop shaft not the other way around.

 

[SteveK]

What is a baseline on the alignment with the old (possibly collapsed ) mounts still in place?
How is this important when the new mounts change almost everything?

Changing the mounts is the easy part of the job.
Have you magnified the pictures to see the coupler is very rusted. The trans is corroded. The boat apparently sat for some time. There was no mention of rough running with trans bouncing around. Baseline to me is seeing the uncoupled shaft and trans position to show the extent of the work ahead.
I see a trans that needs to go to the shop for analysis for damage before worrying about mounts. Why do that work if the trans is beyond repair, it does not look healthy. JMO

 

[Delta Riverat]

When I look at the pics I see the prop shaft so badly misaligned that the gland coupling is curved.

 

[SteveK]

When I look at the pics I see the prop shaft so badly misaligned that the gland coupling is curved.

That as well, is it a bent shaft causing the trans oil leak? Uncouple and find out.

 

[Portage_Bay]

I will jump on the band wagon of not using any sealant. It won't solve the problem and will likely damage the transmission. As for using gear oil, be careful. Use only the recommended lubes. Hydraulically actuated Twin Disc transmissions use single weight engine oil. 30W or 40W.

Here's a link to the manual.
https://twindisc.com/wp-content/uploads/2021/03/MG-MGX-Operators-manual-1016313_RevL_0818_CD.pdf

Yours is almost surely hydraulically actuated. Start on page 2-5 of the manual if you want detailed info on the recommended oils.

A hydraulically actuated twin disc transmission has a hydraulic pump that regulates the pressure on clutch plates. Gunk that system up with sealant or the wrong lube and you've got more problems. Hopefully running it dry has not damaged the transmission.

What AI says about determining of your transmission is hydraulically or mechanically activated.

How to tell if your Twin Disc transmission is hydraulic

Most Twin Disc marine gears, and all power-shift transmissions, are hydraulically actuated, relying on pressurized oil to engage the clutches. If you see any of the following features, your transmission is hydraulic:

• An oil pump: A hydraulic system needs a pump to create fluid pressure. Look for an oil pump mounted to the transmission case.
• An oil cooler and filter: Hydraulic systems require an external oil cooler and filters to regulate fluid temperature and prevent contamination.
• Hydraulic lines: The presence of hydraulic hoses and fittings connecting to the transmission case is a key indicator.
• A dipstick for hydraulic fluid: These transmissions have a dipstick to check the level and condition of the hydraulic fluid.
• Smooth shifting: Hydraulic engagement provides for smooth and responsive gear changes, as opposed to the more abrupt feel of a mechanical clutch.

 

[Mac2]

Based on the above, are you leaning towards replacing the engine and transmission as you noted in your original post? I suppose it will depend on the type of boating you plan to do (read near a tow service) and how long you plan to have the boat. Also, you didn’t mention your skill level and if you’re able to do some of the above tests before making major decisions.

 

[luna]

Stevek, thanks for that.

Now that I know what you meant (just a visual indication of the engine's current misalignment) I realize that when I do an engine alignment I have not missed something important.

I agree that the gearbox needs to come out to be at a minimum, cleaned, sealed and painted. Understanding that the OP has stated "But lifting the motor is not in the cards anytime soon.", I somehow doubt that even undoing the rusted up bolts connecting the shaft coupler to the output shaft is on the table.

With this kind of attitude the boat may well be just another Ganges mooring ball or dock queen.

 

[Choices]

Go to my blog Grandbankschoices, dig around and find my write up. I replaced all my motor mounts with a 2x4. This is relatively cheap. 2nd get a new mechanic. Let sit a week or two then align engine. Decouple shaft from engine before installing new mounts. A wobbling shaft took out your seals, and tranny could be next.

 

[SteveK]

I replaced all my motor mounts with a 2x4
I know exactly what you mean, (using) but others may wonder if the 2x4 was laid flat or on edge and how long the wood will last.

 

[Choices]

I used it as a prybar. Sat on one end it and changed each mount one at a time. Go to the blog for information. We changed three other boats the same way. A one day job if all goes well.

 

[luna]

I wondered how you would use just a 2 x 4 to unbolt the coupler and the engine mounts? What about the exhaust piping, where do you put the 2 x 4? Perhaps this works only in Texas!

 

[Delta Riverat]

The 2x4 just supports the engine so you can change the mounts. Temporary only.