Allison M20 over-full

[nautibake]

Hello,
I understand from this forum the issue with engine (8V71TI in my case) oil leaking through seal into transmission. This is happening on my stbd side (tranny oil black), but not the port side (oil like honey). I poured through the tranny documentation and could not find any reference to a few questions.

Is there some tolerance for over-full (on the side that is getting the extra engine oil via the leaking seal)? Mine is about 1/2" above full (checked warm while idling). We're planning an 18 hour trek back down to Key West via Marco and I'm trying to decide how much oil to pump out of the tranny. I assume it is going to vent out into the bilge at some point so I want to prevent that. Assuming the vent is working properly, is there any potential damage from running over-full a certain amount?

I also couldn't find any reference to how much oil it takes to raise the level from the "low" line on the dipstick to the "full" line?

Any insights appreciated!!
Thank you,
Jerry

 

[Boatsb]

I'd be more concerned about low engine oil

That can run an engine in no time.

 

[nautibake]

I'd be more concerned about low engine oil

That can run an engine in no time.

Thanks Scott - I do keep a close eye on that, just mainly wondering about the over-full condition on the tranny.

 

[bostonhatteras]

overfull m20 will aerate the oil (foam). that will lead to poor lubrication and pressures. aerated oil is impossible to check for level due to being filled with air bubbles and seeming like more oil than whats there. If its not foaming youre ok id say. one of mine leaks just like yours does, just not bad enough to justify changing the seal. every other year oil change takes care of mine.

 

[captddis]

Most likely the rear engine seal is leaking. It will leak engine oil to trans. Check the hydrico pump first but , my money is on the seal.

 

[Fanfare]

Suck it out of the transmission, add about that amount to the engine. Since the oils are the same I suppose you could even add the extra transmission oil back into the engine (the purist in me says don't, however). There are more contaminants from engine combustion in engine oil than in transmission oil but I doubt if short exposure would do much damage. And these are already mixing through the bad seal. I would fix this as soon as convenient as long term the acids in engine oil won't be good for the transmission.

 

[Boatsb]

Most likely the rear engine seal is leaking. It will leak engine oil to trans. Check the hydrico pump first but , my money is on the seal.

Mine too. I've seen the hydreco pump blow a seal and empty the transmission into the engine.

 

[Tim Powell]

Mine too. I've seen the hydreco pump blow a seal and empty the transmission into the engine.

I had the same problem. I sucked all the oil out of the transmission. Changed filter and cleaned screen. Why i do not know but it has not turned black an oil lever ok sence the engines were rebuilt.

 

[mykim]

Hello,
I understand from this forum the issue with engine (8V71TI in my case) oil leaking through seal into transmission. This is happening on my stbd side (tranny oil black), but not the port side (oil like honey). I poured through the tranny documentation and could not find any reference to a few questions.

Is there some tolerance for over-full (on the side that is getting the extra engine oil via the leaking seal)? Mine is about 1/2" above full (checked warm while idling). We're planning an 18 hour trek back down to Key West via Marco and I'm trying to decide how much oil to pump out of the tranny. I assume it is going to vent out into the bilge at some point so I want to prevent that. Assuming the vent is working properly, is there any potential damage from running over-full a certain amount?

I also couldn't find any reference to how much oil it takes to raise the level from the "low" line on the dipstick to the "full" line?


Any insights appreciated!!
Thank you,
Jerry

On Twin Disc Gears being over full is a bad thing. This condition may cause several problems. Such as low pressure, foaming oil and slippage. IMO, the best way to maintain your oil level is 1/3rd above the low mark when the gear box is cold. This allows for expansion.

As for your concern that engine oil is getting into that gear. There is no real avenue for this to happen. Even if the main seal on the back of your engine has gone bad it won't get into the gear. The only possible way would be a bad oil cooler. Even that is unlikely. I'd suggest you pump out the gear completely, pull the filter and suck out the oil at the bottom. Then install a new filter and refill with 40W. Keeping your cold fill line a third up the stick. On your sea trial, when she is good and hot, in neutral, check the level. If it is above the full mark pump some out till it is just a hair below the full line

 

[Boatsb]

Jerry.

The Allison's have a wet bell housing so rear ma8n seal leaks will get into the transmission.

 

[johnjen50]

Ok, but isn't the oil cooler raw water cooled? And if so the oil would get milky, not really black correct?

 

[bostonhatteras]

Ok, but isn't the oil cooler raw water cooled? And if so the oil would get milky, not really black correct?

Fresh water cooled, from bottom of heat exchanger.

 

[Boatsb]

Transmission oil cooled by seawater. Engine Oil cooled by freshwater.

 

[johnjen50]

But still, why would that make the oil black? Wouldn't it make it milky?

 

[rsmith]

Transmission oil cooled by seawater. Engine Oil cooled by freshwater.

Both are fresh water cooled

 

[Fanfare]

Fresh or salt, whatever kind of water turns oil milky. This can also give the oil the consistency and appearance of toothpaste. I don't know what this may do to the flow rate of the oil pump but I'm pretty sure it's not good.

 

[JonHupp]

A bad rear engine seal will give you sooty oil on an M20. I went through this drill last year and had major transmission damage requiring a rebuild at Arcadian in Louisiana.

 

[Fanfare]

Did the sooty oil cause the damage? If so, what happened? Or was the cost to remove and replace the engine seal?

I would expect that the engine oil filter would remove most of the damaging grit in normal operation. Bearings in the engine are about the same as those in the transmission. Water from combustion would be bad, but my old Allisons ran about as hot as the engines so should be boiled off through evaporation. Acids on the bearings (especially the roller bearings) could do harm but the transmission oil is the same 40 weight as engine oil and so well buffered against this, as well as there being a lot of otherwise clean oil in the transmission to dilute.

Of course,once you get the transmission separated from the engine you might as well do the whole thing and be good for another 20 years.

 

[rsmith]

My rear main seals leaked into the transmissions on both engines for 20 years and a lot of hard running. Never had a problem. I rebuilt the engines in 2002 and haven't leaked a drop (into the trans) since. I've got the MH gears tho.
IMHO reguarding transmission failures I see a lot of people move the clutches from forward to reverse and back without stopping in neutral to slow the shafts that's a lot of mass to change direction and hard on the transmission.

 

[JonHupp]

I doubt the oil leak and dirty oil caused my transmission failure. That being said, the transmission oil filter is totally incapable of removing the engine soot from the transmission once it leaks in. Early on, I changed the oil and filters multiple times and, just like the engine, with a leaking seal it was 'dirty' in no time. With a good seals, the tranmission oil looks brand new for years.
You can learn much about your transmission by checking the strainer (for metal and clutch material) and cutting open your used oil filters. Mine was full of metal leading to a big overhaul. Again, I don't know if I can blame the bad oil seal but my other transmission has perfect looking oil and no metal in the strainer or filter...