Gear Oil

On second boat with gear oil well passed the full line on both mains. On one, the oil registers all the way up the dipstick. I’ve brought this to the attention of operators, one of whom used to be a diesel mechanic. Essentially, I’m told there is little to no risk posed here.

My understanding is overfilling engine oil can compromise seals with excess pressure. Is this not the case with gear oil? Or are there dangers posed by overfilling gear oil?


How big is the sump? How much overfilled are they? Are you checking the stick while idle, or engine off?

I would always revert to what the manual says, but in my opinion overfilled by a gallon or two shouldn’t cause any damage. Still doesn’t make it right though. There is a mark there for a reason.


The primary concern with overfilling an engine with oil is the oil level reaching the crankshaft and foaming due to mechanical agitation. It can also cause excess base pressure leading to the seal failure you mention.

Gear oil doesn’t have this issue in most gear designs. It’s not uncommon to have the gear oil read ‘all the way up the dipstick’ due to the dipstick location but it’s unlikely that that is where the oil level is. Try checking it after they come in while the engine is idling. Pay attention to the vessel trim.

It is possible to damage a reduction gear by overfilling it with oil but you would have to work at it. It would be more likely to make a mess.


Like all professionals, there’s the good, the mediocre & the bad. The good diesel mechanics that I have ran across tried to run things as they were designed as described in a manual somewhere. The only times I seen gear boxes ran slap full were when: 1. Ran by dummies who didn’t know any better. 2. Filled by a dummy who wasn’t paying attention when filling. 3. Operated by a lazy person who didn’t want to fix a leak who wanted to lessen the possibility of it running empty.

If you are responsible for those gear boxes find a manual for them & see what the manufacturer recommends. Also, find the person who filled them to the top & ask them to explain the reason.


That’s just plain ignorant- keeping the oil level in a running engine at a wickedly higher level than normal. Let’s see- as our peers above have said above- foaming, agitation, front and rear seal seal breakthrough, increased crankcase pressure, oil splash onto the liners, diminished drainage from the camshaft and rocker galleries… yes this could be quite hazardous on 4 stroke high speed engines… Definitely not a good practice. Foolhardy I would venture…

Gear Oil Sumps- same thing- probably well diminished cooling on splash type- on a hydraulic type transmission or twin disk? Definitely a path toward major problems- not too mention seals, etc…

Thanks for the feedback. They’re cats on a tug, and I did check when in idle. Will look for manual info.

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