Outboard Motor Lower Unit Repair
Milky gear oil on the drain plug means water is getting past a seal, and outboard motor lower unit repair done now costs a fraction of the rebuild you'll need if you run it another season.
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⭐ X.X from N reviews · Done at your boat · Published rates
Milky gear oil — what it actually means
Pull the drain plug and what comes out tells you most of what you need to know.
Clean and honey-coloured — the lower unit is sealed and healthy.
Milky, grey or the colour of coffee with milk — water has got in and emulsified with the oil. A seal has failed. The gears are now running in something that doesn't lubricate properly and does corrode.
Metallic glitter in it — not just cloudy but sparkling. That's gear or bearing material, and the repair has already moved from seals to internals.
Almost empty — oil has been leaking out, which means water has been getting in.
Why this is urgent rather than annoying: water in a gearcase corrodes bearings and gear teeth continuously, not just when running. And in this climate, water left in a lower unit over winter freezes, expands, and cracks the housing — turning a seal job into a replacement gearcase.
Where the water gets in
The prop shaft seal — and usually because of fishing line.
This is the single most common cause we see, and if you fish, it's almost certainly the one that will get you.
Line wraps behind the propeller. The prop keeps turning and starts to eat it, drawing it into the seal behind the hub, and the seal gives up. You will not know it has happened, because all of it is underwater and none of it changes how the boat feels. By the time there's a symptom, there's water in the gearcase and the lower unit is already damaged.
The prevention takes two minutes and nobody does it. Pull the prop off periodically, clear anything wrapped behind it, look at the seal, and regrease the shaft before it goes back on. We do this as standard during oil changes — it's the only reliable way to catch a seal before it fails rather than after.
If your boat fishes and the prop has never been off, that's where we'd start.
The driveshaft seal at the top of the gearcase.
The shift shaft seal, where the linkage passes through the housing.
The drain and fill plug washers. Cheap, single-use, and routinely reused until they leak.
A cracked housing — from a strike, or from water that froze in there over a winter.
How we find it, rather than guess
Pressure and vacuum testing. The gearcase is sealed and pressurised, and we watch whether it holds. A drop tells us there's a leak; where it drops from tells us which seal.
This matters because the alternative is replacing seals in sequence until the symptom stops — which costs more, takes longer, and still doesn't confirm the job is done. A gearcase that holds pressure after the repair is proof, not hope.
Gear oil service
The routine version of this job, and the cheapest insurance on an outboard.
What's involved: drain from the lower plug, inspect what comes out, refill from the bottom until clean oil appears at the vent, fit new washers, check level after it settles.
When: at every annual service at minimum, and always before winter storage — you do not want water sitting in a gearcase through a freeze.
What we tell you: what the old oil looked like. That's the actual value of the service — it's a scheduled inspection that happens to include an oil change.
Done at your boat, in the water, no haul-out. How on-site service works.
Lower unit repair and rebuild
When the oil comes out wrong, the work depends on how far it went.
Seals only — caught early, with clean-running gears and good bearings. The gearcase comes off, seals are replaced, it's pressure tested and refilled.
Seals, bearings and shims — where water has been in there a while. Bearings corrode first and get noisy or rough before the gears do.
Gears — pinion, forward and reverse gears and the clutch dog. Usually the result of running a long time on contaminated oil, or of hard shifting.
A cracked or frozen-damaged housing — at which point we'll talk honestly about a replacement gearcase instead.
While the unit is off, the water pump is right there and worth doing at the same time — the labour overlaps almost entirely.
Shifting problems
Hard to shift, popping out of gear, or won't engage at all — the cause sits somewhere between the control at the helm and the clutch dog in the gearcase.
Some of it is adjustment and cheap. Some of it is a worn clutch dog, which means the unit comes apart.
Full diagnosis: outboard won't shift into gear →
Rebuild or replace?
Worth an honest answer rather than a default.
Rebuild makes sense when the housing is sound and the damage is seals and bearings.
A replacement gearcase often wins when gears are damaged, the housing is cracked, or the motor is older and good used units are available. On some models a serviceable used lower unit costs less than a full rebuild — and we'll tell you when that's the case rather than quoting the bigger job.
The comparison depends on your motor, what's actually wrong, and what parts are available. We do the compare before you commit. Same principle as rebuild versus repower on the powerhead side.
What it costs
Gear oil service is a published flat rate. Lower unit repair is quoted after diagnosis, because "milky oil" covers everything from a $X seal job to a replacement gearcase. See pricing.
Cheapest possible version of this page's advice: change the gear oil on schedule and look at what comes out. Most of the expensive repairs here started as a seal that leaked for a season without anyone noticing.
Frequently asked questions
What does milky gear oil mean on an outboard?
Water has got past a seal and emulsified with the oil. It needs attention before the next trip — water corrodes bearings and gears continuously, and if it freezes over winter it can crack the housing.
Can I still use the boat with milky gear oil?
Briefly, and reluctantly. Every hour makes the repair larger. If the oil is also metallic, stop.
How often should outboard gear oil be changed?
At least annually, with yearly service, and always before winter storage.
What causes a lower unit seal to fail?
Age, and fishing line wrapped behind the prop cutting the prop shaft seal. The second one is far more common than most owners realise and takes thirty seconds to check.
Can gear oil be changed with the boat in the water?
Yes. Drain, fill and inspection all happen with the boat in its slip.
Is it worth rebuilding a lower unit or should I replace it?
Depends on the housing and the gears. Seals and bearings usually favour a rebuild; damaged gears or a cracked housing often favour a good used unit. We compare both before quoting.
There's metal in my gear oil. How bad is that?
Serious. Cloudy oil means a seal; glitter means something inside is wearing away. Stop running it and get it looked at.
Do you do this at my boat?
Yes — gear oil service and diagnosis on site. A full internal rebuild means the unit comes off and goes to the bench, but the boat stays where it is.
Book lower unit service
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