Gearcase · Gear Lube · Drain & Vent Seals · Prop Shaft Seal · Impeller
The lower unit is a sealed gearbox living underwater in salt. Its oil is the only thing between the gears and the Gulf, and the only way to know what is going on inside is to drain it and look. Here is what the service is, what the oil tells you, and how we do it where the boat sits.
The lower unit, the gearcase, holds the forward and reverse gears, the shift mechanism, the prop shaft and the water pump. It runs in a bath of gear oil behind three or four seals, all of them underwater. Yamaha and Mercury both schedule the gear oil change every 100 hours or once a year, with a first change at about 20 hours on a new Yamaha, and before storage.
The service is more than a drain and fill. The old oil is inspected for water and metal, the drain plug magnet is read, both plug gaskets are replaced, the gearcase is filled from the bottom until clean oil runs from the vent, and when water shows up the gearcase is pressure and vacuum tested to find the seal that let it in. The water pump impeller lives in the same housing: Yamaha checks it at 100 hours and replaces it at 300, Mercury replaces it at 300 hours or three years, sooner if the engine ever ran hot.
Why it matters: a prop shaft seal cut by fishing line lets a teaspoon of water in every trip. The oil turns milky, the bearings rust from the inside, and the first symptom most owners notice is a gearcase that costs more than the engine is worth to rebuild.
The lube and the amount are set by the gearcase, not the horsepower. We bring what the manual names for your serial, and the amount is never quoted from a chart.
Both makers put the gear lube on the 100-hour or once-a-year list, whichever comes first, and before storage.
This is why the oil is drained into a clear container and read, not dumped into a pan.
Milky gear oil, a prop that grinds or whines, or a lower unit that drips oil at the dock: do not run the boat until it is looked at. Water in the gearcase does its damage while the boat sits: the bearings rust between trips. A seal is a modest repair; a gearcase is not. If the oil came out milky at the last service and nobody tested the seals, that is the first thing we do.
How often should lower unit oil be changed?
Every 100 hours or once a year, whichever comes first, and before storage. A new Yamaha gets its first change at about 20 hours.
What gear oil goes in a lower unit?
Yamaha: Yamalube Marine Gearcase Lube on most engines, and Yamaha's performance gear lube on the V8, the V6 V MAX SHO and the 4.2L and 4.3L V6 Offshore, where the standard lube is not approved. Mercury: the gear lube Mercury names for your engine family. The serial number settles it before we arrive.
Why is my lower unit oil milky?
Water got in, almost always past a seal: a worn drain or vent washer, a prop shaft seal cut by fishing line, or the shift shaft seal. New oil alone does not fix it; the leak is found with a pressure and vacuum test.
Is metal in the gear oil normal?
A fine gray paste on the drain plug magnet is normal wear. Shiny flakes need a closer look. Chunks mean stop running the engine.
How much gear oil does an outboard hold?
It depends on the gearcase, not the horsepower, so we do not quote a number from a chart. The lube goes in from the bottom until clean oil reaches the vent, and the manual for your serial is the reference.
When does the water pump impeller get replaced?
Yamaha: checked at 100 hours, replaced at 300 hours or three years. Mercury: replaced at 300 hours or three years. Either one sooner if the engine ran hot or the telltale stream has weakened.
Is this part of the Premium Full Service?
Yes. Gear oil, both gaskets, the oil inspection and the water pump kit when due are on the list, parts included, one price per engine. A lower unit that shows water gets the pressure test and a written quote for the seal before any work starts.
Can it be done at the dock or on the lift?
Yes. The engine goes down into the water-off position on the lift, or the boat is pulled onto the lift or trailer for the water pump. We bring the pump, the lube and the gaskets for your serial.
We come to the boat with the gear lube your maker specifies, new drain and vent gaskets for your serial, the fill pump and the test gear. The old oil is read before it leaves with us, the prop comes off to check the shaft, and if water shows the gearcase is pressure tested and you get a written quote for the seal before anything else is done. The water pump is replaced when the schedule or the telltale calls for it.
What it costs: this job is part of the Premium Full Service — the complete outboard service, parts included, one price per engine, in writing before we come. The serial number sets the price, so text us a photo of the plate. Nothing is added at the dock; sales tax is its own line at your county's rate.
Something wrong, not just due? That is a $250 diagnostic visit at the boat in regular hours ($295 after hours, Sundays and holidays): we find the actual cause and quote parts and labor before any work starts.
Dock, lift, marina, driveway or storage lot — we come to the boat across Southwest Florida:
Text us the engine model, the year and where the boat sits, and a photo of the oil if you have one. You get a straight answer, one price per engine for the service, and a day on the calendar.
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More guides: Lower unit leaking oil · Boat won't go into gear · Overheating alarm · Yamaha oil change · Mercury & Verado oil change · The 100 / 300 / 500-hour service explained · The Premium Full Service