Which one you need is decided by what comes out of the drain plug and what the pressure test says. Do not run the engine on emulsified oil. Water in a gearcase rusts bearing races, and in a covered Florida slip in August that starts inside a week.
How to read what comes out of the drain plug
Pull the drain screw and catch the first ounce separately — water is heavier than gear lube, so it comes out first.
| What you see | What it means | What to do |
|---|---|---|
| Clean amber or dark oil, normal smell | Healthy | Service on schedule |
| Faint haze or cloudiness | Early moisture — a seal starting, or a reused drain washer | Seal kit and pressure test now |
| Milky tan, like coffee with cream | Emulsified water, meaningful quantity | Stop running it. Seal service, pressure and vacuum tested |
| Chocolate milk, or water out before oil | Heavy intrusion, running that way a while | Do not run it. Inspect bearings for rust |
| Fine gray paste on the magnetic plug | Normal wear metal | Service normally, note it in the record |
| Shiny flakes or chunks on the plug | Gear or bearing material | Rebuild territory, whatever the oil looks like |
How water gets in
Prop shaft seal, cut by fishing line. The most common cause on a fishing boat. Monofilament wraps behind the prop, works into the seal area and cuts a groove in the lip and sometimes the shaft. It is silent, and it happens to careful people. Pull the prop and look behind it — every time.
Driveshaft seal at the top of the case, next to the water pump and replaced as part of a proper water pump job, along with the shift shaft seal and O-ring.
The drain and fill washers. A reused fifty-cent crush washer is a frequent cause of a wet gearcase — replaced on every service here.
Case damage. A rock, a bunk or a piling can crack a case or distort a seal bore; corrosion pitting does the same more slowly, and boats in wet slips like Palm Harbor Marina get there before lift-kept ones.
Pressure tested before and after — and why both
Every lower unit job is tested before it is opened and again after it is closed. The before-test localizes the leak; the after-test proves the repair, and the reading goes in the service record with a photograph.
Seal service or rebuild? What decides it
The seal service replaces the prop shaft carrier seals, driveshaft seal, shift shaft seal, O-rings and washers, refills with OEM lubricant, and re-tests. It fixes the water problem — not damage the water already did, and not worn gears. You are into a rebuild when there is metal on the plug beyond fine gray paste, when the prop shaft turns rough by hand in neutral, when gear whine changes with load, when the unit jumps out of gear, or when water sat long enough to pit the bearing races. Plenty of gearcases opened for a seal kit need bearings too — hence a band, not a number: clutch dog and bearings at the bottom, a full set of gears in a large case at the top. Written estimate first, call before any overrun beyond 10%.
Won't shift into gear
Water intrusion and shifting faults often appear together but are separate problems. Before condemning a gearcase, the shift system gets checked in this order.
Shift cable and adjustment. Saltwater corrodes cable ends and the barrel adjuster at the engine, and a stretched or seized cable delivers less travel than the mechanism needs — the unit will not fully engage, or will not come out of gear. Cheap fix, checked first. Corroded linkage, a worn shift shaft spline or fishing line on the prop shaft feel the same.
Clutch dog wear. Shifting at high RPM rounds the square corners on the clutch dog and gear ears. Once rounded, the dog rides up the ramps under load and pops out of gear — usually in reverse first. No adjustment fixes a rounded dog.
Shift interrupt circuit. Some engines briefly interrupt spark during a shift to unload the dogs. When that circuit fails you get hard, grinding shifts on a healthy gearcase — an electrical fault dressed as a mechanical one, and a reason to run a standard diagnostic first.
Repair, rebuild, or replace
| Situation | Best move | Range |
|---|
When a rebuild approaches the cost of a sound used or remanufactured gearcase, you get both numbers in writing and you choose.
Questions people actually ask
Can a lower unit be serviced in the water? The seal kit and gear oil service is done at the boat, in the slip or on the lift. A full rebuild is not — the gearcase comes off to a bench, because gears, bearings and shims need clean conditions and measurement. You learn which category your unit is in after the drain-plug inspection and pressure test.
What proof do I get that the repair actually sealed? The after-test reading and a photo of it, plus part numbers, hours and before-and-after photos of the seals and oil, in the digital service record — emailed and texted before we leave the dock. That record stays with the boat, which matters when roughly 78–80% of US boat sales are pre-owned.