Tohatsu gearcase and drivetrain: why a pressure test is the smartest maintenance step
A gearcase pressure and vacuum test can spot leaking seals before water destroys bearings and gears. Here is when to do it, what it tells you, and the key outboard drivetrain parts to understand.
The lower unit on a Tohatsu outboard is a sealed gearbox full of oil. If a seal starts leaking, water gets in, oil emulsifies (milky), bearings corrode, and gears can fail. A simple pressure and vacuum test is the fastest way to confirm the gearcase is watertight before damage happens.
When to pressure test (recommended):
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After prop strikes, rope/fishing line around the prop, or grounding.
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If gear oil looks milky, grey, or smells burnt.
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When you replace seals, water pump parts, or do any gearcase work.
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At least annually if the engine lives in the water or sees heavy use.
What the test tells you:
A good gearcase should hold pressure and vacuum without dropping. If it leaks, you then locate the leak using soapy water around likely points.
Common leak points on outboards:
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Prop shaft seals (often damaged by fishing line or a nicked shaft).
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Drive shaft seal (top of the gearcase, especially after impacts).
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Shift shaft seal (small seal, easy to overlook).
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Drain and vent screw seals (crushed or reused washers).
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Housing or carrier damage after a strike.
Drivetrain basics (what is inside):
The drive shaft turns a pinion gear, which meshes with forward and reverse gears on the prop shaft. Bearings carry load, and seals keep the oil in. Keeping that oil clean and the case sealed is what keeps the entire drivetrain alive.
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