What is a lower unit reseal and when is it necessary?

A lower unit reseal involves replacing worn or damaged seals that protect the gearcase from water intrusion. It becomes necessary if water is found in the gear oil, seals are leaking, or after propeller shaft damage. Resealing helps prevent internal corrosion and costly gear failure.

How do you pressure test a lower unit?

We perform both pressure and vacuum testing using professional tools. This finds even small seal leaks around the prop shaft, drive shaft, shift shaft and drain screws. We test again after the work, before the unit is refilled and the boat goes back in the water.

What happens if I keep running my boat with water in the lower unit?

Running with water-contaminated gear oil can quickly destroy bearings and gears inside the lower unit. This often leads to overheating, metal damage and complete gearcase failure. Early resealing and testing can save the much higher cost of a gearcase rebuild or replacement.

Do you replace all seals or only the leaking ones?

In most cases we recommend replacing all the critical lower unit seals during a reseal: prop shaft, drive shaft and shift shaft seals. Replacing them all at once reduces the risk of future leaks and saves opening the unit again later.

How long does a lower unit reseal service take?

The labor usually takes 2-4 hours. When the job is done depends on parts and on getting the boat out of the water, and we give you a time with the written estimate.

Is pressure testing included after the reseal?

Yes. After resealing we pressure and vacuum test the unit again, before refilling it with gear oil, to confirm the new seals hold.

Do you provide warranty on lower unit reseal work?

Yes. Our workmanship is warranted for 90 days from the invoice date, and new seals (OEM or quality aftermarket) carry their manufacturer's warranty. Your invoice lists the work and the parts.

Marine mechanic using a hand-held pressure tester pump connected to the drain hole of a Mercury

Marine mechanic using a hand-held pressure tester pump connected to the drain hole of a Mercury (2)

Lower Unit Reseal & Pressure Testing

Water in your gear oil is a disaster waiting to happen. We perform professional vacuum and pressure testing to find leaks, replace worn prop shaft seals and test the unit again before it is refilled with gear oil.

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$150 /hr

Labor estimate Usually 2–4 hours

Book sends us a ticket; nothing is charged online. Before any work you get a written estimate for labor, parts and the trip fee, and nothing is done without your OK.

Trip fee By distance, one way: $30 in San Pedro, $50 in Long Beach

San Pedro$30
Wilmington, Terminal Island$40
Long Beach: Shoreline, Rainbow Harbor$50
Long Beach Marina, Alamitos Bay$60
Redondo Beach$60
Seal Beach, Huntington Harbour$75
Marina del Rey$85
Newport Beach$110

Farther away: $25 plus $2.50 a mile, one way from our San Pedro shop. No trip fee when the boat comes to our shop, or for plan members. The work of a visit is at least $150; the trip fee is on top.

Registered in California: Breeze Marine Boat Service LLC, San Pedro
Where we work: At your slip from Marina del Rey to Newport Beach, or in our San Pedro shop
Questions first: (213) 693-0224 or submit a ticket

About this service

Seal Integrity

Hold Pressure.
Keep Water Out.

Milky gear lube is the first sign of disaster. Water in lower unit gear oil rusts gears and destroys bearings. We use precision pressure and vacuum testing to identify failing seals before they cost you a lower unit.

Pressure & Vacuum

We don't just guess. For a lower unit pressure test, we drain the unit and pressurize it to 10-15 PSI to find leaks that let oil out. Then we pull a vacuum to find leaks that suck water in (cooling phase). If it holds both, the seals are sound.

Complete Reseal

When a leak is found, we perform a complete teardown, with the boat out of the water. We replace the prop shaft seals, drive shaft seals beneath the water pump, shift shaft seals, and all carrier O-rings with OEM quality parts.

Leak Detection

Using soapy water and UV dye, we pinpoint the exact failure point. Is it a fishing line cut on the prop shaft? A corroded shift seal? We identify the root cause so the new seals last.

The Reseal Procedure.

A rigorous defense against saltwater.

Drain & Pressure Test

We drain the old lube (checking for water/metal) and hook up the pressure tester. We inflate to 12 PSI and watch for a drop. A drop = a leak.

Disassembly

We pull the bearing carrier and water pump base. We inspect the shafts for grooving (which ruins new seals): light wear is polished out, and a grooved shaft gets a repair sleeve (Speedi-Sleeve) or is replaced.

Seal Installation

Using specialized drivers, we install new double-lip seals. We ensure the springs face the correct direction (oil side vs water side) for maximum protection.

Final Verification

Before adding oil, we test the unit again. It must hold pressure and vacuum for the time the maker specifies, with no drop. Only then do we fill it with the gear lube the maker specifies.

Sealed Tight.

Quality components make the difference.

Pressure test gauge attached to an outboard lower unit

New propeller shaft seals installed in a lower unit

Replacing the shift shaft seal on an outboard gearcase

Test Specs.

We follow factory standards.

Pressure Test 10-15 PSI (no drop, per maker's spec)

Vacuum Test 3-6 inHg (Checks water ingress)

Shaft Runout Checked to prevent seal wobble

Common Questions

What causes seals to fail?

The most common culprit is fishing line wrapped around the prop shaft, which cuts the rubber and makes prop shaft seal replacement necessary. Age, heat cycling, and sitting dry for long periods also cause the rubber to crack.

Can I just refill the oil and ignore it?

No. Water is heavier than oil and settles at the bottom, rusting the gears and bearings. A lower unit leak left alone usually ends in a gearcase failure, which costs far more than a reseal.