Marine gearbox rebuild cost

Gearboxes rarely fail without warning. They slip, they engage slowly, their oil goes milky or dark, or they get noisy — and they are usually run for months in that state before anyone looks.

Rebuild, repair or exchange

Adjustment or external repair. Sometimes the fault is a control cable not reaching full travel, a low oil level, or a leaking cooler. These are cheap and should be ruled out before anything is dismantled.

Rebuild. Box out, stripped, clutch packs, bearings, seals and gaskets replaced, shafts and gears inspected, reassembled and set up.

Exchange unit. A reconditioned gearbox swapped for yours. On common models this is often faster and sometimes cheaper than a rebuild, and it comes with a warranty.

Replacement. A new box, usually only where the model is obsolete or the damage is catastrophic.

Establish which one you are being quoted.

Cost drivers

Removal and refitting. The gearbox is behind the engine, connected to the shaft. Getting it out can mean moving the engine forward, disconnecting the shaft coupling, and working in the tightest part of the boat.

Whether the shaft has to be moved. On some installations the shaft must be pushed aft or drawn to clear the coupling, which can mean a haul-out.

What is worn. Clutch plates and seals are one price. Bearings and gears are another. A failed thrust bearing that has damaged the housing is another again.

Parts availability. Older or less common gearboxes are the constraint.

Cause of failure. If the box failed because of misalignment, fixing the box without fixing the alignment buys a repeat.

Cooler. A gearbox oil cooler in the raw water circuit that has corroded and let seawater in is a separate part and must be replaced, not just flushed.

The alignment point

Say this to whoever quotes: what caused it?

The most common non-wear cause of marine gearbox failure is a shaft that is not aligned to the box. Symptoms appear elsewhere first — vibration, a stern gland that will not settle, coupling bolts loosening, sagging mounts.

A rebuild without an alignment check afterwards is an incomplete job.

Do it while the engine is out

If you are already committed to an engine rebuild or repower and the gearbox has any symptoms at all, doing both at once saves the entire removal cost of the second job. This is one of the few genuine savings available in marine engineering.

Frequently asked questions

Why is my gearbox oil milky? Water is getting in, most commonly through a corroded gearbox oil cooler in the raw water circuit. Stop running it and have the cooler pressure tested.

My gearbox is slow to engage. Is it rebuild time? Not necessarily. Low oil, the wrong oil, a control cable short of full travel, or an idle too low to build pressure are all cheaper causes. Rule them out first.

Is an exchange gearbox better than a rebuild? On common models it is often faster and comes with a warranty. On uncommon ones a rebuild may be the only route.

What does a rebuild include? It should include clutch packs, bearings, seals and gaskets, with shafts and gears inspected and replaced as needed. Ask for the specification.

Will my gearbox fail again? If misalignment caused it and the alignment is not corrected, yes. Insist on an alignment check as part of the job.