What does it cost to rebuild a Mercury outboard?

For a professional powerhead rebuild in the UK, budget roughly £1,500 to £4,000 for a smaller Mercury outboard and £4,000 to £10,000 or more for a larger or more complex engine. Before authorising dismantling, confirm the model, serial number and cause of failure, because a repairable cylinder fault and a damaged crankshaft lead to very different bills.

Check what needs rebuilding

Start with the serial number on the transom bracket and a written diagnosis. Horsepower alone tells you too little: a Mercury 90 two-stroke, 90 FourStroke and 90 OptiMax have different parts, fuel systems and repair costs, and their production periods vary.

Ask for compression readings on every cylinder, followed by appropriate leak-down testing and a borescope inspection. One low reading does not automatically mean the whole powerhead needs rebuilding. On a four-stroke, leaking valves or a head-gasket fault may explain it.

Have the workshop inspect the gear oil, cooling system and general corrosion too. Even a sound powerhead repair offers poor value if the gearcase also needs major work.

Disconnect the battery and isolate the fuel supply before dismantling. Use a rated lifting arrangement to remove the powerhead.

Rebuild costs by engine type

These are broad budgets for professional work, not fixed quotations. Location, VAT, parts availability and the amount of machining can change the total considerably.

• Small portable engines, roughly 2.5 to 30hp: around £800 to £2,500 for substantial internal repairs or a rebuild. At the smallest sizes, replacement often makes better financial sense.
• Conventional two-strokes, roughly 40 to 90hp: budget £1,500 to £3,500 where the block and crankshaft are recoverable.
• Larger two-strokes, including V6 engines: allow approximately £3,000 to £6,000. Extensive cylinder damage or crankshaft replacement can push the bill higher.
• FourStroke engines, roughly 40 to 115hp: allow about £2,500 to £5,500, depending on the condition of the crankshaft, cylinder head and valve train.
• Larger FourStroke and Verado engines: use £5,000 to £10,000 or more as a starting budget while investigating the repair. An exchange powerhead may be more practical.

OptiMax direct-injection equipment can add separate costs, as can the compressor and associated systems on supercharged Verado models. A powerhead rebuild does not automatically renew these components.

What the quote should cover

A rebuild normally covers the engine assembly above the exhaust housing. It does not necessarily include the lower unit, trim system, starter, charging equipment or fuel injection.