Mercruiser 5.7L MPI Rebuild, common faults and everything you need to know
Marine version of the Chevrolet small block 350. The 350 MAG MPI launched in 1998 and was discontinued in September 2015 when Mercury replaced it with the 6.2L V8.
The 5.7L MPI is the marine sibling of the world's most-produced V8. Spare parts for the block itself are cheap through the American speed shop market, but the marine-specific components - manifolds, risers, water pump, the MerCathode system - are more expensive and have to be genuine. The big failure is exhaust manifolds and risers that corrode from the inside; as a rule they are replaced every 5-7 seasons in saltwater. When that is missed, saltwater ends up straight in the cylinders, and hydraulic lock on start-up can destroy the engine. We carry out cylinder head reconditioning, cylinder boring, crankshaft reconditioning, complete engine reconditioning and V8-specialised reconditioning in our own machine shop with 12-month warranty. See also our boat engine rebuild hub page.
The Mercruiser 5.7L at a glance
Drawn from Mercury Marine's official specifications, BoatTest engine reviews, the PerfProTech product database and the Michigan Motorz catalogue.
5.7L MPI variants
| Variant | Power | Drive | Period |
|---|---|---|---|
| 5.7L MPI (base) | 250 hk | Alpha | 1998-2014 |
| 350 MAG MPI | 300 hk | Bravo | 1998-2015 |
| 350 MAG MPI Horizon (inboard) | 300 hk | V-drive / direct | 1998-2015 |
| Tow Sport MPI | 315 hk | Alpha One | ~2005-2015 |
NOTE - discontinued production: Mercury Marine replaced the 5.7L MPI / 350 MAG MPI with the 6.2L V8 in September 2015. Mercury's current sterndrive lineup consists of the 4.5L, 6.2L and 8.2L. Aftermarket blocks (DART and others) are still available for reconditioning. The 260 hp engine is the 5.0L MPI (not the 5.7L); the 320 hp engine is the 6.2L 377 MAG MPI.
Common faults on the 5.7L MPI
Cross-checked against YBW Forum, Club Sea Ray, PowerBoatSupply, Yacht Survey (David Pascoe) and the Mercury Service Manual.
Exhaust manifolds and risers corrode
5-7 seasons • High- Symptoms
- White/black smoke, heavily sooted spark plugs, poor compression, saltwater under the engine.
- Cause
- Manifolds and risers have water cooling passages. Saltwater erodes them from the inside; as a rule they are replaced every 5-7 seasons (Yacht Survey/David Pascoe). Missed service leads to water entering the cylinders.
Water breakthrough into the cylinders - hydraulic lock
Consequence of manifold failure • Critical- Symptoms
- Engine refuses to start after a period of standing, water in the oil, possibly bent connecting rods.
- Cause
- Saltwater enters the cylinders from corroded manifolds or risers. On start-up the engine hydraulic-locks.
Transom assembly corrodes around the pivot pin
The sterndrive's main failure • High- Symptoms
- Stiff steering feel, leakage from the transom, corroded surface around the pivot pin.
- Cause
- Galvanic corrosion between the steel pivot pin and the aluminium transom. It worsens when the MerCathode system is not working properly.
Circulation pump and thermostat age
Common at 1,500+ hours • Medium- Symptoms
- Overheating, alarms from the instrument panel, reduced power under load.
- Cause
- The circulation pump bearing wears, the thermostat sticks or opens late. Thermostat replacement is recommended every two years.
MerCathode system is neglected
Galvanic damage • High- Symptoms
- The sterndrive corrodes despite zinc anodes, transom damage, the propeller pits.
- Cause
- MerCathode is an active protection system for the sterndrive. When it fails (wiring faults, corroded connectors, a flat battery), the zinc anodes no longer direct the galvanic damage correctly and sterndrive corrosion accelerates. It needs an annual test.
5.7L MPI reconditioning prices
V8 engine with 8 cylinders and 2 cylinder heads. Based on 5.7L MPI specifications. Excl. VAT and parts. For V8-specific service see V8 reconditioning.
Cylinder head reconditioning
Bottom end
Full rebuild
Manifolds + risers
Cylinder machining V8
| Service | Price |
|---|---|
| Cylinder boring 8 cyl (101.6 mm) | 6 116 kr |
| Plateau honing 8 cyl | 1 925 kr |
| O-ringing V8 block | 4 200 kr |
Crankshaft work V8
| Service | Price |
|---|---|
| Crankshaft grinding V8 (5 main + 4 rod) | 4 540 kr |
| Balancing rotating assembly V8 | 5 220 kr |
| Cleaning V8 crankshaft | 1 568 kr |
Cylinder head V8 (each)
| Service | Price |
|---|---|
| Resurfacing cast iron cylinder head V8 | 1 590 kr |
| Pressure testing cylinder head V8 | 2 100 kr |
| Valve seat cutting 8 valves | 910 kr |
| Valve guide replacement 8 pcs | 1 580 kr |
See also
Other V8 engines and related services.
Questions and answers about the 5.7L MPI
Is the 5.7L MPI still in production?
No. Mercury Marine replaced the 5.7L MPI / 350 MAG MPI with the 6.2L V8 in September 2015. Mercury's current sterndrive lineup consists of the 4.5L, 6.2L and 8.2L. Aftermarket blocks (DART and others) are still available for reconditioning.
What is the power difference between the variants?
5.7L base: 250 hp (Alpha drive). 350 MAG MPI: 300 hp (Bravo). 350 MAG MPI Horizon (inboard): 300 hp. Tow Sport MPI: 315 hp. The 260 hp engine is the 5.0L MPI, not the 5.7L. The 320 hp engine is the 6.2L 377 MAG MPI, not the 5.7L.
What does it cost to recondition a 5.7L MPI?
Cylinder head reconditioning SEK 12,000-16,000 per head (the engine has two). Complete engine reconditioning SEK 38,000-65,000 for labour, plus parts SEK 12,000-25,000. Manifolds and risers often add SEK 8,000-18,000.
How often do manifolds and risers need replacing?
Every 5-7 seasons in saltwater (Yacht Survey data). In freshwater they can last 10+ years. At annual winter lay-up, endoscopy should be done to catch internal corrosion early.
Are aftermarket blocks as good as GM originals?
For many use cases, yes. DART blocks have thicker cylinder walls and take more power than a standard GM. For standard pleasure boating, reconditioned GM blocks are enough. For performance builds, DART or similar is justified.
What is the MerCathode system?
Mercury's active galvanic protection system for the sterndrive. It creates a counter-galvanic potential that directs the corrosion damage to the zinc anodes instead of the sterndrive. If it fails (wiring faults, corroded connectors), the sterndrive suffers rapid galvanic corrosion. It needs an annual test.
How long does a 5.7L MPI reconditioning take?
Cylinder head reconditioning (both): 2-3 weeks. Complete engine reconditioning: 4-6 weeks in the workshop. V8 engines have 8 cylinders, 2 cylinder heads and more components than 4-cylinder engines.
Do you have a Mercruiser 5.7L that needs looking over?
Call us or send a quote request. Reply within 24 hours. We service all types of boat engines - see boat engine page. All rebuilds with 12-month warranty.