Should You Repower an Old Sterndrive Boat? - 11/15/2018

At some point, a boatowner is going to have to face a choice: Does he buy a new boat to replace his aging vessel or, if he likes the layout and features of his current boat, does he consider repowering it with a new or remanufactured engine?

For owners of sterndrive boats, the decision can be a tricky one – for example, upgrading the engine with one with more horsepower. A big step-up in horsepower would require different gearing in the drive and a new prop at the least. In some situations, a new drive would also be required. That’s not to say that it wouldn’t be worth it, but that call is up to the individual boat owner.

We’ve seen repower projects where a gas engine was replaced with a diesel, but for the sake of this report, we’ll stick with straight gas for gas swaps.

Should You Repower an Old Sterndrive Boat engines
With remanufactured engines and drives more readily available, repowering an old sterndrive with a new one can be a smart alternative.

When and Why Repower?

These are the two toughest questions to answer when it comes to repowering a sterndrive. Most mechanics say that a properly maintained gasoline sterndrive should run 1,000 hours without major issues. Repowering is recommended when an aging engine is starting to show signs of being on its “last legs,” such as longer cranking times, increased fuel and oil consumption, and frequent “little fixes” that take time away from being on the water – or when everything seems to be going at once.

That takes care of the “when.” The “why” is more of a case-by-case basis. If the boat is still in good condition and the owner likes the features, repowering is less expensive than buying a new boat, or even down-sizing. Often, repowering replaces a carbureted engine with a more modern fuel-injected version that improves performance and fuel economy – and is almost always easy to start. Plus, there is now a new-engine warranty.

Should You Repower an Old Sterndrive Boat alpha
The 350 MAG-Alpha from Mercury Remanufacturing is a popular choice for repowering because it has a multi-port fuel injection and more modern engine control module.

Rebuild vs. Repower

It’s important to know the difference between these two terms. In a “rebuild,” the engine in a given boat is taken apart and put back together with new parts. It can be less expensive. The problem is that most repair shops are not likely to tie up a shop bay and a technician during the middle of a busy summer on a rebuild.

The solution is to buy an engine already rebuilt, such as a Mercury Reman engine. All of the Mercury Reman parts are as good as new, some are actually new, and some are even better in the reman engines than the off-the-shelf new engines.

Crate or Long Block?

The next step up is to purchase a “crate” engine from a local aftermarket facility and re-use components such as the exhaust, carburetor/fuel injector and intake manifold from the old motor to complete it. The key with a crate motor is to make sure that it is a marine engine that is up to date with U.S. Coast Guard Safety Standards.

To ensure that he’s getting a product designed for marine use, an owner could go with a Pro Series Remanufactured Long Block from Mercury Remanufacturing. It features hypereutectic pistons that are stronger than factory models, new marine valves and springs, cylinder heads and a water pump specified for marine use, and a new camshaft. The long block is equipped with an oil pan and windage tray as well as a new high-flow oil pump.

Mercury’s Quicksilver division is also offering completely new long block engines.

Should You Repower an Old Sterndrive Boat long block
A long block such as this model from Mercury Remanufacturing basically comes new with everything except the fuel delivery system and exhaust.

Certified Pre-Owned

Mercury Remanufacturing offers two types of “pre-owned” engines: Factory Certified Pre-Owned (CPO) engines and completely rebuilt motors. The certified pre-owned engines offer a low-cost way to replace or upgrade. Mercury Remanufacturing takes in these engines and goes through them with a checklist that meet the company’s criteria for Mercury Factory Certified Pre-Owned.

It should be noted that a CPO engine will show some evidence of use such as scratches and cosmetic corrosion. These motors are available as direct replacements from four-cylinder models up to V-8s so check with your dealer for availability. Factory Certified Pre-Owned engines come with a one-year factory warranty and up to two years of optional coverage is available.

Remanufactured Engines – Are Full of New Parts

Mercury Remanufacturing has a full line of rebuilt motors including a number of models that aren’t available in the “new” Mercury MerCruiser series of new sterndrive packages. A remanufactured model is re-built with a CNC-machined block that has freshly honed cylinders and deck machining for clean surfaces. New cylinder heads include new valves and springs and the engine gets a new marine water pump and a new camshaft. The oil pan, oil pump, and timing chain cover are—you guessed it—new.

Big Power

Now let’s take a look at the repower options starting with the most powerful. The new offerings are the MerCruiser 8.2 L MAG and H.O., which put out 380 and 430-hp, respectively. If someone is looking for more torque and bottom-end grunt, the Mercury Remanufacturing 540 MAG Bravo 4V is an 8.9 L (540-cid) carbureted engine that makes 440-hp.

Also from the remanufacturing division is the 496 H.O. Bravo that puts out 425 hp from an 8.1 L block that is equipped with multi-port fuel injection. Taking a look at the weights of these three engines, the 8.2 MAG H.O. weighs 1,122 pounds (509 kg), while the 540 MAG Bravo 4V 8.9 L engines are 1,025 pounds (465 kg) and the 496 H.O. Bravo tips the scales at 1,131 lbs. (513 kg). All three engines are compatible with a variety of Bravo drives.

Volvo Penta has the V8-430, a 430-hp big block that can be paired with a Duoprop drive. Consumers should contact their local Volvo Penta dealer for more information.

Mercury Reman and Quicksilver engines are handled through factory-authorized local dealers who work directly with the factory.

Should You Repower an Old Sterndrive Boat mag
A carbureted large-displacement motor, the 540-cid Mercury Remanufacturing 540 MAG, could be a good choice for a classic boat.

The Small Blocks – Mercury Marine

Mercury Quicksilver will soon be offering the 3.0L as a 100% new engine that Mercury has tooled and plans to offer as a long block, crate engine, and complete engine. The long block can also be used to service the GM LW6 industrial propane and natural gas engines because it uses hardened seats and Mercury’s exclusive Black-X valves for longer life.

Mercury Reman plans to have something available to repower most of the legacy cast iron in recreational boating. However, their recommendations may not be what some people expect. For example: for 5.0 L models it recommends 5.7 L type products and 7.4L. It highly suggests the 383, 496, and 540 products. This isn’t so much an issue with the engine itself as it is the marinization parts availability and delivering the best quality and value for a repower, according to a company spokesman.

Mercury’s MerCruiser division offers its 6.2-liter V-8 in 300 and 350-hp catalyzed versions. It has multi-port fuel injection and is available with SmartCraft instrumentation and digital controls.

Mercury Remanufacturing has three models that would be considered small blocks. The 350 MAG MPI has a storied history and makes 300-hp with a 5.8-liter (355-cid) block. It has multi-port fuel injection, electronic control module and a Gen II Cool Fuel system. Next up, the 357 MAG Bravo 4V makes 325-hp on a 5.8-liter (357-cid block). Fuel is delivered via a Weber four-barrel carburetor.

Finally, the Mercury Remanufacturing 383 MAG Stroker Sterndrive is a 350-hp model with a 6.3-liter (383-cid) block, multi-port fuel injection and the same ECM as the 350 MAG MPI. The 6.2L weighs 862 lbs. (391 kg) while the 350 MAG MPI and the 383 MAG Stroker Sterndrive check in at 932 lbs. (423 kg), the 357 MAG Bravo 4V is 871 lbs. (395 kg).

The 6.2L, 357 MAG Bravo 4V and 383 MAG Stroker Sterndrive are all compatible with the full Bravo series of drives while the 350 MAG MPI comes with an Alpha drive.

Should You Repower an Old Sterndrive Boat mercruiser
Popular in boats up to 22’ to 23’ long, the MerCruiser 6.2L is available in 300 and 350-hp versions.
Should You Repower an Old Sterndrive Boat stroker
One of the most popular small blocks from Mercury Remanufacturing is the 350-hp 383 MAG Stroker Sterndrive.

Volvo Penta V-8s

Volvo Penta has its widest range of power options in the small-block V-8 range. The V8-380 C-B makes 380-hp at 6000 rpm and has multi-port fuel injection. It is most often paired with a DuoProp drive. The V8-320-E-A is based on a 5.7-liter (350-cid) block and it makes 320-hp at the propshaft. It’s similar to the V8-350-C that is rated 350-hp at 5800 rpm. Volvo also makes a 320-hp engine at 350 cid and a 300-hp model at 323 cid. Finally, Volvo Penta’s 5.0-liter offering is the V8-270-C-A that is based on a 305-cid GM 5.0-liter block.

Local Volvo Penta dealers should be contacted for more information.

Team Six

Six-cylinder engines remain an important and valuable power source for runabouts and deckboats in the 20’ and under range. For decades, the 4.3-liter V-6, which is essentially a 350-cid GM block with two cylinders cut off, was the foundation for offerings that ranged from 190 to 220-hp from MerCruiser and Volvo Penta.

Today, Mercury MerCruiser's V-6 offering is a 4.5-liter engine in 200- and 250-hp models. Mercury Remanufacturing’s 4.3L MPI ECT is offered in 180 and 220-hp versions, and all three engines have the same dimensions, 28” (69.6 cm) x 30” (74 cm) x 22” (55.9 cm). The 4.3-liter engine weighs 775 lbs. (352 kg) while the 4.5-liter weighs 761 lbs. (345 kg). Both are available with a variety of Alpha and Bravo drives.

From Volvo, the V6-280-C makes 280 hp on a 4.3-liter block at 5800 rpm. A V6-240-G is based on the same 4.3-liter block and it makes 240-hp. Volvo Penta also makes the V6-240-G and the V6-200-G on the standard 4.3-liter (262-cid) block.

Should You Repower an Old Sterndrive Boat replacement
The MerCruiser 4.5-liter V-6 replaced the venerable 4.3-liter model that had been in the mode line for decades. Mercury makes this engine itself and does not source the blocks from GM.

The 3.0 L MerCruiser Engine

Mercury Quicksilver will soon offer an all-new 3.0-liter four-cylinder model that puts out 135-hp. The carbureted engine has auto enrichment—better known as automatic choke—for turn-key starting and the engine alone weighs 635 pounds (288 kg). It will be paired with an Alpha drive.

The Drives

Regardless of whether an owner goes with a remanufactured engine or a new one, the drive options are the same. He can choose from an Alpha One, A Bravo One, a Bravo Two or a Bravo III from MerCruiser. It’s easy to cut a bigger hole in the transom to step up from an Alpha to any of the Bravos.

If the re-power project is a Volvo Penta, an owner can choose from the single prop SX drive or the twin-prop DuoProp.

Should You Repower an Old Sterndrive Boat bravo one
The Bravo One has been a workhorse in the MerCruiser drive line for decades, pushing cruisers and offshore performance models with equal efficiency.

Control Freaks. With new Mercury MerCruiser and Volvo Penta repower installations, an owner gets digital throttle and shift on engines above 305 hp. Even with conventional installations, it would be wise to upgrade the cables and controls simply because they would operate more smoothly than a 10- or 15-year-old system. We would recommend replacing the steering as well.

Should You Repower an Old Sterndrive Boat merc monitor
Mercury’s MercMonitor is a multi-function gauge that puts plenty of information on one instrument.


Taking a look at the pros and cons of new Mercury MerCruiser models and those from Mercury Remanufacturing, first there are differences in the warranty. Mercury Remanufacturing engines have a one-year warranty. MerCruiser models have a one-year warranty plus three years against corrosion.

Contact a local Volvo Penta dealer about its warranty policy.


Repowering a sterndrive-powered boat has its own unique challenges. A boatowner must make decisions based on the engine and the drive as well as the overall performance of the boat. With newer, lighter displacement small-block V-8s from Mercury MerCruiser, Mercury Remanufacturing, Quicksilver and Volvo Penta, plus numerous independent re-power shops, an owner can get a more fuel-efficient propulsion system that, more often than not, will boost a given boat’s performance.

A twin-prop drive such as a MerCruiser Bravo III or a Volvo Penta DuoProp will improve low-speed handling and the ability to keep a single-engine boat on plane at lower speeds. That will make a boat more attractive for towing popular watersports such as wake boarding.

For many people, re-powering is a low-cost way to stay in boating, and this option deserves serious consideration.

For more repowering information and to see tests…