Mercury Remanufacturing - 383 MAG Stroker Inboard

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Captain's Report

Mercury Remanufacturing 383 MAG Stroker Inboard

The inboard version of the 383 MAG Stroker is identified by its blue flame arrestor cover. The sterndrive version is orange.

Mission Statement

Mercury Remanufacturing designed the 383 MAG Stroker Inboard to be a lighter, higher-output replacement for a variety of gasoline inboards ranging from small blocks to 454-cu. in. big blocks in single-engine watersports towboats and twin-engine cruisers.

Test Boat

Mike Abendschoen owns a 1987 Bayliner 34 Avanti that measures 34’ (10.4 m) long and weighs about 12,175 lbs. (5,522 kg). He didn’t like the fuel consumption with the original U.S. Marine 454-cid big blocks and Borg Warner V-drive transmissions, but he liked the layout of his old boat. When he started hearing a knock in one of the engines, he talked with Jeffrey Zahn, Sr., Vice President of Weaver’s Marine in Baltimore, MD, about repowering. Zahn and technician Chris Layte suggested a pair of 350-hp Mercury Remanufacturing 383 MAG Stroker Inboards. They explained that the 383 MAG Stroker is based on a small-block engine and that it should provide the improved fuel economy that Abendschoen was looking for.

In addition to replacing the engines, Weaver’s Marine swapped out the Borg-Warner V-drive transmissions with Hurth V-drives and changed the gear ratio from the original 1.5:1 to 2.5:1. Changing the ratio let them prop the boat with larger 20 x 24 four-blade bronze wheels from Millers Island Prop Shop, which is close to Weaver’s in Baltimore. The bigger props improved the fuel efficiency and increased the top speed.

Mercury Remanufacturing 383 MAG Stroker Inboard

The weight savings and increased power that came with the repower breathed new life into this 1987 Bayliner 34 Avanti.


We weren’t able to obtain “before” numbers with the original motors, but Abendschoen said that he rarely ran faster than about 15 mph because the original engines’ four-barrel carburetors would open the secondary venturis and start burning more fuel than he was comfortable paying for. He estimated that at 15 mph, his old engines were burning 25 gph.

Top speed with the new 350-hp engines, transmissions and propellers was 31.9 mph at 4500 rpm. She burned 50.7 gph, giving the boat a range of 113 miles. Best cruise came at 3000 rpm where the 34 Avanti ran 19.3 mph and burned 20.7 gph for a range of 167 miles with a 10% reserve. Abendschoen was elated with the fuel consumption from the new propulsion system.

In acceleration tests, the repowered boat planed in 7.3 seconds and ran to 20 mph in 8.7 seconds before hitting 30 mph in 18.3 seconds. The new engines were also quieter than the old ones. The only time our decibel meter exceeded 80 dBA was at wide-open throttle. Otherwise, the sound levels averaged in the low to mid-70s, which is a comfortable level for having a conversation in normal tones.

Mercury Remanufacturing 383 MAG Stroker Inboard

With her newfound speed and power, the 34 Avanti could outrun some boats.

Big Block Discontinued. The General Motors big block was a workhorse in a variety of marine applications, but that motor is no longer available. In carbureted versions, the 454 produced anywhere from 310 to 365 hp, while fuel-injected models coaxed up to 385-hp out of the engine.

A Lighter, More Powerful Replacement. When considering a block on which to base a replacement for the 454, Mercury Remanufacturing decided to go with a proven 6.2-liter small block V-8 that’s been in the company’s arsenal for at least a decade. While most big blocks weigh 1,100 or 1,200 lbs. (499 kg or 544 kg), the 383 MAG Stroker inboard tips the scales at 932 lbs. (423 kg).

Mercury Remanufacturing 383 MAG Stroker Inboard

With a standard footprint and a 350-hp rating, the 383 MAG Stroker can replace a big block or small block.

Rebuilt Like New. The 383 MAG Stroker Inboard starts out as a used iron block core that’s bored out to 4.030” and the piston stroke is increased to 3.75”. The cylinder deck is machined for flatness and the crank journals are line-honed and prepped for new bearings. The crankshaft is forged from 4340 steel and the graphite-coated hypereutectic pistons have Moly-coated rings for improved durability. The exhaust uses dry-elbow technology to prevent water reversion.

Modern Efficiency. Mercury Remanufacturing equips the 383 MAG Stroker Inboard with the Generation 3 Cool Fuel system that’s designed to prevent vapor lock on hot days and a two-piece long runner intake with a 75 mm throttle body. Mercury’s 555 ECM (electronic control module) manages all the engine systems and is SmartCraft compatible.

Easier Access. In addition to being lighter and more powerful, the 383 MAG Stroker Inboard has regularly serviced accessories positioned for easy maintenance. The oil filter is at the front of the motor, as is the plug for connecting a laptop or scan tool for diagnostics. Additionally, the old engines in the boat had two belts, while the 383 has a single serpentine belt and a routing diagram on the thermostat housing.

Mercury Remanufacturing 383 MAG Stroker Inboard

Our test captain points out the location of the oil filter, which should be easy to change.
Mercury Remanufacturing 383 MAG Stroker Inboard

Bolted to a bracket next to the exhaust riser, the electronic control module is positioned for convenient access.
Mercury Remanufacturing 383 MAG Stroker Inboard

Tucked away behind the water pump pulley, the diagnostics plug pulls off so a technician can plug in a laptop or scan tool.
Mercury Remanufacturing 383 MAG Stroker Inboard

The serpentine belt is easy to check or replace and note the routing diagram on the thermostat housing for reference.

The Installation. Weaver’s Marine has developed a reputation locally as a one-stop shop for repowering. Layte supervised the project on the 3450 Avanti and he said that aside from having to modify the stringer mounts for the engine, everything went as expected. Most MerCruiser, Volvo Penta and other GM-based blocks have the same footprint and usually bolt right in, but the U.S. Marine engines had an unique bolt pattern that required the stringers to be re-done to accommodate the 383 MAG Stroker Inboards’ mounts. Other than the stringers, the project went according to plan.

In a V-drive installation, the engines are basically installed backwards and the transmissions are forward of the motors. The new Hurth transmissions were smaller than the Borg-Warner models. The space savings in the engine compartment gives technicians more room to work on accessories abaft the engines, such as the sea strainers.

Mercury Remanufacturing 383 MAG Stroker Inboard

The 383 MAG Stroker Inboard fit so cleanly that the exhaust risers that our captain is pointing at didn’t have to come off during installation, which is rare.
Mercury Remanufacturing 383 MAG Stroker Inboard

Because the original motors had a non-standard mounting footprint, the stringers had to be modified for the new mounts.
Mercury Remanufacturing 383 MAG Stroker Inboard

Because the new engines are shorter than the old ones, there’s plenty of space to get to the sea strainers and thru-hull pickups.

Updating The Dash. In addition to the engines, Weaver’s Marine upgraded the 3450 Avanti’s dash panel with new Faria gauges in a faux carbon-fiber panel. It doesn’t sound like that big of a deal, but Layte said the new ignition keys are a huge improvement over the old ones because of how smoothly they operate.

Mercury Remanufacturing 383 MAG Stroker Inboard

These are the original instruments on the 3450 Avanti and they look every bit like they came from the 1980s.
Mercury Remanufacturing 383 MAG Stroker Inboard

The new gauges in the simulated carbon-fiber panel give the boat a more current look and are much more functional.
Mercury Remanufacturing 383 MAG Stroker Inboard

The ignition key panel also got the faux carbon fiber treatment and their smooth operation is a bonus.


$11,118.75 for a single 383 MAG Stroker Inboard


1-year Factory Backed Limited Warranty

2 additional Years of Mercury Product Protection available

Mercury Remanufacturing 383 MAG Stroker Inboard

One consideration when looking at repowering is how long the owner plans to keep the boat.


With its combination of lighter weight and more power than many big-block engines, the Mercury Remanufacturing 383 MAG Stroker inboard could be an attractive engine for repowering. The new motors in our Bayliner 3450 Avanti provided the fuel efficiency that the boat owner was looking for as well as a speed increase that virtually anyone can appreciate.

Obviously, repowering is a big decision and one of the biggest considerations is how long does the owner plan to keep the boat.