Replacing a CAT 3408C Tier 0 with a Mitsubishi S6A3-Y1

Replacing a CAT 3408 with a Mitsubishi S6A3

Replacing a CAT 3408 with an S6A3-Y1 typically requires rerouting piping, updated mounting feet, and a review of cooling capacity due to differences in engine length, circuit design, and heat rejection.
While the S6A3 delivers comparable power across the 3408’s continuous duty range, operators must account for the S6A3’s longer footprint, inline configuration, and potential changes to the cooling circuit depending on the vessel’s existing system.

In many installations, the 12-inch reduction in width is an advantage in tight engine rooms common to older 3408 vessels, but the 7-inch increase in length can impact generator spacing.
Across past repower surveys, gearbox fit is not typically a limiting factor due to similar bellhousing size and close RPM alignment, though propeller adjustments may be required based on the actual rating of the existing 3408.

3408C- Core Engine Specs

Attribute	Value
Tier	0
Configuration	V8, 4-Stroke
Displacement	18 L
Cooling	Single Circuit
Dimensions	57.4 x 48.5 x 54.7 (LxWxH)

Continuous Duty Ratings (A/B)

Model	Rating	HP @ RPM	Fuel Burn (GPH)	Mitsubishi Replacement
3408C	A	402 @ 1800	20.5	S6A3-Y1
3408C	A	455 @ 1800	23.0	S6A3-Y1
3408C	B	480 @ 1800	24.4	S6A3-Y1
3408C	B	515 @ 2100	27.3	S6A3-Y1 or S6A3-Y3

Comparison to the S6A3

Attribute	CAT 3408C	Mitsubishi S6A3	Delta
Tier	0	Tier 1
Power	See above	483HP @ 1840RPM
Configuration	V8, 4-Stroke	Inline-6, 4-Stroke
Displacement	18 L	18.56 L
Length	57.4 in	64.4 in	+7.0 in
Width	48.5 in	36.5 in	–12.0 in
Height	54.7 in	54.0 in	-0.7 in

Common Repower Scenarios

These engines are commonly found on push boats and tug boats.
The 400–500 HP range is the normal operating band for workboats running 3408s.
Many of these boats are small and tight on engine room space (1000 HP twin-screw or less).

Common Modifications to Plan For

Piping Changes

Most installations will require rerouting of engine piping due to the change from a V8 to an inline-6 package and altered connection points.

Expect new raw water and jacket water plumbing runs
On older vessels, clearance around the forward end can be tight, especially where a generator is mounted directly ahead of the 3408
Standard practice is to mock up piping paths before final mounting to confirm access and serviceability

In nearly all repowers surveyed, piping is not a drop-in replacement, and should be planned as a routine modification.

Cooling Circuit Capacity

The CAT 3408 is a single-circuit, fully mechanical engine. The S6A3 interface is similar, but heat rejection may be higher, depending on rating and vessel design.

Since published CAT data for heat rejection is limited, the cooling circuit must be evaluated on the vessel, not by model alone:

Verify flow rate and cooler capacity
Inspect existing channel iron coolers
Expect potential capacity upgrades to match the S6A3’s heat load at the selected duty rating

Most vessels will be close on jacket water capacity, but additional cooling capability is a common requirement during installation.

Propeller Adjustment

The S6A3-Y1 produces 483 HP at 1840 RPM, while legacy 3408s in the field may be running anywhere from 402 to 515 HP depending on overhaul history and uprating.

Because of this variability, propeller work is often case-dependent:

Confirm the actual operating horsepower of the current 3408
Use existing propeller data as the baseline (diameter, pitch, gear ratio)
Plan for pitch changes if the legacy engine has been uprated beyond its original rating

When operators report “500 horsepower,” that becomes the functional baseline for selecting the S6A3 rating and determining whether a prop change is needed.

Engine Origin & Design History of the 3408

Introduced in the 1970s
Started as fully mechanical fuel injection
Later versions (1990s) had electronic fuel injection, but the engine was still mostly mechanical
The V8 version came after the 6-cylinder, and the 12-cylinder followed

Gear Compatibility

The CAT 3408 and Mitsubishi S6A3 share the same bellhousing size and operate at nearly identical speeds, so the existing gearbox is typically retained during a repower.
This reduces project scope and allows the focus to be on piping, cooling, and propeller alignment.

Target Markets

Push Boats
Tug Boats
Crew Boats
Fishing Vessels

3408 Operators IRR as of April 2022

Operator Name	Units
Terral River Service, Inc.	15
Crosby Marine Transportation LLC	8
Pine Bluff Sand & Gravel Co.	7
Luhr Bros., Inc.	6
American Coal Sales Co.	4
American Electric Power Co.	4
Evansville Marine Service, Inc.	4
Magnolia Midstream LLC	3
Cummings Marine Service	3
Monteiro Dock, Inc.	2
River Docks, Inc.	2
Ohio River Salvage, Inc.	2
Choctaw Transportation Co.	2

Sales Strategy

Overhaul cost now approaches $100,000
A full rebuild on a legacy 3408 can cost close to $100K — a major expense for an engine that may have been overhauled four, five, or even six times already.
Engines are old, blocks are worn
Many 3408s in service are 25–30 years old with very high total hours, and the block has already been taken to multiple rebuild limits.
New Tier 1 engine = same money, zero hours
For roughly the same cost as another overhaul, operators can install a brand-new S6A3, reset life to zero hours, and avoid repeating the rebuild cycle.
Parts availability is becoming a pain point
The 3408 is no longer produced; parts are sourced globally and lead times are getting longer, adding downtime and uncertainty during overhauls.
OEM support is limited
CAT no longer supports the platform the way it did when the engines were in production, creating frustration around responsiveness and service.
Laborde support is the differentiator
The value proposition is not just the engine — it’s Laborde’s reliable support, fast response, and customer focus, which customers are missing from the OEM.
Make the financial logic clear
The strategy is simple: “Don’t spend $100K on a worn-out 3408 — spend it once on a new engine.”
The cost is similar, but the outcome is completely different.

Repower Planning Summary

Piping rerouting due to the change from a V8 to an inline-6 footprint and different connection points
Additional cooling capacity may be required, depending on vessel cooler design and Mitsubishi heat rejection at the selected duty rating
Propeller pitch adjustments may be needed if the in-service 3408 has been uprated beyond its original A/B rating
Mounting feet modifications to adapt from the 3408 cradle to the S6A3 mounting pattern
Length increase of ~7 inches can impact generator clearance in tight engine rooms
Width reduction of ~12 inches improves access and service space on small vessels
Close RPM alignment (1800 vs. 1840) allows reuse of existing gear with matching bellhousing size

General Observations

Many 3408s in service have been overhauled multiple times and are often operating above their original A/B rating, so repower decisions are typically based on the vessel’s current horsepower rather than the factory tag.
The narrower footprint of the S6A3 improves access in tight engine rooms common to smaller push boats, while the additional length can affect clearance if a generator is mounted directly forward of the main engine.
Gearbox fit is usually not a limitation in 3408 repowers due to matching bellhousing size and close RPM alignment, so project scope is normally centered on piping changes, cooling capacity, and propeller adjustments.