Replacing a CAT C18 with a Mitsubishi S6R-Y3

Replacing a CAT C18 with a Mitsubishi S6R

Replacing a CAT C18 with a Mitsubishi S6R-Y3 requires careful planning around cooling capacity, exhaust and air piping growth, and vertical clearance in the engine room. The S6R-Y3 delivers more horsepower and a larger displacement than the C18, but it stands roughly 12 inches taller, which can create challenges in vessels with low overhead or second-deck structures. Gear sizing, bell housing compatibility, and higher heat rejection loads also need to be evaluated. For operators willing to address these upfront changes, the S6R-Y3 offers a high-torque, mechanically reliable solution with stronger long-term durability and simpler lifecycle support compared to the electronically complex C18.

Jump to: 

• Specifications & Dimensional Differences
• Cooling Differences
• Common Modifications to Plan For
• Challenges to Overcome
• Third Party Feedback & Common C18 Pain Points
• Repower Planning Summary

Specifications & Dimensional Differences

Common Modifications to Plan For

Cooling System

• The CAT C18 (Tier 3) is a dual-circuit cooled engine (JW + AC).

• The S6R-Y3 has higher heat rejection in both circuits due to its larger displacement and higher horsepower.

• New coolers will be required; existing C18 coolers will not meet S6R heat loads.

• Even though the vessel already has dual circuits, both circuits must be upsized to match S6R requirements.

Exhaust & Air Inlet Piping

• S6R-Y3 requires 10-inch exhaust piping.

• Most C18 vessels are equipped with 8-inch exhaust systems, which will need to be enlarged.

• Air inlet piping on C18 boats is typically 5–6 inches; S6R requires larger diameter air piping to support higher airflow.

• These increases are consistent with moving from an 18L engine to a 24.5L engine.

Physical Dimensions & Fit

• The S6R-Y3 is 13 inches taller than the C18.

• Many C18 vessels (especially 1200 HP class boats) have second-deck structures or overhead framing that extend over the engines.

• The added height creates high risk for exhaust outlet interference and overhead clearance issues.

• Although the S6R is slightly shorter in length, height is the primary fit concern.

Gearbox Compatibility

• Most C18 repowers use the Twin Disc 5222, which is also compatible with the S6R-Y3.

• If the vessel has a smaller gear model, horsepower limitations may be exceeded at 684 HP.

• Gear ratio is not an issue because both engines run 1800 RPM.

• Some repowers may require gear replacement if the existing gear cannot handle the higher torque/HP.

Bell Housing

• CAT C18 units may have #1 bell housing or Single-0, depending on build.

• The S6R-Y3 uses Single-0.

• Because visual identification is difficult, repower surveys must include:

  • Photos of engine serial tags

  • Photos of gear tags

  • Measurements of bolt spacing

  • Verification with gear vendor

• A bell housing adapter may be required if the boat’s C18 uses a different housing.

Challenges to Overcome

• Strong CAT dealer network presence
  CAT locations are widespread, creating a perception of easy support even though many branches are not marine-focused.

• Long-standing customer loyalty to CAT
  Many operators have used CAT engines for generations, making them naturally hesitant to consider alternatives.

• Market perception of CAT as the default option
  Despite parts delays and short engine life, CAT’s brand reputation still influences purchasing decisions in many fleets.

Third-Party Feedback & Common C18 Pain Points

• Long lead times on new engines
  New C18 availability is extremely limited — often over a year from the factory. Operators with failures have few replacement options.

• Parts delays impacting planned maintenance
  Marine parts routinely take 10–12 weeks or more, forcing operators to plan overhauls six months in advance just to secure components.

• Limited alternative supply
  Aside from occasional rebuilt units from brokers, there are very few sources for replacement engines. Most fleets are left waiting or buying from scattered shops.

• Shorter engine life in commercial service
  Operators report 18,000–20,000 hours before replacement, well below expectations for true heavy-duty inland towboat applications.

• Originally designed as pleasure craft engines
  The C-series platform started as a high-performance recreational engine. Even with commercial A-ratings, the design is not inherently suited for pushboat duty cycles.

• High lifecycle cost
  Electronic complexity, expensive parts, and dealer-only service requirements make the C18 costly to maintain and operate over its life.

• Lower uptime and limited service flexibility
  Marine customers struggle with part shortages and technician availability, while mechanical engines like Mitsubishi offer faster service and greater self-maintenance capability.

Repower Planning Summary

Primary Required Changes

• Upgrade both cooling circuits

• Increase exhaust piping size

• Increase air inlet piping size

• Address vertical clearance / height limitations

• Verify or replace existing gear

• Confirm bell housing compatibility

• Upsize system components for larger displacement

General Observations

• The S6R-Y3 is a larger, heavier-duty engine than the C18, offering more horsepower and displacement.

• The primary repower challenge is height — the S6R-Y3 is roughly 13 inches taller, creating the biggest fit concern.

• Supporting systems on C18 boats (cooling, exhaust, air piping, and sometimes gears) are generally undersized for the S6R-Y3 and require upgrades.