Specifications and Identification for Twin Disc MG-5091 Marine Gear

April 3, 2025

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For commercial vessel operators, boatyard technicians, and marine mechanics, accurate knowledge of the specifications and identification details for the Twin Disc MG-5091 Marine Gear is critical to ensure proper installation, maintenance, repair, and replacement of parts. This marine gearbox has powered some of the most durable workboats and service vessels in the industry. Whether you’re troubleshooting performance issues, ordering overhaul kits, or ensuring alignment compatibility, knowing the complete specifications and how to correctly identify your unit is essential for uptime and performance.

This guide provides a comprehensive breakdown of the Twin Disc MG-5091’s technical specifications, including weight, dimensions, reduction ratios, input torque capabilities, and rotation orientation. Additionally, we’ll cover how to correctly locate and interpret the serial number tag, which is vital when ordering parts or confirming unit compatibility.

Let’s begin by diving into the exact gearbox specs.

Gearbox Specifications for Twin Disc MG-5091 Marine Gear

Overview of the MG-5091 Model Series

The MG-5091 is a marine transmission built by Twin Disc specifically for continuous-duty applications in medium- to heavy-horsepower marine engines. It features a reverse and reduction gearbox that is hydraulically actuated and engineered to perform reliably over long durations at sea. With various configurations available depending on reduction ratio and clutch options, understanding the specific specs of your unit ensures optimal installation and performance.

This model is frequently paired with marine diesel engines from major brands such as Cummins, Caterpillar, and Detroit Diesel in fishing vessels, tugboats, passenger ferries, and offshore support craft.

Weight of Twin Disc MG-5091 Marine Gear

Weight plays a key role when installing marine gearboxes. Overhead crane ratings, mounting bracket fabrication, and hull balance calculations all depend on knowing the accurate dry and wet weights of the unit.

Dry Weight (Approximate):
Between 730 lbs to 860 lbs, depending on ratio and configuration.
Wet Weight (With Oil):
Typically 780 lbs to 910 lbs, depending on gear ratio and cooling system attachments.

Weight Considerations:

Oil coolers and power take-off (PTO) attachments add weight.
Higher reduction ratios with larger planetary gear sets are heavier.
Always confirm actual installed weight when installing in tight compartments or small engine rooms.

For shipping purposes, the crated weight is approximately 920 to 950 lbs. If you’re ordering a rebuilt or replacement unit, ensure your receiving dock or hoist system is rated for this range.

Dimensions of Twin Disc MG-5091 Marine Gear

Proper dimensioning ensures clean engine room layout, precise alignment with propeller shafts, and adequate clearance for maintenance and ventilation.

Standard MG-5091 Dimensions:

Length (Bellhousing to Output Flange): ~28.5 inches
Width (Widest Housing Section): ~21.0 inches
Height (Top of Housing to Oil Pan Base): ~25.0 inches
Output Shaft Flange Diameter: SAE 4-bolt or 6-bolt pattern depending on spec
Mounting Bolt Pattern: Standard SAE No. 1 bellhousing
Centerline to Mounting Pad Height: Approx. 8.75 inches

Important Installation Considerations:

Always confirm your reduction ratio’s dimensions — higher ratio gearboxes are slightly longer.
PTO and auxiliary cooling line fittings may increase height and width.
Clearance must be provided for vent caps, shift control linkages, and oil fill points.

For repowering older vessels, be sure to compare existing footprint with these dimensions to avoid misalignment with engine stringers or mounts.

Gear Ratios Available for Twin Disc MG-5091 Marine Gear

Gear ratios determine how engine RPM translates into propeller shaft speed, influencing torque and vessel maneuverability. Twin Disc MG-5091 gearboxes come in a variety of reduction ratios to suit different marine propulsion needs.

Common MG-5091 Gear Ratios Include:

1.50:1
1.75:1
2.00:1
2.47:1
2.96:1
3.00:1

How to Choose a Gear Ratio:

Lower ratios like 1.50:1 allow for higher-speed vessels with less torque at the prop.
Higher ratios like 2.96:1 or 3.00:1 increase torque for heavy-duty propulsion needs (tugboats, trawlers, pushboats).
Gear ratio affects propeller sizing — higher reduction = larger diameter, higher-pitch props.

Important Tip:

Always verify the ratio stamped on your transmission data plate before ordering parts or pairing with a new engine. Even a slight difference in reduction can cause major performance issues or component damage.

Input Power Rating for Twin Disc MG-5091 Marine Gear

Understanding the power rating is essential for matching your engine’s output to the transmission’s capabilities. The MG-5091 is engineered to handle substantial torque loads from commercial diesel engines operating at mid-to-high RPM ranges.

Typical Input Power Ratings (Continuous Duty):

Horsepower Range: 400 HP to 600 HP
Torque Capacity: Up to 1,500 lb-ft, depending on gear ratio
Maximum Input Speed: ~3,000 RPM
Idle Clutch Engagement Speed: ~600–700 RPM
Maximum Shift Pressure: 250 psi (approximate, consult service manual for testing)

Factors Affecting Input Rating:

Duty cycle: continuous vs. intermittent service
Reduction ratio: higher ratios generally lower max input torque tolerance
Cooling: insufficient oil cooling may de-rate effective power limits

When evaluating a replacement or repower, confirm that your engine’s max torque does not exceed your MG-5091’s rating — especially when operating at max load or in warm water conditions.

Rotation Direction for Twin Disc MG-5091 Marine Gear

The direction of engine and output shaft rotation is critical to ensure that the propeller turns correctly and efficiently. Incorrect rotation can result in reduced thrust or serious drivetrain damage.

Standard Rotation Configurations:

Engine Input Rotation: Clockwise (viewed from flywheel end)
Output Shaft Rotation:
- Forward Gear: Counterclockwise (viewed from output shaft end)
- Reverse Gear: Clockwise (opposite)

Twin Disc MG-5091 can be ordered with reversible rotation depending on engine and application needs.

To Confirm Your Setup:

Look at your engine’s rotation direction (often stamped on the tag).
Refer to the output shaft rotation required by your propeller.
Match gear ratio and rotation to ensure the propeller spins in the correct direction for vessel movement.

If retrofitting or replacing an older unit, verify rotation matches your propeller and engine specs — mixing these up can severely compromise performance.

Serial Number Identification for Twin Disc MG-5091 Marine Gear

Correctly identifying your transmission model and configuration is essential for ordering compatible replacement parts, seal kits, or complete rebuild kits. The most accurate way to do this is by reading the serial number plate on the transmission itself.

Where to Locate the Serial Number Plate

On the Twin Disc MG-5091, the identification plate is typically a metal tag riveted or bolted directly to the main housing, often on the top or side of the gear case.

Common Serial Number Plate Locations:

Top of the housing, near the input bellhousing
Driver’s side of the gearbox (port or starboard depending on install)
Near the oil fill cap or hydraulic pressure test ports
In some rebuilds, it may be relocated — check all visible flat housing surfaces

The plate will be metal (often aluminum or stainless steel) with engraved or stamped numbers. Older units may have painted or worn plates — clean gently with solvent to read details.

How to Interpret the Twin Disc MG-5091 Serial Number Tag

The identification plate provides a complete breakdown of your specific gearbox’s configuration. Here’s what you’ll typically see:

1. Model Number

Clearly identifies the series — e.g., “MG-5091”
May also list extended model numbers like “MG-5091A” or “MG-5091RV” for reverse rotation

2. Serial Number

Unique number assigned to each unit at the factory
Essential when ordering exact replacement parts or warranty claims
Often begins with two letters, followed by six digits (e.g., “AB123456”)

3. Gear Ratio

Ratio listed as 2.00:1, 2.47:1, etc.
Must match replacement or rebuild components exactly

4. Rotation Direction

Indicated by “LH” or “RH” (Left Hand or Right Hand)
Alternatively marked as “Standard” or “Reverse”

5. Twin Disc Factory Code or Build Date

Helps identify manufacturing batch
Can assist with identifying model updates or design changes

Why This Information Matters

Every part inside the MG-5091 — from the clutch plates to the bearings to the seal kits — is ratio and configuration-specific. If you order a 2.47:1 clutch set for a 3.00:1 transmission, it may not fit, or worse, may cause internal damage.

Use the model, serial number, and gear ratio to verify:

Rebuild kit compatibility
Oil cooler line routing
Output flange compatibility
Bellhousing alignment patterns
Clutch pressure specifications

When working with a parts supplier like Diesel Pro Power, providing this plate data guarantees the correct parts are shipped — reducing downtime and installation delays.

Tips for Mechanics and Fleet Managers

Take a clear photo of the ID plate before disassembly.
Record the model, ratio, and serial number in your maintenance log.
Tag and label gearboxes pulled from the vessel for rebuild.
If the plate is damaged or missing, contact a Twin Disc expert with casting numbers and photos to help match your model.

Conclusion

Specifications and Identification for Twin Disc MG-5091 Marine Gear

Whether you’re performing routine maintenance, ordering parts, or planning a full rebuild, understanding the exact specifications and identification markers of your Twin Disc MG-5091 is critical. This gearbox is designed to perform under the most demanding conditions, but only when maintained and supported with the correct components.

Here’s what to remember:

Confirm the exact gear ratio before ordering any internal components.
Identify rotation direction to ensure proper propeller compatibility.
Use accurate serial number plate data when sourcing seal kits or rebuild kits.
Know the weight and dimensions to plan for installation, removal, and replacement.

If your serial number plate is missing or unreadable, contact a marine transmission expert. At Diesel Pro Power, our team can help cross-reference casting numbers and physical measurements to ensure you get the right parts the first time.

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Specifications and Identification for Twin Disc MG-5091 Marine Gear