General Specifications For Allison M & MH Marine Transmissions

April 8, 2025

Allison MH Transmission) — Allison MH Transmission

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Physical Dimensions

Understanding the physical dimensions of the Allison M and MH marine transmissions is critical for anyone performing installation, servicing, or component replacement. These gearboxes are engineered for compactness relative to their torque-handling capabilities, allowing efficient use of space in often cramped engine rooms. Proper awareness of length, width, height, and mounting footprint ensures a precise fit—especially when retrofitting into vessels or replacing worn-out units.

Allison M Series Dimensions

The M Series is built for lighter-duty applications, often seen in pleasure crafts and small commercial vessels. While there are variants, the most common Allison M Series transmission units feature the following approximate dimensions:

Overall Length: 20 to 26 inches
Width Across Mounting Flanges: 14 to 16 inches
Height from Base to Top Cover: 14 to 18 inches
Mounting Footprint: Standard SAE bell housing configurations
Bolt Hole Pattern: Typically SAE #2 or #3, depending on engine match

Because M Series transmissions have minimal external control hardware, they can be installed in tighter spaces than some of their competitors. This design makes them ideal for twin-engine installations or refits where every inch of clearance matters.

Allison MH Series Dimensions

The MH Series is significantly larger and more robust than the M Series, reflecting its commercial-grade duty cycle and hydraulic control features. These units are widely used in tugs, ferries, fishing boats, and utility craft.

Typical MH Series dimensions include:

Overall Length: 28 to 36 inches
Width Across Mounting Flanges: 18 to 22 inches
Height from Base to Top Cover: 20 to 26 inches
Mounting Footprint: Extended flange with reinforced bolt patterns
Housing Style: Cast iron or reinforced aluminum alloy (in some versions)

Due to its added complexity and size, the MH Series may require reinforced engine beds or extended shaft coupling systems in certain applications. The design accommodates external hydraulic lines, cooling loops, and in some configurations, remote oil filter mounts.

Weight

Transmission weight is a key consideration for handling, lifting, mounting, and vessel balance. Both the Allison M and MH Series are heavier than equivalent recreational units due to the durability of their internal components, which are engineered for extended marine service life.

Allison M Series Weight Specifications

Dry Weight (without oil): ~180 to 260 lbs
Wet Weight (with full oil and filter): ~210 to 290 lbs

M Series units are typically light enough to be handled with two people using hoists, engine cranes, or lift tables. These weights include internal clutches and gears but do not account for any auxiliary mounting hardware or shaft couplings.

Allison MH Series Weight Specifications

Dry Weight (without oil): ~320 to 460 lbs
Wet Weight (with oil, filter, and cooler lines): ~370 to 520 lbs

Due to their robust housing, thick gearsets, and built-in hydraulic systems, MH Series transmissions are significantly heavier. These units should be handled with lifting hoists rated above 1,000 lbs for safety. Most commercial vessels are equipped with gantry cranes or A-frames for moving heavy drivetrain components during servicing.

When planning installation or reinstallation, always verify vessel trim and buoyancy charts to ensure balance is maintained after a transmission swap.

Input/Output Shaft Specifications

The heart of any marine transmission lies in its input and output shafts. These components connect the engine’s power to the propeller via reduction gearing and clutch modulation. Proper shaft alignment, lubrication, and coupling selection are essential to prevent premature wear or catastrophic drivetrain failure.

Allison M Series Input/Output Shaft Details

Input Shaft:

Shaft Diameter: ~1.25 to 1.5 inches
Spline Type: SAE standard 10- or 12-spline
Material: Hardened steel with corrosion-resistant coatings
Coupling: Mates directly with engine flywheel via flexible coupling or dampener plate

Output Shaft:

Shaft Diameter: ~1.5 to 1.75 inches
Flange Type: SAE or custom bolt pattern flanges
Rotation: Available in clockwise or counterclockwise depending on model
Optional Configurations: Output flanges can be adapted to reduction gears, V-drive setups, or direct propeller couplings

These shafts are precision-machined and balanced. Wear to spline teeth or scoring along the shaft body indicates improper alignment or inadequate lubrication.

Allison MH Series Input/Output Shaft Details

Input Shaft:

Shaft Diameter: ~1.75 to 2 inches
Spline Type: SAE heavy-duty 10-spline or straight keyed shafts
Material: Case-hardened alloy steel
Mounting Interface: SAE flywheel housing (typically #1 or #2)

Output Shaft:

Shaft Diameter: ~2 to 2.5 inches
Flange Options: Heavy-duty 6-bolt or 8-bolt output flanges
Rotation Direction: Configurable for LH or RH propeller applications
Additional Features: Often fitted with thrust bearing assemblies and multi-bearing output support housings

MH Series transmissions are designed to absorb high levels of axial and radial load, which is essential in commercial applications with oversized propellers or high thrust output.

Diesel Pro Power supplies both new and replacement input/output shafts, keyed couplings, and flange kits for Allison marine gearboxes. All components meet or exceed OEM tolerances.

Rotation Direction and Gear Ratios

Understanding gear ratio and rotation direction is vital for correct propeller performance. Propeller pitch and rotation direction must match transmission output direction to ensure proper thrust and fuel efficiency.

Rotation Direction

Allison M Series:

Available in left-hand (counterclockwise) or right-hand (clockwise) output configurations
Some models are bi-directional, allowing reversible output via internal gear setup
Confirm rotation direction by viewing from the stern, looking forward

Allison MH Series:

Typically configured for unidirectional output, but optional gearsets allow reverse builds
Used in both single-screw and twin-screw propulsion systems
Important: Use correct propeller for assigned rotation—installing the wrong prop leads to cavitation and poor thrust

Rotation direction is stamped on the transmission ID plate and can also be verified by turning the output flange by hand while engine is off and transmission is in gear.

Gear Ratios

Gear ratio is the reduction between input RPM (engine) and output RPM (propeller shaft). Allison marine transmissions are offered in various reduction ratios depending on vessel type and engine match.

Typical ratios include:

1.22:1 – Direct drive, minimal reduction (used in high-RPM light craft)
1.50:1 – Common for moderate prop sizes and average cruise speeds
1.97:1 – Most popular for diesel-powered vessels between 300–500 HP
2.50:1 or 2.96:1 – Used in large-displacement slow-turning props
3.08:1 and above – High-reduction ratios for heavy-duty commercial vessels requiring low-speed torque

How to choose the right ratio:

Match to engine’s torque curve and cruising RPM
Consider hull design and propeller efficiency
Avoid overloading engine at cruise or under-spinning prop at idle

If in doubt, Diesel Pro Power can assist with proper ratio selection for engine repowers or gear replacements.

Torque Handling and Horsepower Limits

Torque and horsepower handling define the transmission’s mechanical limit—exceeding these values can result in bearing wear, clutch slippage, and gear failure. Understanding these limits helps in matching engines to transmissions, especially during repowers or performance upgrades.

Allison M Series Capabilities

Horsepower Range:

Typically rated for up to 250 HP continuous, 300 HP intermittent

Torque Rating:

Rated between 400 and 600 lb-ft, depending on model and ratio

These ratings assume clean oil, proper cooling, and correct installation. The M Series is ideal for recreational diesel engines such as:

Detroit Diesel 4-71
Cummins 6BT
Perkins 6.354
John Deere 4045 and 6068 series

Allison MH Series Capabilities

Horsepower Range:

Handles 300 to 450 HP continuous, up to 500 HP intermittent

Torque Rating:

Typically between 800 and 1,200 lb-ft
Some configurations rated higher with oil cooler upgrades

The MH Series is matched with heavier engines such as:

Detroit Diesel 6-71, 8V71, 6V92
Cummins N855 or KTA19
Caterpillar 3208, 3306
John Deere PowerTech 9.0 and 13.5L

In all cases, the transmission must be properly matched to engine torque—not just peak horsepower. Diesel Pro Power provides compatibility support to ensure the transmission you select can handle your engine’s output curve.

Final Notes on Specification Use

These general specifications serve as a technical baseline for marine operators, mechanics, and repower specialists. While actual specs may vary slightly based on model year and configuration, the key takeaway is this:

Allison M Series is a compact, mid-torque, flexible option for yachts and small craft
Allison MH Series is a powerhouse gearbox for demanding commercial and heavy marine use

When servicing or replacing units, always refer to:

Transmission model number and ratio tag (usually located on the case)
Serial number records
Oil cooler size and fittings
Bell housing SAE standard for engine interface

By combining this data with proper marine transmission maintenance and high-quality aftermarket parts, you’ll ensure long service life and reliable operation.

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General Specifications For Allison M & MH Marine Transmissions