Introduction

When servicing Allison M & MH Marine Transmissions, having access to a high-quality rebuild kit and reliable aftermarket parts is essential for restoring transmission performance and extending service life. Whether you’re dealing with worn clutch plates, degraded seals, or failing bearings, the right parts ensure precision and durability in marine environments. This comprehensive guide will cover what’s typically included in a rebuild kit, the advantages of aftermarket components, common replacement parts, and important specifications to help you get your transmission back in peak condition.

Disclaimer: For torque specifications, disassembly procedures, and advanced tolerances, always refer to the OEM Allison Service Manual SM1984EN or consult a certified marine technician.

What’s Included in a Standard Rebuild Kit For Allison M & MH Marine Transmissions

A complete overhaul kit (rebuild kit) for Allison M &  or an Overhaul Kit For Allison MH marine transmissions will typically include:

Clutch-Related Components

• Forward and reverse clutch friction plates

• Clutch piston seal rings (Teflon or Viton)

• Piston return springs (reverse clutch only – typically 16 springs)

• Snap rings for clutch assemblies

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Gaskets and Seals

• Oil pan gasket

• Selector valve body gasket

• Shaft seals (input, output)

• Hook-type seal rings for the input shaft

• O-rings and seal expanders

• Breather cap seal or replacement

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Bearings and Bushings

• Input shaft ball bearing or roller bearing (model-dependent)

• Output shaft bearing(s) – single or double row, or tapered roller depending on the model

• Pinion gear bearing

• Needle roller bearings and thrust washers for the reverse planetary carrier

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Hardware

• Cotter pins, split pins for backplate retention

• Replacement snap rings

• Replacement bolts (bearing retainer and oil pan bolts)

• Lockwashers

These kits are tailored by model, such as M10, M15, M20, MH25, MH37, MH45, etc., and gear ratios (e.g., 1.5:1, 2.0:1, 3.7:1).

Advantages of Aftermarket Parts Over OEM

At Diesel Pro Power, we specialize in premium aftermarket parts for Allison marine transmissions that:

• Match or exceed OEM performance using high-tolerance materials.

• Offer lower lead times and greater availability.

• Provide cost-effective solutions without sacrificing durability.

• Are backed by warranty, including our OEM matching warranty on rebuild kits.

Due to the age of many Allison M & MH units and the fact that these manuals and parts are no longer in mainstream production, aftermarket solutions are often your only realistic option for full restorations.

Common Wear and Replacement Parts

The following components frequently require replacement during a rebuild, and are available individually if a full kit isn’t needed:

Clutch Components

• Reverse clutch friction plate: Minimum thickness 0.176 in (4.47 mm), groove depth 0.005 in (0.13 mm)

• Forward clutch plate: Not rebuilt in transmission; part of engine flywheel assembly

• Reverse piston springs: Replace all 16 as a set

Bearings

• Input shaft ball bearing (double-row or single-row, depending on model)

• Output shaft tapered roller bearing set or ball bearings

• Pinion gear bearings (always inspect for metal contamination)

Seals and Rings

• Teflon seal rings (must be replaced; never reuse)

• Hook-type seal rings (replace if worn or loose)

• Oil pump drive seals and output shaft seals (replace any that show signs of wear or leakage)

Thrust Washers

• Reverse planetary carrier thrust washers: Minimum thickness 0.120 in (3.05 mm)

• Input shaft thrust washers: Typically bronze/steel combo, minimum 0.086 in (2.18 mm)

Gaskets and Fasteners

• Oil pan and bearing retainer gaskets

• Selector valve gasket

• Snap rings and split pins

• Cotter pin for output shaft nut (ball bearing models)

• Stake nuts for tapered bearing models

Recommended Fluids and Filter Replacements

• Oil Type: Use the same oil as the engine (SAE 40, 30, or 15W-40)

• Oil Capacity:

  • Shallow profile units: 6 quarts (5.5 L)

  • Deep profile units: 6.5 quarts (6 L)

• Filter Element: Always replace the full-flow oil filter element during service

• Strainer: Clean thoroughly; replace the gasket

Transmission Models Supported

This parts overview covers rebuild kits for the following Allison marine transmission models:

M Series

MH Series

Ensure correct identification using the transmission nameplate, which includes:

• Model Number

• Assembly Number (Part No.)

• Serial Number

The nameplate is typically located on the rear upper side of the transmission housing.

How to Identify the Correct Rebuild Kit

Before ordering a rebuild kit:

1. Locate your nameplate (see image or reference Fig. 1-4 in OEM manual).

2. Confirm gear ratio and whether your unit is Right-hand (R) or Left-hand (L) rotation.

3. Note whether you have a shallow or deep oil pan profile, as this affects gasket and fluid specs.

4. Confirm if your transmission uses ball bearings or tapered roller bearings on the output shaft. Early M Series used ball bearings; later and most MH Series use tapered bearings.

If in doubt, contact Diesel Pro Power for compatibility assistance using your transmission’s model and serial number.

Additional Parts Available at Diesel Pro Power

We stock and ship the following Allison marine  transmission parts worldwide:

• Clutch plate kits

• Reverse planetary assemblies

• Selector valve components

• Oil pumps and gears

• Bearing retainers

• Output flanges

• Input shafts and sealing rings

• Oil pans and drain plugs

• Preservative kits for long-term storage

We also offer complete rebuilt Allison marine transmissions for those who prefer not to do a full rebuild themselves or who can’t afford the downtime.

FAQ – Frequently Asked Questions

Q: Can I reuse the piston return springs in the reverse clutch?
A: No. Springs weaken over time and may compromise clutch engagement. Replace all 16 as a set.

Q: What oil pressure should I see during operation?
A: Forward @ 1800 RPM: Minimum 130 psi (896 kPa); Reverse @ 1500 RPM: Minimum 110 psi (758 kPa).

Q: Do I need to preheat the oil in cold climates?
A: Yes. Below 50°F (10°C), preheat is recommended before shifting into gear.

Q: How often should I change the oil and filter?
A: Every 500 hours or annually, whichever comes first.

Q: Are the forward clutch components included in the rebuild kit?
A: No. The forward clutch is located inside the flywheel and serviced through the engine manufacturer.

Additional Resources for Allison M & MH Marine Transmissions

Practical Guide To Servicing Allison M & MH Marine Transmissions

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