July 17, 2025
Introduction
What’s Included in a Standard Rebuild Kit For Allison M & MH Marine Transmissions
Clutch-Related Components
Gaskets and Seals
Bearings and Bushings
Hardware
Advantages of Aftermarket Parts Over OEM
Common Wear and Replacement Parts
Clutch Components
Bearings
Seals and Rings
Thrust Washers
Gaskets and Fasteners
Recommended Fluids and Filter Replacements
Transmission Models Supported
M Series
MH Series
How to Identify the Correct Rebuild Kit
Additional Parts Available at Diesel Pro Power
FAQ – Frequently Asked Questions
Additional Resources for Allison M & MH Marine Transmissions
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July 17, 2025
Introduction
1. Manual & Tools – Essential Reference and Equipment
2. Rebuilt Allison M & MH Gears – When to Replace the Entire Transmission
3. Overhaul Kit – Complete Seal and Gasket Replacement
4. Individual Seals – Fixing Localized Leaks
5. Clutch Plate – Worn or Burnt Clutches
6. Hydraulic Pump – Pressure Delivery System
7. Selector Valve & More – Control System Component
8. Transmission Bearings – Reducing Friction and Load Support
9. Sleeve, Ring – Shaft Support and Load Distribution
10. Locknut – Retaining Components Under Torque
11. Washer – Load Spreading and Thrust Control
12. Oil Strainer – Protecting the Hydraulic System
13. Spring & Bolt – Clutch and Valve Assemblies
14. Oil Cooler & Filter – Managing Heat and Cleanliness
15. Coupling – Connecting the Propeller Shaft
Final Notes On Component Replacement
When to Choose Individual Parts vs. Rebuild Kits
Conclusion
Additional Resources for Allison M & MH Marine Transmissions
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July 17, 2025
Introduction
General Operation FAQs For Allison M & MH Marine Transmissions
1. What do the designations "M" and "MH" mean?
2. What types of gear reductions are available?
3. Can these transmissions accommodate both left-hand and right-hand engine rotation?
4. Are these transmissions suitable for both pleasure and commercial vessels?
5. What kind of clutches are used?
Installation and Setup FAQs For Allison M & MH Marine Transmissions
6. How is the transmission mounted?
7. Does the transmission use the same oil as the engine?
8. What is the oil capacity?
9. Where is the oil dipstick located?
10. What type of oil filter is used?
Operation and Usage FAQs For Allison M & MH Marine Transmissions
11. Can you shift from forward to reverse at any speed?
12. What happens if I try to tow the boat with the engine off?
13. Is full torque shifting recommended?
14. What does the selector valve do?
15. How is the selector valve actuated?
Maintenance FAQs For Allison M & MH Marine Transmissions
16. How often should the oil be changed?
17. What is the correct oil level check procedure?
18. What are signs of oil contamination?
19. Can I reuse oil if it looks clean?
20. How should I store a transmission long-term?
Emergency Use and Troubleshooting FAQs For Allison M & MH Marine Transmissions
21. What is the "Get Home" emergency feature?
22. Can I engage reverse in emergency mode?
23. What should I do if the oil pressure is low?
24. Why is the transmission overheating?
25. What are symptoms of worn clutch plates?
Disassembly and Overhaul FAQs For Allison M & MH Marine Transmissions
26. Are special tools required to overhaul the transmission?
27. What parts are typically replaced during a rebuild?
28. What bearing inspection criteria should I follow?
29. Can I reuse clutch plates?
30. How do I clean and inspect oil passages?
Technical Specifications FAQs For Allison M & MH Marine Transmissions
31. What is the max input torque rating?
32. What’s the max input speed?
33. What is the oil pressure in forward?
34. What is the oil pressure in reverse?
35. What are the recommended shaft horsepower limits?
Torque, Fasteners & Assembly FAQs For Allison M & MH Marine Transmissions
36. What torque spec is used for the output flange nut?
37. Are spanner nuts used?
38. How is backlash adjusted?
39. Should I reuse a staked nut?
40. Is Loctite or thread sealant recommended?
Clutch and Hydraulic System FAQs For Allison M & MH Marine Transmissions
41. How are the clutches engaged?
42. What happens if a dump valve fails?
43. Are clutches adjustable?
44. Can I rebuild the flywheel clutch?
45. How does the hydraulic system work?
Identification & Model-Specific FAQs For Allison M & MH Marine Transmissions
46. Where is the serial number located?
47. What does the “R” in MH25R mean?
48. Can I upgrade from an M to an MH model?
49. Is there a parts catalog available?
50. Where can I find overhaul procedures?
Closing Note
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April 8, 2025
Disconnecting Cooling and Control Systems
Step-by-Step Cooling System Disconnection
Drain Oil Cooler and Hoses
Disconnect Temperature and Pressure Sensors
Remove Control Linkages and Shift Cables
Remove Breather and Fill Tube Cap
Supporting the Weight Safely
Required Tools and Equipment
Lifting Techniques and Safety Tips
Removing Fasteners and Couplings
Engine to Transmission Fasteners
Shaft Coupling and Propeller Shaft Detachment
Block the Propeller Shaft
Remove Output Flange Bolts
Check for Wear
Extracting from Engine Room
Planning the Extraction Route
Reinstallation Alignment Tips
Preparation Before Reinstallation
Aligning Input Shaft and Bell Housing
Coupling the Output Shaft
Final Pre-Start Checklist
Summary
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April 8, 2025
Slipping in Gear
No Engagement in Forward or Reverse
Overheating
Noise Under Load
Oil Contamination or Leakage
Additional Problem Scenarios and Remedies
Prevention Tips and Best Practices
Conclusion: When to Seek Professional Help
Disclaimer
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April 8, 2025
Inspecting and Adjusting Shift Linkage
Free-Play and Detent Engagement
Correct Neutral and Gear Alignment
Replacing Worn Bushings or Cables
Bushings
Cables
Verifying Cockpit-to-Transmission Shift Timing
Maintenance Tips for Extended Shift System Life
Common Problems and Troubleshooting
Final Verification and Sea Trial
Disclaimer
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