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Parts Catalog for Twin Disc MG5091 Marine Transmissions
Rebuilt Twin Disc MG5091 Marine Transmissions
Plate Kit For Twin Disc MG5091 Marine Transmission
Gasket Kits For Twin Disc MG5091 Marine Transmission

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The Twin Disc MG-5091 Marine Gear is one of the most trusted marine transmissions in commercial, industrial, and workboat applications. Whether you’re operating, maintaining, repairing, or rebuilding the MG-5091, having fast answers to common questions can save hours of downtime and prevent costly mistakes.

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This comprehensive FAQ covers installation, operation, maintenance, troubleshooting, rebuilds, technical specifications, fluid guidelines, and much more — designed for captains, technicians, mechanics, and fleet managers worldwide.

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General Information

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1. What type of marine transmission is the Twin Disc MG-5091?
It is a mechanical, hydraulically actuated marine gear, typically used in medium-duty to heavy-duty marine propulsion systems.

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2. Is the MG-5091 reversible?
Yes. It features hydraulic forward and reverse clutches, allowing for full reversal of output shaft direction.

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3. What vessels use the MG-5091?
It is commonly found in tugboats, shrimp trawlers, ferries, patrol boats, and commercial fishing vessels.

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4. What is the typical horsepower rating for the MG-5091?
The MG-5091 can handle up to 660 HP (at 1800 RPM), depending on gear ratio and application.

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5. How is the MG-5091 cooled?
Through an external oil-to-water or oil-to-air cooler, connected via oil ports on the housing.

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Specifications

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6. What gear ratios are available for the MG-5091?
Common ratios include 1.50:1, 1.75:1, 2.00:1, 2.47:1, 2.95:1, and 3.5:1, depending on the model configuration.

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7. What is the input rotation direction?
The MG-5091 is designed for clockwise (right-hand) input rotation when viewed from the flywheel end.

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8. What is the weight of the transmission?
The dry weight is approximately 925 lbs (420 kg), depending on configuration and ratio.

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9. What is the maximum input speed?
The rated maximum input speed is 2100 RPM, although it may vary slightly by ratio.

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10. Does the MG-5091 come with a PTO (Power Take-Off)?
Yes, some versions include an optional PTO (live or clutched) for driving hydraulic pumps or generators.

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Oil & Lubrication

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11. What type of oil does the MG-5091 use?
Use SAE 30 or SAE 40 non-detergent oil approved for use in marine transmissions.

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12. Can I use automatic transmission fluid (ATF)?
No. ATF is not compatible with MG-5091 clutches and will cause slippage.

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13. What is the oil capacity of the MG-5091?
It ranges from 2.75 to 3.5 gallons (10.5 to 13.2 liters) depending on the cooler and lines.

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14. How often should I change the oil?

• Initial change: After 25 hours
• Regular change: Every 300–400 hours or annually
  

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15. Should I use synthetic oil?
Synthetic oil may be used if it meets Twin Disc viscosity and friction specifications, but always consult with Diesel Pro Power for guidance.

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Maintenance

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16. What’s included in a typical maintenance kit?
Diesel Pro Power’s kit includes filters, gaskets, seals, and O-rings needed for routine servicing.

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17. How often should I check the oil level?
Daily before startup and after extended operation.

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18. When should the oil filter be changed?
Change it every oil change or at least every 300 hours.

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19. How do I check shaft alignment?
Use a dial indicator or feeler gauge to ensure runout is <0.003″ TIR at the coupling.

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20. What’s the best way to flush the transmission?
Drain completely, fill with flushing oil, run 10 minutes, then drain and refill with clean oil.

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Installation

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21. What is the correct alignment tolerance?
Shafts must be aligned within 0.002″–0.003″ total indicated runout.

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22. What is soft foot and why does it matter?
Soft foot refers to uneven mounting pad contact, which causes housing distortion and internal misalignment. It must be corrected using shims.

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23. What mounting hardware should I use?
Use Grade 8 bolts, lock washers, and thread locker on all mounts and flanges.

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24. What is the torque spec for housing bolts?
3/8” bolts = 30–35 ft-lbs, 1/2” bolts = 75–85 ft-lbs

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25. How do I torque the output flange?
Torque to 125–135 ft-lbs for 5/8″ bolts, and follow a star pattern.

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Troubleshooting

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26. Why does my MG-5091 slip in forward?
Likely causes: worn clutch plates, low oil pressure, or contaminated oil.

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27. What causes delayed engagement?
Low hydraulic pressure, air in the system, or incorrect clutch clearance.

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28. I hear whining or grinding—what’s the issue?
Possible bearing wear, gear backlash out of spec, or oil starvation.

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29. Oil is leaking from the output seal—what do I do?
Inspect the seal lip, shaft surface, and replace the output seal if necessary.

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30. Pressure is low on startup. Should I be concerned?
Yes. Pressure should build within 5 seconds. Check the pump, filter, and fluid level.

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Rebuilds

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31. When should I rebuild the MG-5091?
At 8,000–12,000 hours, or earlier if experiencing slippage, overheating, or abnormal noise.

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32. What’s in a Diesel Pro Power rebuild kit?
Includes clutch plates, gaskets, seals, bearings, O-rings, and more — everything needed for a full rebuild.

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33. Can I rebuild it myself?
Yes, if you have mechanical experience, correct tools, and follow OEM torque and clearance specs.

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34. What tools do I need?

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• Torque wrench
• Dial indicator
• Seal installers
• Pullers
• Feeler gauges
  

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35. Do I need to reshim anything?
Yes — backlash and endplay may require shimming to OEM tolerances.

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Technical Tolerances

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36. What is the clutch clearance spec?
0.015″ to 0.030″ total clearance.

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37. What’s acceptable backlash between gear teeth?
0.004″ to 0.010″, depending on the gear set.

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38. What is acceptable input shaft runout?
No more than 0.002″–0.004″ total indicated runout.

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39. What’s the output shaft endplay spec?
Typically 0.004″ to 0.008″.

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40. How do I check backlash and endplay?
Use a dial indicator mounted to a magnetic base while rotating or shifting shafts.

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Storage & Preservation

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41. Can I store the MG-5091 without oil?
Only after applying rust inhibitors to internal components. Never store with used oil.

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42. How do I protect it from corrosion?

• Drain oil
• Apply vapor-phase or contact rust inhibitors
• Seal breather and shaft ends
• Store in low-humidity environment
  

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43. How often should I rotate the shaft during layup?
At least once per month to prevent clutch and bearing damage.

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44. What should I do before startup after layup?

• Drain rust inhibitor oil
• Refill with clean oil
• Check all seals
• Perform oil pressure test
• Shift to forward/reverse at idle
  

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45. Can I fog the gearbox with marine fogging oil?
Yes, but it must be flushed before return to service.

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Parts & Sourcing

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46. Where can I buy aftermarket parts for the MG-5091?
At Diesel Pro Power, offering fast shipping worldwide.

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47. Are Diesel Pro’s rebuild kits compatible with OEM specs?
Yes — our parts meet or exceed OEM tolerances and durability.

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48. How quickly can I get parts?
Most parts ship same day, globally.

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49. Do I need my serial number to order?
Recommended but not always required. It helps match gear ratio, clutch configuration, and shaft design.

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50. Does Diesel Pro offer tech support for installation?
Yes — visit dieselpro.com/help for expert assistance by phone, email, or chat.

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Parts Catalog for Twin Disc MG5091 Marine Transmissions
Rebuilt Twin Disc MG5091 Marine Transmissions
Plate Kit For Twin Disc MG5091 Marine Transmission
Gasket Kits For Twin Disc MG5091 Marine Transmission