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Reassembling the Twin Disc MG520 Marine Gear is a precision process. After hours of teardown, cleaning, and inspection, every component must now be reinstalled with accuracy, discipline, and attention to detail. This section provides a comprehensive reassembly guide, including best practices, general torque procedures, and step-by-step guidance for installing seals and gaskets without warping or leakage.

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⚠️ Note: Always refer to the official Twin Disc MG520 OEM Service Manual for exact torque values, component clearances, and specific installation diagrams. This guide is a general framework optimized for real-world servicing while staying within the limits of legal usage and copyright compliance.

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Parts Catalog for Twin Disc MG520 Marine Transmissions
Rebuilt Twin Disc MG520 Marine Transmissions
Plate Kit For Twin Disc MG520 Marine Transmission
Gasket Kits For Twin Disc MG520 Marine Transmission

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Best Practices for Reassembly For Twin Disc MG520 Marine Gear

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1. Cleanliness Is Mandatory

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Contamination is the number one cause of premature failure in reassembled marine transmissions. A speck of grit in a bearing seat or a stray metal shaving in the oil gallery can ruin an otherwise perfect overhaul.

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Before You Begin:

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• Wipe down all components with lint-free cloths soaked in solvent.
• Blow out oil passages, cooler lines, and galleries with compressed air.
• Keep parts organized by subassembly: input, clutch pack, output, etc.
• Store clean components on rubber mats, foam trays, or wrapped in plastic until ready for use.
• Use clean nitrile gloves to avoid oil contamination or fingerprint residue.
  

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2. Dry Fit Critical Assemblies

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Before applying sealants or torquing bolts:

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• Dry-fit components to confirm:
  

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  • Correct part orientation
  • Free movement of rotating shafts
  • Proper alignment of bolt holes and mating surfaces
    

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• This is especially important for:
  

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  • Clutch packs
  • Pressure plates
  • Shaft couplings
  • Output flanges
  • Valve bodies
    

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3. Apply Assembly Lubricant Appropriately

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• Use marine-grade assembly lube or transmission oil to lubricate:
  

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  • Bearings
  • Shaft journals
  • Gear teeth
  • Clutch pistons
  • Seal lips
• Avoid over-lubricating friction materials. Never get oil on friction plates or steel clutch plates.
  

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4. Alignment Checks

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Misalignment can cause:

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• Shaft runout
• Clutch dragging
• Oil pump cavitation
• Premature bearing wear
  

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How to Avoid It:

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• Use a dial indicator to confirm shaft runout is within spec before final bearing preload is applied.
  

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• Align:
  • Input shaft to engine flywheel
  • Output shaft to propeller shaft or shaft brake
  • Control valves and actuator levers
    

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• If shaft-to-shaft alignment cannot be confirmed visually, use laser alignment tools or feeler gauges at coupling faces.
  

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5. Replace All Wear Items

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Any of the following should never be reused during reassembly:

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• Shaft seals
• O-rings
• Gaskets
• Snap rings (if distorted)
• Lock washers
• Friction plates (if thickness is below spec)
• Any bolt or plug showing thread deformation or corrosion

Always install a full overhaul kit, preferably from a trusted aftermarket provider like Diesel Pro Power.

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Torque Sequences and Values (General Reference) For Twin Disc MG520 Marine Gear

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Proper torque application is more than just tightening bolts—it’s about even clamping, gasket sealing, and component centering.

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1. General Torque Guidelines (Non-Specific)

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Bolt Diameter	Grade 8 Bolt (ft-lbs)	Grade 5 Bolt (ft-lbs)
1/4″-20	12	9
5/16″-18	25	17
3/8″-16	45	33
7/16″-14	70	55
1/2″-13	105	80
9/16″-12	150	115
5/8″-11	210	165
3/4″-10	375	300

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⚠️ ALWAYS check OEM manual for exact torque specs. Use these as general references only.

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2. Recommended Torque Wrench Procedures

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• Use a calibrated torque wrench capable of the correct torque range.
• Apply torque in three stages:
  1. Snug (hand-tight)
  2. Mid-range (~50% of final torque)
  3. Final torque
• Use crisscross or star patterns when torquing covers, housings, flanges, and valve bodies.
• After torquing, mark bolt heads with paint to verify which bolts have been completed.
  

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3. Torque Application Tips

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• Never torque dry bolts into aluminum or cast iron—use anti-seize or thread lubricant where appropriate.
• Be cautious with low-torque gaskets—over-torquing can crush or warp mating surfaces.
• Torque bolts only when both surfaces are fully flush—do not use torque to “pull in” misaligned parts.
  

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Seal and Gasket Installation For Twin Disc MG520 Marine Gear

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Proper seal and gasket installation is vital to ensure fluid containment, pressure integrity, and internal cleanliness. This is especially critical in the MG520’s high-pressure clutch circuits and rotating shaft assemblies.

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1. Gasket Preparation and Installation

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Gasket Types Found in MG520:

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• Paper or Fiber Gaskets: Used between valve body covers, housing flanges, inspection ports.
• Cork-Rubber Composite: Rare; typically used on access panels.
• Metallic or Graphite Gaskets: Found in high-temperature zones or flange seals.
• O-Ring Gaskets: Installed in cooler ports, shaft bores, and control valve ports.
  

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Installation Steps:

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1. Clean the Surface Thoroughly
   • Use a plastic scraper to remove old material
   • Finish with solvent-wipe (acetone or brake cleaner)
     

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2. Check for Flatness
   • Use a straightedge across the gasket face
   • If warped, machine or replace the mating surface
     

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3. Apply Sealant (Only If Recommended)
   • Only apply RTV or anaerobic sealants if OEM instructions specify
   • Avoid excessive application—squeeze-out can block oil passages
     

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4. Position Gasket Carefully
   • Use alignment dowels or gasket adhesive to hold position
   • Don’t overtighten bolts during initial seating
     

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5. Torque in Sequence
   • Tighten evenly to ensure uniform gasket compression
     

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Common Mistakes to Avoid:

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• Over-compressing paper gaskets—this causes early failure and weeping
• Applying sealant on gaskets not rated for it
• Allowing foreign material between gasket and metal
• Failing to re-torque after heat cycle (if specified)
  

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2. Shaft Seal Installation

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Seals are critical in the MG520 because they keep transmission oil inside and water/dirt out, especially around rotating input/output shafts.

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Types of Shaft Seals:

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• Lip Seals (Radial Shaft Seals): Used for both input and output shafts
• Face Seals (Rare in MG520): Found in some PTO variants
  

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Installation Procedure:

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1. Clean and Deburr Shaft and Bore
   • Use fine emery cloth to remove any sharp edges
   • Polish seal bore with solvent and soft cloth
     

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2. Apply Light Oil
   • Coat the shaft and seal inner lip with marine gear oil
     

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3. Use a Seal Driver
   • Tap the seal evenly into place using a tool that contacts only the outer casing
   • Do not use a socket or hammer directly—it risks deformation
     

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4. Check for Flush Fit
   • Seal should be flush or slightly recessed—never proud of the housing face
     

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5. Verify Lip Contact
   • Rotate shaft to ensure smooth movement
   • Lip should maintain gentle contact without binding
     

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Post-Installation Tip:

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Allow seals to rest for 30 minutes before running the unit. This allows the rubber to settle and prevents lip roll-over during initial shaft spin-up.

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3. O-Ring and Valve Body Seals

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O-rings are used in cooler ports, control valves, and hydraulic pistons. Even minor nicks or swelling will cause internal pressure loss or leakage.

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O-Ring Installation Tips:

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• Use only O-rings rated for petroleum-based oils (NBR or Viton).
• Lightly coat O-rings with assembly lube or oil before seating.
• Install by rolling into place—not stretching.
• Avoid sharp tools or picks that can nick the rubber.
• Seat O-rings evenly and confirm they do not twist or bulge.
  

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Sealant Application Dos and Don’ts

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✅ Use Sealant When:

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• Installing inspection plugs, pipe threads, or cooler fittings
• OEM specifies RTV or anaerobic sealants in gear joint interfaces
• Shaft spline protection is needed (anti-seize with sealing properties)
  

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❌ Don’t Use Sealant:

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• On paper gaskets unless OEM says so
• On bearing seats or bushing bores
• As a substitute for proper gaskets
• Around shaft seals (seal installs dry—sealant can damage the lip)
  

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Final Checks Before Startup For Twin Disc MG520 Marine Gear

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Before powering up the transmission, perform these essential checks:

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1. Confirm Torque Application
   

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   • Check every major bolt, especially:
     • Housing flanges
     • Gear covers
     • Valve body bolts
     • Shaft nuts
       

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2. Verify Seal and Gasket Fit
   • Look for visible gaps, pinched edges, or misaligned bolt patterns
     

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3. Rotate Input/Output Shafts
   • Ensure free movement with no binding or abnormal resistance
     

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4. Fill with Recommended Oil
   • Use SAE 30W or 40W non-detergent transmission oil (per MG520 spec)
   • Verify oil level on dipstick or sight glass
     

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5. Prime the Oil Pump (if necessary)
   • Some gear designs require pre-lubing of internal passages
     

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6. Reconnect and Secure All Lines
   • Hydraulic controls, cooling lines, and electrical connections
     

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7. Bleed Air from System
   • Run the gear at idle and shift through forward/reverse to remove air pockets
   • Recheck oil level after running
     

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Conclusion: Reassembly Is a Test of Precision and Patience For Twin Disc MG520 Marine Gear

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Reassembling the Twin Disc MG520 isn’t just about putting parts back together—it’s about restoring factory-level integrity with every turn of the wrench. The attention you give to cleanliness, alignment, torque, and sealing directly determines the success of your service.

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By following the best practices in this guide—alongside torque procedures and smart seal/gasket strategies—you’ll ensure your MG520 delivers trouble-free operation, smooth shifting, and years of reliable marine propulsion.

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Need a full rebuild kit, torque specs guide, or premium seals for your Twin Disc MG520? Visit Diesel Pro Power for trusted aftermarket parts and support.

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Parts Catalog for Twin Disc MG520 Marine Transmissions
Rebuilt Twin Disc MG520 Marine Transmissions
Plate Kit For Twin Disc MG520 Marine Transmission
Gasket Kits For Twin Disc MG520 Marine Transmission