Free US Calls: 1-888-433-4735
International: 305-545-5588
Parts Catalog for Twin Disc MG521 Marine Transmissions
Rebuilt Twin Disc MG521 Marine Transmissions
Plate Kit For Twin Disc MG521 Marine Transmission
Gasket Kits For Twin Disc MG521 Marine Transmission
1. What type of oil should I use in the Twin Disc MG521?
Use SAE 30 non-detergent mineral oil or a Twin Disc-approved transmission fluid. Avoid multi-grade engine oils as they can cause clutch slippage.
2. How much oil does the MG521 hold?
Approximately 2.5 to 3.5 gallons (9.5 to 13.2 liters), depending on model configuration and oil cooler size.
3. How often should I change the oil?
Change the oil every 250 hours of operation or annually, whichever comes first. More frequently in commercial or high-load use.
4. How often should I change the oil filter?
At every oil change. Use a marine-rated filter compatible with your MG521 model.
5. What is the correct operating oil pressure?
Oil pressure should be between 200–250 PSI during normal operation. A drop below 180 PSI may indicate a problem.
6. What causes delayed engagement in gear?
Likely causes include low oil level, air in the hydraulic system, contaminated oil, or worn clutch plates.
7. How do I bleed air from the control system?
Loosen the hydraulic actuator bleed screw while cycling the shift lever until air bubbles stop and fluid runs clear.
8. Can I run the MG521 dry without oil?
No. Running the unit without oil will cause immediate and irreversible damage to internal components.
9. What are signs my clutch plates need replacement?
Slipping, burnt oil smell, excessive engagement delay, and visible discoloration or warping of plates.
10. How do I inspect the clutch plates?
Disassemble the clutch pack and measure plate thickness. Check for glaze, cracks, and spline wear.
11. How do I know if my bearings are bad?
Listen for whining or grinding, check for shaft play, and inspect bearings for pitting or overheating signs.
12. What’s the difference between forward and reverse clutch plates?
Some models use the same plates, while others use differently sized or splined plates. Always match to serial number.
13. Are Diesel Pro Power’s clutch plates interchangeable with OEM?
Yes. All Diesel Pro Power MG521 clutch plates meet or exceed OEM design and materials.
14. How often should I check alignment?
Check alignment during installation, after any vibration issue, and during every major service.
15. What causes excessive vibration in the MG521?
Possible causes include engine misalignment, worn flexible coupling, bent output shaft, or bad bearings.
16. How tight should mounting bolts be torqued?
Bell housing bolts should be torqued between 75–85 lb-ft. Always follow the torque chart provided in your rebuild kit.
17. Can I install the MG521 vertically?
No. The MG521 is designed for horizontal mounting only unless specified by Twin Disc for a special configuration.
18. Is there a filter inside the MG521?
Yes, most models use a spin-on or cartridge-type filter mounted externally. Always replace with approved types.
19. How do I prime the MG521 after rebuilding?
Rotate the input shaft by hand after oil refill, or crank the engine without ignition to build oil pressure.
20. Can I use synthetic oil in the MG521?
Only if approved by Twin Disc for your application. Most units are designed for non-detergent mineral oil.
21. What are common signs of oil contamination?
Dark oil, burnt smell, metal flakes in the drain pan, and erratic shifting.
22. What should I do if my MG521 overheats?
Check oil level, inspect the oil cooler and lines, look for slipping clutches, and test pressure relief valves.
23. What are acceptable gear backlash values?
Typically between 0.004″ and 0.012″, depending on gear set. Check service manual or contact Diesel Pro for your model.
24. How do I measure gear backlash?
Technician uses a starrett dial indicator to measure the backlash between the gears on a Detroit Diesel engine.
Use a dial indicator on one gear while rocking its mate back and forth to measure movement.
25. What causes noisy operation?
Misaligned gears, worn bearings, low oil, or cracked gear teeth.
26. Can I operate the MG521 with a damaged output seal?
No. You risk oil loss and contamination. Replace the seal before operating.
27. What’s included in a standard rebuild kit?
Clutch plates, bearings, seals, gaskets, filters, washers, snap rings, and instructions.
28. Does Diesel Pro Power offer technical support for installations?
Yes. Diesel Pro offers live chat, phone support, and step-by-step guides for MG521 installs and rebuilds.
29. Can I use aftermarket bearings in the MG521?
Yes, if sourced from a trusted supplier like Diesel Pro Power. All bearings match OEM tolerances.
30. How do I disconnect the output flange?
Remove flange bolts in a crisscross pattern and use a puller if necessary to avoid damaging the shaft.
31. What tools are required for MG521 disassembly?
Standard socket set, bearing puller, snap ring pliers, seal drivers, torque wrench, and dial indicator.
32. What’s the weight of the MG521?
Approximately 400–550 lbs depending on the model. Always use a hoist or crane for removal.
33. Can I install the MG521 without alignment tools?
Technician uses a starrett dial indicator to measure the backlash between the gears on a Detroit Diesel engine.
You can, but a dial indicator is strongly recommended for shaft alignment to prevent vibration and seal damage.
34. How do I recheck alignment after installation?
Measure offset and angular misalignment at the output flange using feeler gauges and a dial indicator.
35. Should I replace the flexible coupling during a rebuild?
Yes, especially if it shows wear, cracking, or hardening. It protects against engine shock and vibration.
36. What causes the MG521 to slip in forward but not reverse?
Worn forward clutch plates, damaged forward servo, or a pressure issue isolated to the forward circuit.
37. Is there a rebuild video available?
Yes. Diesel Pro has MG521 rebuild guides and videos on YouTube and at dieselpro.com/help.
38. What is the difference between a plate kit and a rebuild kit?
A plate kit includes only clutch discs; a rebuild kit includes seals, gaskets, bearings, and more.
39. Should I resurface the gear teeth if worn?
No. If gear teeth are worn or pitted, the entire gear should be replaced to maintain balance and strength.
40. Do I need to remove the engine to service the MG521?
No, but you must decouple the transmission and use lifting equipment for access.
41. What if I can’t identify my MG521 model?
Contact Diesel Pro Power with photos or serial numbers. Our team will help match the exact kit.
42. Are MG521 rebuild kits in stock?
Yes. Diesel Pro keeps all standard and master kits in stock with same-day shipping available.
43. What is the warranty on Diesel Pro’s MG521 parts?
OEM-matching warranty covering materials and manufacturing for up to one year.
44. Can I order individual parts instead of a full kit?
Yes. Diesel Pro sells clutch plates, seals, bearings, filters, and more as standalone parts.
45. What’s the lead time for international orders?
Typically 3–5 business days via DHL or FedEx, depending on customs and destination.
46. Can I rebuild my MG521 on the vessel?
Yes, but only if space, tools, and cleanliness can be maintained. Otherwise, remove and rebuild in a workshop.
47. What’s the best way to clean old clutch debris?
Use lint-free rags and solvent. Avoid compressed air that can embed debris in bearings.
48. What’s the typical lifespan of a rebuilt MG521?
3,000–5,000 hours depending on load, service, and oil change frequency.
49. Can the MG521 be upgraded with newer control systems?
Yes. Diesel Pro Power offers EC300 integration components for MG521 models.
50. Where can I buy MG521 parts online?
Visit dieselpro.com to browse rebuild kits, plate kits, and all service components.
Rebuilt Twin Disc MG521 Marine Transmissions
Plate Kit For Twin Disc MG521 Marine Transmission
Gasket Kits For Twin Disc MG521 Marine Transmission
Videos About Twin Disc Transmissions
6 Reasons Your Twin Disc Transmission Has Low Oil Pressure
7 Reasons Your Twin Disc Transmission Is Overheating
3 Reasons Your Clutch Plates in Your Twin Disc Transmission Are Making Excessive Noise
Bull Gear On A Twin Disc Transmission
Rebuilt Twin Disc Transmissions
