Parts for the
SM420 Transmission

The SM420 has proven to be one of the best transmissions ever built. Its watershed design made it a greatly respected predecessor to following designs.

We offer nearly all parts at the component level, and our master rebuild kit, which includes the most thorough collection of parts offered anywhere.

The Master Kit
Our master rebuild kit has been assembled for our customer's rebuilds as well as our own. It is the most complete kit we know of an it features all the typical consumable and wear item components.

Rebuild Requirements
It is very common for our customers to do top-level rebuild work, using good methods and by following our included instruction guide.

We suggest the rebuilder have access to a set of snap ring pliers, and a small arbor press, in addition to standard shop tools.

Other Components
Nearly all parts for the SM420, including gears, shafts, etc. are available new and used and affordably so. Contact us.

 

Master Rebuild with synchros, SM420, 1947-1953 Part: #420MR-1 Weight: 10 lbs. Price: $213.20 Buy: Master rebuild with synchros, SM420, 1954-1963 Part: #420MR-2 Weight: 10 lbs. Price: $213.20 Buy: Master rebuild with synchros, SM 420, 1964-1967 Part: #420MR-3 Weight: 10 lbs. Price: $213.20 Buy:

 

 

All asterixed (*) items are already included in our master rebuild kit, above. 1. Expansion Plug 2. Roll Pin 3, Third and Fourth Speed Shifter Yoke 4. First and Second Speed Shifter Yoke 5. Reverse Shifter Head 6. Third and Fourth Speed Shifter Shaft 7. First and Second Speed Shifter Shaft 8. Reverse Shifter Shaft 9. Yoke Detent Ball 10. Yoke Detent Ball Spring 11. Transmission Cover 12. Reverse Shifter Compression Pin 13. Reverse Shifter Compression Pin Spring 14. Expansion Plug 15. Shifter Interlock Plate 16. Shifter Interlock Pin 18. Lock Spring 19. Slotted Hexagon Nut 20. Reverse Shifter Lever 21. Reverse Shifter Lever Eccentric 22. Clutch Gear Bearing Retainer 23. Clutch Gear Bearing Retainer Lip Seal * 24. Clutch Gear Stem Snap Ring * 25. Clutch Gear Bearing Retainer Ring * 26. Clutch Gear Bearing * 27. Clutch Gear 28. Transmission Mainshaft Pilot Roller Bearings * 29. Transmission Mainshaft Pilot Roller Bearing Retainer 30. Transmission Cover Gasket * 31. Transmission Case 32. Input Shaft (Clutch Gear) Synchronizer Cone * 33. Third and Fourth Speed Clutch Hub 34. Third and Fourth Speed Clutch Key Spring 35. Third and Fourth Speed Clutch Keys (also called pawls, dogs or blockers) 36. Third and Fourth Speed Clutch Sleeve 37. Third Speed Gear Synchronizer Cone * 38. Third Speed Gear 39. Third Speed Gear Mainshaft Bushing 40. Second Speed Gear Thrust Washer * 41. Second Speed Constant Mesh Gear 42. Second Speed Gear Synchronizing Ring Retainer 43. Second Speed Gear Synchronizing Ring 44. First and Reverse Gear Assembly 45. Transmission Rear Bearing Retainer Gasket * 46. Transmission Rear Bearing Retainer 47. Oil Seal * 48. UniversaI Joint Front Flange 49. Special Washer 50. Third and Fourth Speed Clutch Hub Retainer Ring * 51. Transmission Mainshaft Pilot Roller Bearing Spacer 52. Transmission Mainshaft 53. Transmission Mainshaft Bearing * 54. Reverse Idler Bushing 55. Reverse Idler Gear 56. Reverse Idler Gear Shaft 57. Roll Pin 58. Drain and Filler Plugs 59. Magnetic Chip Collector (Bonded to Case) 60. Power Take Off Cover 61. Power Take Off Cover Gasket * 62. Countershaft Front Bearing (#2412 for 3/4-ton version, #2412 or #1206 for 1-ton version) * 63. Countershaft Assembly 64. Countershaft Rear Bearing Oil Deflector 65. Countershaft Rear Bearing * 66. Countershaft Bearing Snap Ring * 67. Snap Ring *

Key items of interest:

Kit, PTO cover, six bolt style, billet Part: #K-PTO Weight: 1 lbs. Price: $33.06 Buy:

 

Kit 420FS, SM420 front seal & billet retainer conversion Part: #420FS Weight: 8.0 lbs. Price: $122.72 Buy: Notes: Please label and ship your existing input gear to Novak's physical address as found on our Contact page. We will associate it with your order and proceed.

Early SM420 Front Seal Conversion
The early SM420 transmissions (1947-1953) feature a necked-down retainer sleeve bore and screw-back threads to push oil rearward back into the transmission during operation; akin to a pump. This is in lieu of a neoprene seal as used in modern transmissions. This type of system is revealed by the presence of a dripper hole at 6 o'clock in the retainer tube, whose role it is to drip any escaped oil down into the bellhousing instead of towards the clutch disc. This early method works only tolerably well. Parking downhill with the engine switched off can be especially problematic.

We've developed a clean way to fit the SM420 with a front seal. We grind a precision cylindrical seal journal into a customer's existing shaft. We then provide our billet steel SM420 front retainer and a neoprene seal. This has the benefit of providing an excellent seal at the front of the transmission, without sacrificing the flushing and cooling action of gear oil to the main input bearing. It also replaces the brittle, cast iron front retainer with a far more durable version.