Parts for the

SM420 Transmission

sm420_transmissionThe SM420 has proven to be one of the best truck transmissions ever build, laying the groundwork for transmission design for the next several decades.

We offer many 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 and it features all the typical consumable and wear items including components for a full lid rebuild.

Note that the 1st-to-2nd gear synchronizer ring is not included in the master kit like for other transmissions, as GM manufactured the synchro into the 1st sliding gear. Because most drivers start out in 2nd gear and do not regularly shift from 1st into 2nd, this synchro ring usually outlasts the others. However, Novak can install a new synchro ring into your first gear if you would like to send it to us. Cost is $125. and we require about four business days to turn it around. Please inquire if interested.

SM420 Transmission Rebuild Kit
  • Kit, rebuild, master, SM420 transmission, SM420, 1947-1953

  • Kit, rebuild, master, SM420 transmission, 1954-1963 version

  • Kit, rebuild, master, SM420 transmission, 1964-1967 versions

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

Many parts for the SM420 are available new and used; however, gears and input shafts can be very difficult to source, primarily due to having been out of production for several decades. Please refer to the parts list below to determine availability. Contact us.

Related Products

Billet PTO Cover
Shifter Retention Kit, SM420, SM465, T176, NP435
Shifter stick for the SM420 transmission
Shifter stick for the SM420 & SM465 transmission
Turn-Key SM420
SM420 front billet retainer

All asterixed (*) items are already included in our master rebuild kit, above.

All items marked with a cross (†) are unavailable or nearly impossible to source

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 *
17. Lock Spring
18. Slotted Hexagon Nut
19. Reverse Shifter Lever
20. Reverse Shifter Lever Eccentric
21. Clutch Gear Bearing Retainer
22. Clutch Gear Bearing Retainer Lip Seal *
23. Clutch Gear Stem Snap Ring *
24. Clutch Gear Bearing Retainer Ring *
25. Clutch Gear Bearing *
26. Clutch Gear / Input Shaft †
27. Transmission Mainshaft Pilot Roller Bearings *
28. Transmission Mainshaft Pilot Roller Bearing Retainer
29. Transmission Cover Gasket *
30. Transmission Case
31. Input Shaft (Clutch Gear) Synchronizer Cone *
32. Third and Fourth Speed Clutch Hub
33. Third and Fourth Speed Clutch Key Spring
34. Third and Fourth Speed Clutch Keys (also called pawls, dogs or blockers)
35. Third and Fourth Speed Clutch Sleeve
36. Third Speed Gear Synchronizer Cone *
37. Third Speed Gear †
38. Third Speed Gear Mainshaft Bushing
39. Second Speed Gear Thrust Washer *
40. Second Speed Constant Mesh Gear †
41. Second Speed Gear Synchronizing Ring Retainer
42. Second Speed Gear Synchronizing Ring
43. First and Reverse Gear Assembly †
44. Transmission Rear Bearing Retainer Gasket *
45. Transmission Rear Bearing Retainer
46. Oil Seal *
47. UniversaI Joint Front Flange
48. Special Washer
49. Third and Fourth Speed Clutch Hub Retainer Ring *
50. Transmission Mainshaft Pilot Roller Bearing Spacer
51. Transmission Mainshaft
52. Transmission Mainshaft Bearing *
53. Reverse Idler Bushing
54. Reverse Idler Gear
55. Reverse Idler Gear Shaft
56. Roll Pin
57. Drain and Filler Plugs
58. Magnetic Chip Collector (Bonded to Case)
59. Power Take Off Cover
60. Power Take Off Cover Gasket *
61. Countershaft Front Bearing (#2412 for 3/4-ton version, #2412 or #1206 for 1-ton version) *
62. Countershaft Assembly
63. Countershaft Rear Bearing Oil Deflector
64. Countershaft Rear Bearing *
65. Countershaft Bearing Snap Ring *
66. Snap Ring *

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.

SM420 Front Input Seal Upgrade

Kit 420GM, SM420 front seal & billet retainer conversion

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