Complete SM420 Transmission Package

The SM420 is a legend, and remains one of the greatest Jeep conversion transmissions of all-time.


sm420_fds_w_423_421_adapterOur Muncie 420 is professionally and meticulously rebuilt. We use the best components, including bearings, synchronizers, bushings, seals and gaskets built by our SM420 specialists and experts. Any required updates and improvements are performed, including an industry exclusive remanufacture of the integrated first-to-second gear syncrho ring.

The 420 will be built using a GM style mainshaft or a Novak conversion mainshaft, as per your application. Included and installed are the Novak transfer case adapter and crossmember urethane mount assembly.

A new, straight transmission shifter cane is included and ready for the installer to bend to the particular application and driver's preference (if you wish you use your factory stick, let us know and we'll apply a discount).

Applications & Compatibility

Factory Style Build vs. Adapted Build

Our factory style builds are OEM style replacement transmissions that may require the use of the vehicle owner's original tailhousing (either 2wd or 4wd) to complete the installation. 

Adapted builds are designed primarily for installation into Jeep vehicles and include the installed aftermarket Novak mainshaft, adapter kit, rear mount, and pilot bushing as indicated for the customer's scenario.


An example of a Novak adapter, installed and ready.

Transfer Case Adaptability 

Your 420 can be ordered ready to join to the following Jeep transfer cases:

We can also do a full rebuild of your Jeep transfer case if you would like to ship it to us. We can provide any upgrade components for it that you wish. It will be pre-installed to the transmission and ready to lift into your Jeep.

Bellhousing Compatibility 

We will also configure your front input section for a variety of OEM or conversion bellhousings including:

  • GM 4-11/16" bore bellhousings
  • GM 5-1/8" bore bellhousings
  • AMC I6 (258 & 4.0L) & V8 bellhousings

We can often source a bellhousing or use your particular GM or AMC bellhousing as required. Conversions to AMC bellhousings will include necessary machining for Crank Position Sensors (CPS), face pockets (if necessary) and the bellhousing rear face bolt pattern. The pilot bushing is included as specified.


Your SM420's Construction:

We start with a great core transmission and do a full disassembly and breakdown

Detailed parts inspection and measurement are performed

The main case and top cover are hot-cleaned, media blasted, primed and enameled with a tough, industrial grade finish

The top shifter cover is fully rebuilt with new springs, poppets and hardware

Full gearbox reassembly using all new bearings, bushings, synchronizers, snap rings - all consumable transmission parts - are replaced and new hard parts are installed wherever indicated

A new reverse switch with simple spade style terminals is installed, even on models that originally didn't feature one

Your transfer case and bellhousing (if available) are installed and configured

The transmission undergoes a 24 hour leakdown test, after which it is fully stress-tested through its full operational range on our 300 HP test stand


Your transmission package is palleted, wrapped, fully insured and ready to bolt into your application. Novak will seek the best competitive shipping rates with our extensive trucking networks. It will arrive at a location of your choosing, though we strongly recommend a business location with a dock / forklift for delivery, as residential and liftgate fees can add up to an extra $100. per delivery.


Pricing is generally as follows:

  • SM420 factory style build: $1590.
    • SM420 to GM engines, with a Novak transfer case adapter assembly, including pilot bushing, a new straight shifter stick and urethane mount: $2090.
    • SM420 to AMC engines, with a Novak transfer case adapter assembly, including special front retainer and pilot bushing, a new straight shifter stick and urethane mount: $2190.

Individuals considering the SM420 for many conversion situations can also consider the more modern and more affordable SM465 transmission package.


Because of the many variations involved, we encourage you to contact us directly by phone or e-mail. We will help you put together your transmission to best fit your requirements. 


Parts for the

SM420 Transmission

sm420_transmissionThe 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 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.

IMPORTANT: For #SM420-1 and SM420-2 kits, the 3rd - 4th synchro rings are not available and not included as they once were. However, we are planning to manufacture these items shortly and will make them available as soon as they are available.

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 and affordably so; 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.


Key Items of Interest:





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.