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: $1750.
    • SM420 to GM engines, with a Novak transfer case adapter assembly, including pilot bushing, a new straight shifter stick and urethane mount: $2250.
    • 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: $2350.

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.