Adapting the

GM Muncie SM420 Transmission

to the

Jeep Models 207, 231, 242, 249 & 241OR Transfer Cases

Adapter kit for 4L80E to Jeep transfer cases

Kit 421-23, adapting SM420 transmissions to the Jeep NP 207, 231, 241, 241OR, 242 and 249 transfer cases, 23 splines

Kit Contents

  • Adapter housing
  • Transmission mainshaft, 23 spline output
  • Seal extension
  • Seal
  • Transmission to adapter gasket
  • Adapter to transfer case gasket
  • Steel mounting base foot
  • Fasteners
  • Instructions

Our SM420 to the Jeep New Process (or New Venture) transfer cases conversion makes for an absolutely strong and capable assembly both on-road and off-road. It features a superior crawling gear, broad gear span, and compatibility with great engines.

The Novak SM420 to Jeep transfer cases adapter is compact at only 3.2" long for a combined transmission and adapter length of 13.9". The adapter itself is cast and machined of 356-T6 aluminum alloy, featuring thick flanges, a heavy cross-section, and strength-adding gussets. It is drilled for over 20 degrees of transfer case clocking locations, though the provided factory angle will suffice for most New Process transfer cases.

The new Novak manufactured 23 spline output shaft is machined from premium alloy steel and hardened for maximum strength and longevity. Ready to be installed in your Borg-Warner transmission and then spline directly into the OEM style input gear of your NP transfer case.

This transfer case adapter's design features a modular mounting and support system and mounting base, ready to accept our industry standard urethane isolator mount, available for purchase below.

SM420 to Jeep transfer case adapter
The Novak adapter helps pack a whole lot of four-speed punch into a small space, making for longer rear driveshafts and better angles.

Applications & Compatibility

Compatible Transmissions

SM420 transmission with adapter

Any version of the SM420, including 2wd, 4wd early, & 4wd late styles are compatible with this adapter assembly. There are no advantages between the 2wd vs. 4wd versions of this transmission.

Mainshaft replacement is largely straightforward. Good work is within easy reach of the careful home mechanic and instructions are included. We recommend at least our basic gasket kit. Better yet, this is a good time for a more thorough master rebuild and update of the transmission.

Some installers consider one of our complete Novak remanufactured and upgraded transmission packages. This is a professionally built and dyno-tested transmission, delivered ready to bolt into your Jeep build.

Compatible NP & NVG Transfer Cases

The adapter kit is designed for compatibility with 23 spline Jeep NP207, NP231, NP242, NP249, NP241 & NP241OR "RockTrac" transfer cases. Below is a summary of spline counts and lengths usually encountered.

Transfer case seal spacer
New Process input gears come in varying spline counts and lengths. Whichever you have, we've got you covered. A seal spacer sleeve is included and pre-installed in your Novak adapter in the event your transfer case has a short input gear.
  • NP 231 - The NP231 may have 21 or 23 splines and a long or short style input gear
  • NP 242 - The Jeep version NP242 may have 21 splines for the 1987-1990 versions. When connected to the 4.0L HO engine (1991- ) the 242 will have 23 splines as will the Dodge and later V8 Grand Cherokee, AMG versions are 32 splines long
  • 249 - The NP249 always has 23 splines, but may have a long or short style input gear
  • 241OR - The RockTrac will always have a 23 spline, short style input gear. Its gear is unique and not interchangeable with any of the other versions due to the lower 4:1 ratio
  • NP207 - The NP207 may have 21 or 23 splines. Many versions feature extra-long input gears whose stick-out lengths (as measured from the front edge of the input gear to the transfer case front mounting face) longer than 2.2". These will require shortening with a cut-off saw and/or lathe and finished with edge breaking chamfers. New 23 spline NP207 input gears are available in Related Products below.

A more detailed discussion of New Process input gears is found here.

No 21 Spline Version?

The 21 spline shaft is substantially weaker than the 23 spline version. 21 spline versions were only used in the 4 cylinder applications and some very early automatic 6-cylinder versions. Novak does not make a 21 spline version of this adapter kit because of this weakness, vs. the exceptional torque created by the compound low gear of this transmission.

Our recommendation is to change the transfer case input gear to a 23 spline version. See Related Products, below.

Compatible Jeeps

Many of these conversions are for the YJ Jeeps, in which the SM420 is a terrific match. Even earlier Jeeps that are being retrofitted with the NP231 or similar later transfer cases accept the SM420 well. However, TJ and XJ Jeeps with their significant dash consoles may pose an interference problem with the SM420's shifter and top tower. The 420 is a shorter and taller transmission than factory options in these later Jeeps. Individuals should be aware of this caveat.

Compatible Engines

Any engine that can be made to work with the SM465 will work with this conversion. They include:

Related Products

Urethane Rear Mount for Crossmember
Parts for the SM420
Transfer Case Input Gear, 23 Spline
Transfer Case Input Gear, 23 Spline
NP207 Input Gear 23 Spline
Shifter stick for the SM420 transmission
Shifter Retention Kit, SM420, SM465, T176, NP435
1976-1986 Jeep floor tunnel cover
Billet PTO Cover
NP231 transfer case parts
NP242 transfer case parts
Adapting the SM420 to AMC 2.5L Engines
Turn-Key SM420

Installation Requirements


The transmission shaft installation takes more specialized tools, heavy snap ring pliers, a press to remove and replace bearings, etc. Normal mechanics tools are required for the balance of the installation.

Clocking Information & Driveshaft Clearance

Our adapter is drilled for multiple clocking locations and driver’s side drop transfer cases usually clear nicely in most all applications.

Engines & Engine Mounting

GM Engine Mounts for Jeeps

Factory engine placement on YJ Jeeps is perfect for the SM420 transmission, whether using an I4 or an I6 engine. If installing an AMC or GM V8, your engine mounts should permit about a 1-1/4" offset towards the driver's side. Novak's engine mounts , featuring excellent vibration dampening, superior strength, and impressive adjustability - adjustable even after the engine is installed. The design, strength, and affordability of our mounts are second to none.


Driveshaft length changes are often required. Most conversions to these transmissions will require that the rear driveshaft be modified to be longer and front driveshaft shorter.

Some installers, concerned about the expense of new or modified driveshafts, attempt to let the existing driveshafts dictate engine, transmission and transfer case location, often to the detriment of the project. Our recommendation is to prioritize the correct position of drivetrain components over saving a few dollars which is usually regretted in the long run with compromised positioning.

Jeeps that require extensive travel or specialty-built driveshafts have this option available through several fabricators across the nation. These are normally specified after placement of the new transmission and measured at vehicle ride height. As the rear driveline gets shorter, it is often advantageous to us a Double Cardan or “CV style” rear shaft with the correct geometry at the axle to minimize vibrations and possible binding.


Use of factory axles is completely acceptable with this conversion. Axle upgrades are not necessary, but they may be chosen for reasons external to this transmission upgrade.

Also See