Adapting the

GM Muncie SM420

to the

Jeep Dana Model 300 & FSJ New Process Transfer Cases

Our SM420 to Dana 300 conversion makes for a low geared, and exceptionally durable assembly both on-road and off-road. It still a popular manual shift transmission conversion into Jeeps, providing a compact geartrain, a low crawling gear, compatibility with excellent engines, and ease of installation.

The Novak SM420 to Jeep Model 300 transfer case 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 a high-grade aluminum alloy, featuring thick flanges, a heavy cross-section and strength-adding gussets.

Our adapter is drilled for multiple clocking locations. This will permit the installer to optimize the angle of the transfer case. However, shifters, clearances and other considerations may be limiting factors. 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 in Related Products, below.

Any version of the SM420 can be made to be compatible with this adapter assembly and as such, there are two options to this transmission to transfer case conversion. Neither one method nor the other is any stronger than the other, so choose based on the installation factors discussed, as follows:

Option I: SM420 10 Spline to Dana 300 Transfer Case Input Shaft

Adapter kit for SM465 to Jeep transfer cases

Kit 423-10, adapting SM420 transmissions to Jeep Dana 300 transfer cases. 10 spline transfer case input shaft version

Nearly every SM420 features a 10 spline output shaft protruding 2-5/8" from the rear of the case and is compatible with this kit. Disassembly of the transmission is unnecessary. However, partial disassembly of the transfer case is required to install the provided 10 spline Dana 300 input shaft. Note that there are rare versions of the SM420 which feature a 35 spline threaded output shaft, for which we recommend the Option II kit, below. Check your version before ordering.

The input shaft installation is a 1-2 hour bench job and usually includes either a simple gasket and seal replacement or preferably a full rebuild of the Dana 300. It is not requisite that you rebuild your Dana 300 to successfully complete the conversion. However, it is a good idea if your transfer case leaks or has many years of service. See our select Dana 300 gaskets, master rebuild kits and upgrade components in Related Products, below.

This kit version is not compatible with FSJ New Process transfer cases. See below.

Kit Contents

  • Adapter housing
  • Transfer case input shaft, 10 spline / 30 spline
  • Seal
  • Gaskets
  • Fasteners
  • Steel mounting foot base for crossmember mount
  • Instructions

Option II: SM420 Replacement Main Shaft with 23 Spline Output

Adapter kit for SM465 to Jeep transfer cases

Kit 423-23, adapting SM420 transmissions to Jeep Dana 300 & FSJ NP transfer cases, 23 spline transmission mainshaft version

A new, 23 spline mainshaft is provided, which splines directly into the factory Dana 300 and FSJ New Process transfer cases, with no internal modifications to these transfer cases required. SM420 mainshaft replacement is required. These are somewhat technical rebuilds but good work is within reach of the careful home mechanic and instructions are included. We recommend at least our basic gasket & seal kit. Better yet, this may be a good time for a more thorough master rebuild and update of the transmission. See Related Products, below.

Nearly *all versions of the SM420 are compatible with this method, including rarer instances of that feature a 35 spline threaded end output shaft. you may choose this option if you have the 35 spline version, are unsure what your transmission is or may be, are rebuilding the transmission anyway, or simply would like a new mainshaft in the SM420. This is the shaft we use in our Novak-built transmission as mentioned below. This is the only version that will work with the 208, 219, 228 & 229 series transfer case.

*Rare instances of the early SM420 feature 22 splines on the mainshaft 1st-2nd gear journal, before GM revised this to the "skip-tooth", 11 spline version mainshaft with precision ground journal. If your 420 features this shaft, your 1st sliding gear will not work with the new Novak mainshaft. Acquiring the correct (and vastly more popular) 11-spline first gear would be required here, or choose Option I above if you can.

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.

Kit Contents

  • Adapter housing
  • Transmission mainshaft with 23 output splines
  • Seal
  • Gaskets
  • Fasteners
  • Steel mounting foot base for crossmember mount
  • Instructions

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 Jeeps

Compatible with most Jeeps that originally featured the D300 and FSJ transfer cases. This combination may also work in early CJ's, though the floor tunnel may require rework. Wagons, Commando's, Full Size Jeeps, etc. are all good candidiates for this conversion. XJ / MJ Jeeps aren't as receptive to the SM420, given their tight tunnels and dash interference with the shifter.

Compatible Engines

Any engine that can be made to work with the SM420 will work with this conversion. They include:

Related Products

Urethane Rear Mount for Crossmember
Parts for the SM420
Dana 300 twin stick shifter
Shifter stick for the SM420 transmission
Shifter Retention Kit, SM420, SM465, T176, NP435
1976-1986 Jeep floor tunnel cover
Billet PTO Cover
Dana 300 Rebuild Kit
Dana 300 Super Short Tailhousing
Billet aluminum Dana transfer case pan
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.

Engines & Engine Mounting

GM Engine Mounts for Jeeps

Factory engine offset and placement on Buick V6 Jeeps (1967-1971) is great for the SM420 transmission, as well as AMC I6 and V8 in 1972-1979 CJ models. If installing a GM V8, your engine mounts should permit about a 1-1/4" offset towards the driver's side. Novak's engine mounts, feature 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. Many conversions to these transmissions will require that the rear driveshaft be modified to be longer (usually welcomed) 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