Adapting the
GM Muncie SM420 Transmission
to the
Jeep Models 207, 231, 242, 249 & 241OR Transfer Cases

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.

Kit 421, adapting the SM420 transmission to the Jeep NP 207, 231, 241, 241OR, 242 and 249 transfer cases Part #: 421* Item in Production Weight: 15.92 Price: $559.00 Buy: Includes the adapter, SM420 mainshaft, mount base, fastening hardware, seal, gaskets and instructions ** Note that our mainshaft is the first gear splines "skip-tooth" version mainshaft with 11 splines. A few rare SM420's feature 22 teeth. Check for compatibility prior to ordering. * This item may still be ordered. We are currently in the production process, and expect this item to ship soon .

The adapter kit is designed for compatibility with the Jeep NP207, NP231, NP242, NP249 & NP241OR "RockTrac" transfer cases.

The Adapter
The Novak SM420 to Jeep transfer cases adapter is very 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 and a heavy cross-section.

Transmission Versions
Any version (2wd or 4wd) and year of the SM420 can be adapted to these Jeep transfer cases. There is not a particular advantage to 2wd or 4wd versions.

Applicable 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 inteference 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.

Transmission Mainshaft Replacement
This SM420 conversion kit comes with a new output shaft to be installed in your Muncie transmission that was originally a 2wd or 4wd version - either works equally well. The Novak mainshaft is of a premium alloy steel, precision splined and specially hardened for strength and against wear, providing a superior interchange. This shaft connects the transmission output directly into the existing 23 spline NP / NVG input gear for the strongest connection. Transfer case disassembly is not typically required unless you have a 21 spline version (see note below).

Mainshaft replacement is largely straightforward. Good work is within easy reach of the careful home mechanic and instructions are included. We also recommend our basic gasket & seal kit, or our all-inclusive master rebuild kit, depending on your assessment of your transmission's condition. See our SM420 parts page for information on these items.

Transfer Case Input Gears

NP 231 The NP231 may have 21 or 23 splines and a long or short style input gear NP 242 The 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 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. 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) is longer than 2.2". These will require shortening with a cut-off saw and/or lathe and finished with edge breaking chamfers	New Process input gears come in varying spline counts and lengths. If you have a 21 spline input gear, you will need to install a 23 spline version for this SM420 conversion. A seal spacer sleeve (image, below) is included and pre-installed in your Novak adapter in the event your transfer case has a short input gear.
21 vs. 23 splines Our SM420 to NP231 transfer case conversion kit is available only with a 23 spline output shaft. Because of the torque the SM420 can produce, we have seen limited instances where 21 spline shafts have failed where 23 would not have. You may purchase from us a 23 spline input gear for your NP2xx in conjunction with this 23 spline version of our kit.

Transfer Case Strength and Condition
The New Process / New Venture (NP / NVG) transfer cases are very well matched to the SM420. They are uncommonly strong, feature good gearing and are compact and servicable. They are capable of power ranging up through V6s, V8s and even the Chevy Big Blocks.

It is not requisite that you rebuild your transfer case to successfully complete the conversion, but it may be a good time to do so if your transfer case leaks or has many miles of service. We've made our 231 service kits and our all-inclusive master rebuild kits available for this purpose.

Engine Compatabilities & Engine Mounting
AMC Jeep engines are compatible with the GM SM420 transmission. See Adapting GM Transmissions to AMC Bellhousings. AMC (258, 4.0, etc.) engines may be left in the factory location in virtually all SM420 installations.

An SM420 installation is often done in conjunction with an engine conversion into the Jeep. These engines include the Chevrolet Small Block V6 & V8, from Generations I, II, & III, 1955-2006. Carbureted, TBI and TPI engines are all compatible with this transmission.

Novak's MM Series engine mounts, featuring excellent vibration dampening, superior strength and impressive adjustability - adjustable even after the engine is installed. The design and strength and affordability of our mounts are second to none.

Engine Crank to Transmission Pilot Bushing
This transmission can be married to stock (i.e., AMC I6) as well as conversion (i.e., GM V6 / V8) engines, and a custom pilot bushing will be required in many instances. Some common choices with the SM420 transmission are listed below:

>Bushing, pilot, AMC cranks to the SM420 Bushing, pilot, AMC I6 & V8 cranks (1972-2006) to GM SM420 & SM465 transmissions Part #: PB-3181295 Weight: 0.13 Price: $21.40 Buy: Porous bronze bushing in an aluminum housing. Installs in the crank outer bore. >Bushing, pilot, GM Gen. I-III (1955-2000) cranks to SM420 Bushing, pilot, GM Gen I-III (1955-2000) cranks to GM Saginaw Muncie & Jeep T86, T14 (1968-1971), T176, T4, T5, AX15 (1988-1991) transmissions Part #: PB-3901295 Weight: 0.11 Price: $5.72 Buy: Porous bronze material. Installs in the crank inner bore. >Bushing, pilot, GM Gen. III+ (1997-present) cranks to SM420 Bushing, pilot, GM Gen. III (2001- ) cranks to GM Gen. I transmissions, including Jeep T86, T14 (1966-1971) & T176, T4, T5 & AX15 (1988-1991) Part #: PB-6071295 Weight: 0.18 Price: $24.80 Buy: Porous bronze bushing in an aluminum housing. Installs in the crank outer bore For pilot bushings not listed, see our full collection.

 

Mount, rear, transmission, polyurethane Chevy, Jeep, Novak style Part #: RMU Weight: 1.60 Price: $30.82 Buy:

Transmission Rear Mount
This transfer case adapter's design is based on Novak's Universal Series adapters that feature a modular mounting and support system. Included with the adapter kit is a steel base that is a highly configurable mounting foot, and is ready to accept our industry standard urethane isolator mount, available for purchase (below). This is our preferred rear mount unit. Our kit is designed for it, and its usage is essential to protect powertrain components from the flex and jar typical of any automobile, but especially Jeeps.

Whether you are assembling onto a stock cross member / skidpan or if you are setting up a high-clearance skid plate, you will find it to be a clean and simple process.

Transfer Case Clocking
Our adapter is clocked optimally for slightly better ground clearance than factory locations, which is actually quite good. The Jeep transfer case belly will sit a fair amount higher than the transmission itself.

Many of our customers upgrade their NP / NVG shifter while doing the conversion. Our #SK2X makes the conversion easier than it's ever been.

Transfer Case Shifter
This transmission conversion will allow for the retention of the factory shifters, or allow you to upgrade to the Novak #SK2X shifter assembly for a simpler conversion and a clean shifting, direct linkage system.

Transmission Shifter
Use of the OEM style transmission shifter for the T18 is easily done. The installer may opt to bend the stick to fit the particular Jeep application. We can supply a new, straight stick that replaces the factory unit. This stick can be custom bent to the driver's preference and will sometimes restore a more solid feeling to the shifting of an SM420.

Super short! The Novak adapter helps pack a whole lot of four-speed punch into a small space, making for longer rear driveshafts and better angles.

Yoke & Driveshaft Clearance
Use of the factory size driveshaft and yoke is recommended, but there is room for larger units if desired. Our adapter design has maximized clearance between the transmission and yoke. No transmission modifications are required for clearance on this wise.

Driveshafts
Driveshaft length changes are sometimes required, as this geartrain combination is typically shorter than most factory Jeep combinations. Some potential conversion engine installations sit further ahead than OEM engines in the Jeep's bay and will allow for better driveshaft angles, especially when done with this very short combination.

A turn-key Muncie 420, professionally built, adapted and delivered to your door - ready for a variety of engines and Jeep transfer cases. Read more...

Some of our customers, fearful at the perceived expense of new or modified driveshafts, attempt to let the existing driveshafts dictate engine, transmission and transfer case location, sometimes to the detriment of the project. Driveshaft modifications are usually inexpensive when performed by driveline, RV or tractor implement specialists. New driveshafts are an option but seldom a requirement in regards to the actual successful conversion. Jeeps that require extensive travel or specialty-built driveshafts have this option available through several fabricators across the nation.

Axles
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.