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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.

Adapting the T18 & T19 to Jeep Transfer Cases
Kit 421, adapting the SM420 transmission to the Jeep NP / NVG 207, 231, 242, 249 and 241OR transfer cases
Part: #421
Weight: 17 lbs.
Price: $492.00
Options:
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Notes: Includes the adapter, transmission mainshaft, seals, gaskets, fastening hardware and instructions

Seal SpacerChoose a seal spacer sleeve if your transfer case has a short input gear (discussed below). If you're not sure of the length of your input gear, you may order the spacer and then return it for full credit if not used in your conversion.
Spacer, seal, for short style NP / NVG input shafts (stickout of less than 2.1")
Part: #SS231
Weight: 0.4 lbs.
Price: $19.03
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Notes: Consists of a seal spacer, required if you are retaining a short 231 input gear in your conversion

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.

Transfer Case Input Gears
The included transmission mainshaft splines in directly into the OEM Jeep input gears, and transfer case disassembly is not typically required. Transfer case input gear styles vary by length and spline count. If your transfer case has a short style input gear, add our #SS231 seal spacer, above.

Jeep transfer case planetary input gear
  • 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. This will require the seal sleeve spacer, above.
  • 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. Whichever you have, we've got you covered.

 

A 231 transfer caseTransfer 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 Jeep Motor Mounts
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 cranks to SM420 / SM465 transmissions
Part: #A6932S
Weight: 0.16 lbs.
Price: $23.19
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>Bushing, pilot, GM Gen. I-III (1955-2000) cranks to SM420

Bushing, pilot, GM Gen I-III (1955-2000) cranks to GM transmissions
Part: #6914
Weight: 0.11 lbs.
Price: $5.72
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Notes:

Also compatible with:

  • '66-'71 T86 & T14 Transmissions
  • '80-'86 T176 Transmissions
  • '80-'86 SR4, T4 & T5 Transmissions
  • '88-'94 AX15 Transmission

>Bushing, pilot, GM Gen. III (2001-present) cranks to SM420

Bushing, pilot, GM Gen. III (2001- ) cranks to GM Gen. I transmissions
Part: #A6914G1-G3
Weight: 0.25 lbs.
Price: $21.32
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Notes:

Also compatible with:

  • '66-'71 T86 & T14 Transmissions
  • '80-'86 T176 Transmissions
  • '80-'86 SR4, T4 & T5 Transmissions
  • '88-'91 AX15 Transmission

For pilot bushings not listed, see our full collection.

 

Urethan Rear Mount for the Adapter Kit
Mount, rear, transmission, polyurethane Chevy / Jeep / Novak style
Part: #RMU
Weight: 1 lbs.
Price: $38.27
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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.

Jeep transfer case shifter
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.

SM-420, adapted
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 complete, adapted 420 ready to ship
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.