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.
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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.
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
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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. |
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 21 vs. 23 splinesOur 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. |
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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.
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| 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, GM Gen. I-III (1955-2000) cranks to SM420>Bushing, pilot, GM Gen. III (2001-present) cranks to SM420For pilot bushings not listed, see our full collection. |
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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.
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| 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.
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| 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.
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| 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.