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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.

Product & Features

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***Please read 21 spline notes below before purchase*** 

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 356T6 aluminum heat treated alloy, featuring thick flanges, a heavy cross-section, and strength-adding gussets.

Transfer Case Clocking & Rotation Options 

Our adapter is drilled for multiple clocking locations from near-factory (20 degrees down) to near-flat (3 degrees down). However, pan clearances and front axle width may inhibit the flatter options.

Rear Mount Provision 

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 and is available for purchase below.

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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.

The new Novak manufactured 23 (or 21) 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.

Contents

The kit contains aluminum adapter housing, transmission output shaft, seal with extension, transfer case to adapter gasket, transmission to adapter gasket, fastening hardware, steel mounting base, and instructions.

Applications & Compatibility

sm420_fds_w_423_421_adapterTransmissions

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. Many consider one of our complete Novak remanufactured and upgraded transmission packages. The advantage of a superior, professionally built transmission, tested and ready to bolt in delivered to you is definitely worthy of consideration. 

NP & NVG Transfer Cases 

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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.

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

  • 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) is longer than 2.2". These will require shortening with a cut-off saw and/or lathe and finished with edge breaking chamfers

Strength of 21 spline versions and warranty exclusion

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The 21 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. WE DO NOT OFFER A WARRANTY ON ANY OF THE 21 SPLINE SHAFTS because of this weakness. Our recommendation is to change the input to a 23 spline version as sold below and don’t put a drivetrain “fuse” in a bad location. On lower powered V6 applications in a lightweight vehicle driven conservatively you may get away with it.

Output Shaft Strength

broken_advance_adapters_shaftWe’ve seen a handful of shafts broken like this one, and heard our customers speak of more failures of output shafts made by other adapter companies. This is why we go through the trouble and expense of making our output shafts from a solid bar of exceptionally strong 300M alloy steel, instead of lower grade alloys or friction welding salvage GM shafts. A second, sometimes less obvious but pernicious problem is the issue of warpage during friction welding and heat treating - which cannot be corrected when the shaft journals are already ground and worn at or below factory specifications. Novak shafts receive a final hard turn of bearing journals after the heat treat process with special tooling and cooling process to ensure the ultimate finish. The Novak shaft is produced to factory specifications with stress risers removed using much stronger steel alloy. Our 4-axis CNC cut of the spiral gear, oil feed passages and other features are no small tasks. This attention to detail, maximized strength through design and material along with precise tolerances provide the ultimate parts for your project.

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.

Engines

Any engine that can be made to work with the chosen transmission will work with this adapter. We have available parts to adapt several versions to various engines. See the SM420 information page linked below for that discussion. A procedure adapting to the Jeep engines is shown here.

Related Products

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. This is our preferred rear mount unit. Our kits are designed for it, and its usage is essential to protect powertrain components from the flex and movement typical of any automobile, but especially Jeeps.

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

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GM Transmission Pilot Bushing

A pilot bushing is always required for installation choose the correct version for your application from these choices

AMC Cranks



GM Cranks



SM420 Rebuild Kits 

Though not necessarily required, it is our recommendation to do a full rebuild on these units while you are in the transmission. These are the best parts available. Additional hard parts are no longer manufactured and get increasingly more difficult to obtain. If you have pieces that need to be replaced, it is often best to look at the full transmission option above or keep additional cores on hand. If you need other parts, you are welcome to call in. Please don’t be offended if our sales tech chuckles when you ask for first or second gear or simply quotes you the price of a complete unit. Our instructions accompany all rebuild kits. At the very least you will want the gaskets, but we do recommend the full rebuild kit.

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Gasket Kit

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Sticks

Straight sticks are available if yours is work or you simply want to start with a clean slate to get the bends right.

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Retainer Kit

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Billet PTO Cover 

Why not add a little “Bling” to that transmission? These beautiful billet covers also seal up substantially better than the stock stamped steel piece. Depending on the transmission you may need 1 or 2 of them.

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Transmission Tunnel Cover

Because the transmission and transfer case shifters are often in a different location than factory in these Jeeps, this new transmission tunnel cover gives the installer a clean slate. There are no screw holes nor transmission & transfer case shifter holes, allowing the installer to drill and cut them as necessary. An easy way to get a perfect fit is to use the old cover as a template, making cuts as necessary, then overlay on the new aluminum piece for perfect holes.

The finish is natural brushed aluminum, which looks pretty terrific, but it can easily be primed and painted to suit, if desired.

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New Process Transfer Case Shifter Options 

Often the shifters are linked to your old transmission, body, or both. As things move or even just to have a better setup these shifter solutions prove invaluable and extremely popular.

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New Process Rebuild Kits 

The New Process (or later New Venture Gear) transfer cases are unusually strong for their mass and size. As long as your chain isn’t stretched and you keep up with fluid levels they will take a surprising amount of power and abuse. Like anything else, time and use takes its toll and these rebuild kits consist of the best available components including top quality chains and bearings. The rebuild process is not overly difficult, and if you are there anyway to put in an input gear, our advice is to rebuild, especially considering that the chains can stretch with use.

The 231 kits are divided early and late about 1993-1994. However there are no guarantees that things are original or that the change was made as a clean break 93 down and 94 up. If in question, the input bearing width on these models is a good indicator. Early uses a 24mm wide bearing and late is 16mm wide.

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The Rubicon 241OR rebuild kit is available here.


The 242 kits are a little more pragmatic with chain use. Most of the light Jeep models are the same but there are some variations in those that used a V8 as well as the Liberty and AMG versions. These kits are offered as rebuild components and the chains separately. The same guidance applies to year breaks as the 231 for the rebuild kits. Chains are model and style specific.

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Chains for the 242 had 2 pitch sizes, 2 joint styles and 2 widths and all in different combinations. 

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Chain Style


Use the above illustration in the event you are not certain about the provenance of your NP242 transfer case chain joint type.
 

This is the most common chain used in the XJ and other light duty Jeeps.


The WJ Grand Cherokee and Dodge V8 applications commonly used this wider chain.


The Liberty is a unique animal with this version and rebuild kits vary as well.


Some Jeep XJ & ZJ (Europe), NP242ECE models used this chain.


The H1 AMG HMMWV, Humvee, Hummer version is 1-1/4” wide and the coarse pitch.


23 Spline Input Gears

If you have a 21 spline input transfer case, our recommendation is to replace the input gear with a 23 spline version. The NP/NVG input gears are divided early and late about 1993-1994. However there are no guarantees that the transfer case is original or that the gear change was made as a clean break 93 down and 94 up. If in question, the input bearing width on these models is a good indicator. “E” Early uses a 24mm wide bearing and “L” late is 16mm wide. The photos show the differences in width of the bearing location and the stepped shoulder between the bearing and the gear. 23 spline input gears are available to replace the anemic 21 spline found in all too many applications. Please select the appropriate gear below.

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Installation Requirements

Tools

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.

Placement 

Engine placement in these applications is an effort in compromise to find the best position overall as a package. Generally the engine will be about 1” or even more towards the left, (driver’s side in the USA) away from the front differential in a CJ application. This gives the best balance of weight, and more importantly clearance, for the driveshaft going to the front axle. Your tight spots will be steering to exhaust on the left (hold that as tight as you can) and clearance for the front driveshaft on the right. On a driver’s side drop transfer case, usually the later Jeeps, things often get a little easier as steering and front driveshaft are pushing you the same direction. Fore and aft position will vary with the Jeep model and engine.

Have a CJ5 and Gen I with a rear distributor? You’ll be better off a little forward for more rear driveshaft length and clearance for that HEI. If you are in a longer Jeep and using an LS engine with no distributor, you’ll have more fan clearance and better balance if you hold it to the rear. Usually for ground clearance tuck things up nicely for height to avoid damage to oil pans and other life giving parts in the Jeep. On later Jeeps with a Driver’s side drop transfer case and front differential, things are a little easier as the steering and offset are both pushing the drivetrain to the passenger’s side. Again, that 1 to 1-1/2” dimension is usually where you want to be. Common sense and taking a step back to look at things overall goes a long way.

Clocking Information & Driveshaft to Pan 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

mmtj-g3If converting to a V8 in conjunction with this upgrade, aftermarket engine mounts are available to ease its installation. Because of the broad spectrum of engines and vehicle combinations used with this adapter, it would be difficult to list them all here. Whatever your application, Novak likely has the perfect solution. From a 225 Buick V6 in a CJ2A to an LS3 in your JK, we’ve got you covered. 

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.

Driveshafts

Driveshaft length changes are often required. Most conversions to these transmissions will require that the rear driveshaft be modified to be shorter and front driveshaft longer. 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, usually to the detriment of the project. Our recommendation is to always value the correct position of drivetrain components over saving a few dollars which is usually regretted in the long run with compromised positioning. Sometimes the application allows use of the existing shafts without modification. While that is perfectly acceptable, it should not be the first priority. Driveshaft modifications and rebalancing can be affordable when performed by driveline, RV or tractor implement specialists. New driveshafts are an option, but not necessarily a requirement in regards to the actual successful conversion if your existing driveshafts are in good condition. 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. Novak can also accommodate your needs on this if provided with dimensions for the shafts at a reasonable rate.

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.

Also See