Adapting the

GM Muncie SM420

to the

Jeep Dana Model 300 Transfer Case

Our SM420 to Dana 300 conversion makes for an extremely strong, and capable assembly both on-road and off-road. It is one of the most persistently popular manual shift transmission conversions into Jeeps, providing a very compact geartrain, a supreme crawling gear, compatibility with excellent engines, and ease of installation.

Product & Features


The Adapter

The Novak SM420 to Jeep Model 300 transfer case 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 features a modular mounting and support system and mounting base, ready to accept our industry standard urethane isolator mount, available for purchase below.


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 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 D300 or with the input version of the kit it will spline on to the short 10 spline output of your SM420.


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

Applications & Compatibility

Jeep New Process 208, 219, 228 & 229 Transfer Cases

We've been known to recommend replacing the older style FSJ New Process transfer cases (especially the troublesome 219, 228 and 229) with a newer, more modern version such as the part-time 231J with HD SYE kit installed and upgrades as appropriate. 

However, if your transfer case is in good condition, then move forward with the conversion.

Output Shaft Version “A” 


Nearly *all versions of the SM420, including rarer instances of that feature a 35 spline threaded end output shaft, are compatible with this adapter assembly. 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 also the best choice if you choose a Novak built transmission as mentioned below. This is the only version that will work with the early 208 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. 

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 & seal 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.

sm420_fds_w_423_421_adapterTransfer Case

All Jeep Dana 300 (and FSJ New Process) transfer case input shafts / gears feature 23 splines. Our included Mainshaft will spline in directly to these input shafts with no transfer case modifications required.

Input Shaft Version “B”


Only the short 10 spline SM420 style transmission output is compatible with this adapter assembly. This is the most common version of SM420. Disassembly of the transmission is unnecessary as the 10 spline style output shaft is correct for this kit version. Partial disassembly of the transfer case is necessary to install a new matching 10 spline Dana 300 input shaft machined from triple-alloy gear steel which provides a superior upgrade.

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 a tired D300. It is not requisite that you rebuild your Dana 300 to successfully complete the conversion. However, it is certainly worth consideration especially if your transfer case leaks or has many years of service. Our superior quality Dana 300 gaskets & seals kits and our all-inclusive master rebuild kits with stronger Novak manufactured center pin are highly recommended for this purpose.


Compatible with most Jeeps that originally featured the D300 transfer case. If you are retrofitting both, keep in mind length and shifter location to ensure clearance for both. 


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 has a structural base for an 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.


D300 Transfer Case Shifter

The stock D300 shifter may be used, possibly requiring minor modifications. Often it is advantageous to use a new twin stick style shifter, especially if you are able to remove the interlock pins during the transfer case rebuild process. For the Jeep Dana 300, we recommend two twin stick shifters. One features an adjustable length.


Another great option for the D300 is our Dana 300 Transfer Case Twin-Stick Shifter Kit which is a solid, high quality option but lacks the forward adjustability of the transfer case shifters. It has the option of billet knobs showing the engraved shift pattern.



All would require the


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.


Gasket Kit



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


Retainer Kit


Billet PTO Cover 

These billet aluminum, anodized billet covers seal up substantially better than the factory stamped steel covers. Check your transmission to see if it requires one or two of them. 



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.

This tunnel cover is compatible only with 1976-1986 CJ Jeeps.



D300 Rebuild and Gasket Options


Gasket and Seals Kit

Dana 300 Billet Bottom Cover Pan


These rigid and precisely machined billet bottom pans are the answer to a few Dana transfer case dilemmas. The thick, smooth-finished flange replaces the thin, warped factory pans and they virtually assure a drip-free seal.

Two designs are available for the Dana 300: The shallower depth, low-profile version for Jeeps requiring greater ground clearance. This version increases fluid capacity over the factory pan by about 6 oz. The extended depth version that maximizes the fluid capacity for the most punishing service. This version increases fluid capacity over the factory pan by about 16 oz.


 Dana 300 Ultra-Short HD Rear Output Assembly

If you can accommodate the electronic speedometer or don’t need a speedometer option, this kit will give you the ultimate in strength and short length. Full details are here.


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.


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.


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 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.

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. 


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