AdaptersAdapting the
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, compatiability with excellent engines and ease of installation.
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Option I: Transfer Case Input Shaft VersionThe 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 a high-grade aluminum alloy, featuring thick flanges, a heavy cross-section and strength-adding gussets.
SM420 Versions
Any version (2wd or 4wd) and year of the SM420 can be adapted to the Dana 300.
Option I: Transfer Case Input Shaft Version
Our easiest and most common SM420 to Dana 300 adapter is for the very grand majority of the SM420's that feature a 10-spline rear output shaft that protrudes 2-5/8" from the rear face of the transmission case. These include both 2wd or 4wd styles of 420.
These versions of the SM420 are all ready to bolt up to our assembly as the required style of output shaft is already installed. This kit is our #423 adapter assembly (right, upper), which includes a new 10 spline input shaft for the Dana 300 transfer case. This shaft is compatible with and splines directly to the OEM output shaft. The included Dana 300 input shaft is machined from triple-alloy gear steel and provides a superior upgrade.
It is not requisite that you rebuild your Dana 300 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 Dana 300 gaskets & seals kits and our all inclusive master rebuild kits available for this purpose.
Option II: Transmission Output Shaft Version
Rarer instances of the 420 feature a 35 spline, threaded end output shaft. Some installers also opt to change either this 35 spline shaft or the 10 spline shaft - if worn - in favor of our new one. Use of our #423R kit is recommended in these instances (above box, lower). The included, 23 spline SM420 output shaft is machined from triple-alloy gear steel and provides a superior upgrade. This shaft is compatible with and splines in directly to the OEM style input shaft in the 300 transfer case. This requires disassembly of the transmission. This is commonly done by our customers. The basic installation is a bench job and may include simple gasket and seal replacements. Some choose this as the time for a more thorough master rebuild of the Muncie transmission.
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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 #423 kits are 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 straightforward process. Some factory crossmembers may require minor trimming at the front lip and / or vertical spacing of the rear mount, as per the Jeep's particular tub clearance and crossmember configuration.
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 & V8) 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:
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>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 SM420 For 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.
Transfer Case Shifter
This transmission conversion will allow for the retention of the factory shifters, or allow you to upgrade to a twin-stick shifter.
Transmission Shifter
Use of the OEM style transmission shifter for the SM420 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. |
Clocking Information & Driveshaft Clearance
Our adapter is drilled for multiple clocking locations from near-factory (20 degrees down) to near-flat (3 degrees down). We feel it is optimal to install in the provided position between these, but all three options are available. The flatter rotation option may require Dana 300 shifter modifications. The middle location provided for the SM420 is likely optimal.
Driveshafts
Driveshaft length changes are often required, as this geartrain combination is typically shorter than others. Some GM engine installations sit further ahead in the bay and will allow for better driveshaft angles, especially when done with this very short combination.
If replacing a T176 with this transmission, and using the AMC engine, you may be able to keep your front and rear driveshafts. If replacing an SR4, T4 or T5, you may be able to use a T176 length driveshafts.
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.
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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. |
Engines & Engine Mounting
An SM-420 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.
In any scenario, it is often preferable that the engine be installed (as the Jeep engines usually are) with an offset of 1" to 1-1/2" to the driver's side. Other engine mounts on the market - especially the bolt-in versions - do not take this into account and make other placement decisions for the installer that do not take the full powertrain into account. Novak offers what we have found to be the most intelligent engine mounting system for Jeeps available, allowing the installer to make an easy and satisfying decision as to powertrain placement with the SM-420 and the Dana 300 both considered.
The design and strength and affordability of our mounts are second to none.
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| A turn-key SM420, professionally built, adapted and delivered to your door - ready for a variety of engines and Jeep transfer cases. Read more... |
AMC engines may be left in the factory location in virtually all 420 installations.
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.