Adapting the
Muncie SM420Transmission
to the
Dana Spicer Models 18 & 20 Transfer Cases
Our SM420 to Dana 18 & Dana 20 conversion creates a robust geartrain assembly for broad use in severe terrain or tame roads. It is one of the most steadily popular manual shift transmission conversions into Jeeps, providing a very compact geartrain, a supreme crawling gear, compatiability with several engines and ease of installation.
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The Adapter
The Novak SM420 to Jeep transfer cases adapter is 4.4" long for a combined transmission and adapter length of 15.2". The adapter itself is cast and machined of a high-grade aluminum alloy, featuring thick flanges and a heavy cross-section.
Transmission Versions
Any versions (2wd or 4wd) and year of the Muncie 420 can be adapted to these transfer cases. There are no inherent advantages to the 2wd vs. 4wd transmission.
Transfer Case Versions
All Jeep models of the Dana Spicer 18 and Dana Spicer 20 transfer cases are compatible with this adapter assembly. IH versions of these transfer cases are also compatible.
Transfer Case Input Gear
The included adapter shaft is splined for either six-spline input gears, or ten-spline input gears.
Six-spline gears are predominately most common, as found with the Jeep T84, T90, T86, T89, T98, T18, T15 & T150. The rare ten-spline version is only availalble on the 1966 - 1974 transfer cases against the Jeep T14 transmission and this version of the Dana 18 will only have the 4" (or "large case") bore.
All Warn type overdrives are compatible with this conversion. Spline count information is the same.
Factory Jeep TH400 to Dana 20 combinations had a fifteen-spline input gear, which must be replaced by a compatible six-spline version.
International Harvester Dana 20's that had the TF-727 automatic had a 23-spline input gear, which must be replaced by a compatible six-spline version.
Consult our informative Dana gears interchange chart if necessary.
Transfer Case Bore Size
The diameter of the transfer case center input locating bore will be either 3-5/32" or 4".
- Most Dana 18's have a 3-5/32" bore, including those found against the T84, T90, T89 or T98.
- Some Dana 18's have a 4" bore, including those found against the T86 & T14 transmissions (Buick V6 engines), 1966-1971.
- All Dana 20's have a 4" bore, regardless of the transmission they were joined to.
Transfer Case Strength and Condition
The Dana transfer cases are very well matched to the 420. They are uncommonly strong, 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 Dana 18 & 20 gaskets & seals kits and our all- inclusive master rebuild kits available for this purpose.
Engine Compatabilities & Engine Mounting
Chevy, Buick and other GM engines (and most of their bellhousings) are natively compatible with the SM420 transmission.
AMC Jeep engines can also be compatible with the 420 transmission. See Adapting the SM420 Transmission to AMC Bellhousings. AMC (258, 304, etc.) engines may be left in the factory location in virtually all these installations.
A 420 installation is sometimes 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:
>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 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.
Clocking Information & Driveshaft Clearance
The Novak adapter maintains factory clocking. The Jeep transfer case should not hang any lower than the base of the SM420 transmission.
Transfer Case Shifter
This transmission conversion will allow for the retention of the factory Dana 18 twin-stick shifters, Dana 18 single-stick shifter, and many of the Dana 20 shifters. Some tube-mounted Dana 20 shifters may need to be modified, or possibly replaced with the Twin-Stix Shifter Kit.
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 a 420.
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This adapter provides clearance for the transfer case shifter and standard size front driveshaft yoke, without grinding of the transmission case. This also adds to the ease of installation. Driveshaft length changes are usually required. This conversion does not require disassembly of the transmission, and the adapter comes fully assembled. |
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 longer than most factory Jeep combinations. Some potential conversion engine installations sit further ahead than OEM engines in the Jeep's bay.
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| A turn-key 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.
| J. Heikka's writeup of this installation is an excellent guide. Read it here on the Early CJ forum. |