Adapting the

GM Muncie SM420

to the

Jeep Dana Models 18 & 20 Transfer Cases

Adapter kit for 4L80E to Jeep transfer cases

Kit 422, adapting SM420 transmissions to Jeep & IH Dana 18 & 20 transfer cases

Kit contents:

  • Adapter housing
  • Transfer case adapter shaft, 6 or 10 spline
  • Seal
  • Gasket
  • Fastening hardware
  • Bearing adapter, 3-5/32" or 4"
  • Instructions

Novak's 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, compatibility with several engines, and ease of installation.

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 356-T6 aluminum alloy, featuring thick flanges, a heavy cross-section, and strengthening gussets.

This transfer case adapter's design features a cast mounting base, ready to accept our industry standard urethane isolator mount, available in Related Products, below.

Nearly every SM420 features a 10-spline output shaft protruding 2-5/8" from the rear of the case and is compatible with this kit. Disassembly of the transmission is unnecessary. Note that there are rare versions of the SM420 which feature a 35 spline threaded output shaft that is incompatible with this conversion. We keep good used mainshafts on hand. Contact us if you need one.

Applications & Compatibility

Compatible Transmissions

SM420 transmission with adapter

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. With the exception of the rare HD 35-spline output shaft version, all SM420's are compatible.

Mainshaft replacement is not required for this conversion. Great results are in reach of the careful home mechanic and instructions are included. We recommend at least our basic gasket kit - see Related Products, below. In some situations, this is a good time for a more thorough master rebuild of the transmission.

Many customers request our complete Novak remanufactured and upgraded SM420 transmission packages. This professionally built transmission is dyno-tested, delivered to you and ready to bolt in.

Compatible Transfer Cases

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. The correct input bore and spline count must be observed. This adapter is not suitable for the Ford Bronco transfer case as it is of an entirely different design in attachment and drop.

Transfer Case Input Gear

dana_18_20_input_gearOur adapter shaft is splined for six spline input gears.

  • Six-spline gears (shown) are most common and usually found behind Jeep T84, T90, T86, T89, T98, T18, T15 & T150.
  • The ten-spline version was only available on the 1966 - 1974 transfer cases against the Jeep T14 transmission
  • 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.

All Warn and Saturn type overdrives are compatible with this conversion as long as the spline count is correct. We do not have replacement gears for the overdrive version.

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.

Compatible Jeeps

SM420 with Novak to Dana adapter

This is compatible with most Jeeps as it is one of our shorter 4-speed combinations, and is reasonably low-profile in regard to the tunnel, though some modifications to sheet metal and shifter boots may be required.

Compatible Engines

Any engine that can be made to work with the SM420 will work with this conversion. They include:

Related Products

Urethane Rear Mount for Crossmember
Parts for the SM420
Shifter stick for the SM420 transmission
Shifter stick for the SM420 & SM465 transmission
Shifter Retention Kit, SM420, SM465, T176, NP435
Billet PTO Cover
Dana 18 Billet PTO Cover
Dana 18 & 20 Rebuild Kit
Dana 18 & 20 Input Gears
Billet aluminum Dana transfer case pan
Turn-Key SM420

Installation Requirements


Normal mechanics tools are required for the installation.

Engines & Engine Mounting

GM Engine Mounts for Jeeps

Factory engine offset and placement on Buick V6 Jeeps (1967-1971) is great for the SM420 transmission, as well as AMC I6 and V8 in 1972-1979 CJ models. If installing a GM V8, your engine mounts should permit about a 1-1/4" offset towards the driver's side. Novak's engine mounts, feature 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. Many conversions to these transmissions will require that the rear driveshaft be modified to be longer (usually welcomed) and front driveshaft shorter.

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.

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