Novak Conversions Adapter 422-3-6 Kit

Adapting SM420 transmissions to Jeep & IH Dana 18 transfer cases with 3-5/32? input bore & 6 spline input shaft. (10 spline shaft also available.)

Adapting the GM Muncie SM420 to the Jeep Dana Models 18 & 20 Transfer Cases

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.

Product & Features

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

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.

Applications & Compatibility

Transmissions

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. 

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. 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.

The new Novak manufactured 6 or 10 spline adapter 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 the D18 or D20 input gear attaches directly to it.

Contents

The kit contains aluminum adapter housing, transmission output shaft, seal, transfer case to adapter gasket, transmission to adapter gasket, fastening hardware, steel mounting base, and instructions. The adapter comes assembled to the extent possible.

Transfer Case Input Gear

Our adapter shaft is splined for six spline input gears.

• Six-spline gears (shown, right) 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.

Jeeps 

This is compatible with most Jeeps as it is one of our shorter 4-speed combinations. 

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.