Adapting the

Turbo Hydramatic 400 Transmission

to the

Jeep Dana Model 18 & 20 Transfer Cases

TH400 to Jeep Dana transfer case

Our TH400 to Dana 18 & Dana 20 conversion makes for an absolutely strong and capable assembly both on-road and off-road, and it remains one of the great automatic transmission to transfer case combinations.

The Novak TH400 to Jeep Model 18 & 20 transfer case adapter is compact at 2.5" in length. This adapter assembly is the shortest in the industry, providing a combined transmission and adapter length of 26.9" (equivalent to a 19.9" manual transmission / bellhousing combination).

The adapter itself is cast and precision machined aluminum alloy, featuring thick flanges, gussets and a heavy cross-section. An integrated mounting foot is provided for a urethane rear mount unit, available in Related Products, below.


Kit 124, adapting the TH400 transmission (all versions) to the Jeep & IH Dana Spicer Models 18 & 20 transfer cases

The adapter features the only twin-bearing design available in OEM and aftermarket versions. This prevents adapter shaft misalignment within the assembly that would allow lateral gear movement within the transfer case, which could cause case wear damage or even gear breakage. Double seals ensure fluid-tight operation for many years.

Kit contents:

  • Adapter housing
  • Transfer case adapter shaft, 6 or 10 spline
  • Seals
  • Gaskets
  • Bearings
  • Fastening hardware
  • Bearing adapter, 3-5/32" or 4"
  • Instructions
Novak adapter isntalled to Dana 20

Applications & Compatibility

TH400 Versions

All versions and years of the TH400 can be adapted to these Jeep transfer cases. Chevrolet, Buick / Oldsmobile / Pontiac and Universal versions all work equally well. Many manufacturers sourced the TH400 from GM, even companies like Ferrari and Jaguar used them. As discussed above, 2wd or 4wd versions all work equally well with the correct adapter option.

Some customers request our complete Novak remanufactured and upgraded transmission packages. This master-built, professionally prepared transmission is constructed to your specifications, tested, delivered and ready to lift into your Jeep.


Compatible Engines

Any engine that can be made to work with the TH350 will work with this adapter. Natively compatible engines such as the Small Block Chevy, Chevy V6, Big Block Chevy, and even the 1997-2006 Vortec / LS / Gen III+ engines popular choices. However, be aware of engine crank spacing issues with Gen III+ engines when used against Gen I transmissions such as the TH400.

It's generally better to keep the TH400 with it's Gen I & II GM engines as per factory. Consider the 4L80E if you have the vehicle length to handle it, or the 4L60E built to Level II+ if you need rowdy strength and are going with Gen III+ engines. That said, the TH400 can be made to be compatible with them all.

AMC engines can be adapted via a front adapter such as our 437 series kits.

Compatible Jeeps

Adapted TH400 to Jeep Dana Transfer Case, yoke

This conversion is compatible with most Jeeps. These are strong transmissions native to 3/4 ton truck applications. Full Size Jeeps such as Wagoneers, J-Trucks, etc. are also conversion candidates for them.

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.

OEM TH400 to Dana 20


If you have the factory OEM 8624113 TH400 to Dana 20 adapter but it is broken or showing some wear, our upgraded replacement for that piece may a good option. Read more…

Related Products

Urethane Rear Mount for Crossmember
Auto transmission cooler
1976-1986 Jeep floor tunnel cover
Twin Stick Boot
Dana 18 & 20 Rebuild Kit
Dana 20 billet rear bearing cap
Billet aluminum Dana transfer case pan
Dana 18 Billet PTO Cover
Dana 18 & 20 Input Gears
TurnKey TH400 Transmission Package

Installation Requirements


TH400 to Dana 18

The automatic transmission takes more specialized tools, knowledge, gaskets, seals and snap ring pliers, etc. Transfer case input installation takes normal tools along with some larger snapring pliers. Installation after the output shaft or input gear isin place requires normal mechanics tools.

Powertrain Placement

TH400 to Dana 20 shifters

As these usually are installed as part of an engine swap setting the powertrain to the proper pitch angle is recommended - usually tilted down ~5 (3-7 acceptable) degrees. Offset will be to the right (passengers’) side because of the driver drop transfer case and front differential (usually 1 to 1-1/2”). Consult our engine conversion guides for more specific recommendations on placement if the engine is being replaced.

Engines & Engine Mounting

Engine placement in these Jeeps should consider the following points:

  • Left-right offset location is similar to factory at ~1-1/4" to the driver's side
  • Avoid installing the engine too low (axle clearance and other issues)
  • Avoid installing the engine too high (transmission tunnel interference, etc.)
  • Set your engine to 4-7 degrees of tilt rearwards, like the factory engine & powertrain
  • It is usually best that Chevy and Gen III+ V8 engines install approximately 3" further forward, using the back of the engine block face as the datum
  • When mocking up your engine installation location, pay special attention to radiator and fan clearance.
  • Firewall modifications are not expected.

Since Jeeps featuring the T18 transmission are "high-hood", clearance for classic Small Block and high Vortec (truck style) intakes and accessories is not a problem.

GM Engine Mounts for JeepsNovak engine mounts are available to ease the installation of your engine.

Novak's engine mounts, feature vibration dampening, superior strength and useful adjustability - adjustable even after the engine is installed. The design, strength, and affordability of our mounts are second to none.


Driveshaft length changes may be required. Chevy engines (with their rear distributors) can install around 3" further forward than AMC engines. It is also recommended that LS family engines be installed 3" further forward than the factory block (using the back face of the block as the datum point) for exhaust clearance, firewall access, etc.

Adapter for TH400

Because of the engine placement and transfer case SYE kits are frequently installed with many of these conversions, you may end up with a longer rear driveshaft (very desirable for lifted Jeeps) and a shorter front driveshaft.

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.

TH400 with Dana 18, overhead view


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

AMC TH400, with transfer case adapter