Adapting the
Turbo Hydramatic 400 Transmission
to the
Jeep Dana Model 18 & 20 Transfer Cases
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.
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The Adapter
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 machined of a high-grade aluminum alloy, featuring thick flanges and a heavy cross-section.
The adapter features the only twin-bearing design available in the OEM and other 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.
TH400 Versions
These kits are for all years and versions of the popular HydraMatic 400, including Chevy, Buick and other GM, AMC Jeep, etc. and work with 2wd & 4wd versions.
Transmission Output Shaft
Every #124 kit comes with a new output shaft for the TH400, whether for 2wd or 4wd transmissions. 2wd versions work equally well as 4wd versions. The Novak output shaft is of an alloy steel, precision splined and hardened for a superior interchange. This shaft connects the TH400 output direct to the included adapter shaft for the strongest cobination. Installation requires disassembly of the transmission main assembly, and we recommend the services of a transmission professional. The basic installation is 3-4 hour bench job and may include simple gasket and seal replacements. Some choose this as the time for a more thorough master rebuild and update.
Transfer Case Input Gear
Our transmission mainshaft is splined for six-spline input gears.
Six-spline gears (shown, right) 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.
- 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 type overdrives are compatible with this conversion. Spline count information is the same.
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.
Transmission Rear Mount
This transfer case adapter's design features an integrated base that is ready to accept our industry standard urethane isolator mount, available for purchase (below). This is our strongest and preferred rear mount unit. Our #124 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.
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This mount will sit onto your stock cross member / skidpan, into which you will drill two attachment holes. You will find it to be a clean and simple process.
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 essentially all of Dana 20 shifters, including this twin-stick shifter kit.
Transmission Shifter
A broad variety of transmission shifters are compatible with this conversion, ranging from console shifters (i.e.; Art Carr, B&M, etc.), to direct-mount stick shifters (i.e.; Gennie, Lokar, etc.).
Driveshafts
Use of the factory diameter driveshaft and yoke is recommended. Our adapter design has maximized clearance between the transmission pan and yoke, but it is necessary that you install your GM engine offset 1-1/4" to the driver's side. If you are installing against a factory AMC engine with an AMC TH400 or a GM TH400 using our #437-AMC adapter kit, your factory engine should already have this proper offset.
| The length of the Hydramatic 400 makes its placement into an 80" wheelbase Jeep (CJ5) prohibitive, but is an excellent fit into CJ7s, Utility Series Trucks and Wagons, as well as all of the Full Size Jeeps. |
Driveshaft length changes are often required, as this geartrain combination is typically longer than factory Jeep combinations. However, some GM engine installations will allow for the transfer case to remain in its OEM location.
Quadratrac replacement projects (using the Dana 18) will require removal of the Double-Cardan (CV) style u-joint on the front driveshaft, in favor of a conventional single u-joint matching the Dana 18 front output yoke.
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.
Transmission Cooling
For every 10 degrees cooler you can keep the transmission, you can add 25,000 miles to its service life. Therefore, use of a transmission oil cooler is advisable for this automatic transmission.
Use of a transmission temperature gauge is also a good idea. These are widely available and generally inexpensive.
With the increase in popularity of the newer, hotter burning, fuel-injected engines, it is increasingly wiser to run a stand-alone, dedicated transmission fluid cooler.
Engines & Engine Mounting
A TH400 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.
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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. |
In any scenario, it is critical 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 TH400 and the Dana 300 both considered.
Axles
Use of factory axles is completely acceptable with this conversion. Axle upgrades are not necessary to the conversion itself, but they may be chosen for reasons external to this transmission upgrade.
| Those hoping to repair their QuadraTrac often opt to swap in a Model 18 transfer case. Our Kit #124 makes this worthwhile upgrade possible. The Dana 18 makes for a great replacement transfer case for the Borg Warner 1339 Quadra Trac transfer case. It has the same offset and drop, allowing for the retention of the front and rear axles. The strength of the Dana 18 if properly maintained is very adequate for even the heavier Jeeps and their V8's. |
Summary
The Novak #124 adapter brings together two all-time classic gear-boxes for a most impressive combination. We guarantee it to be the strongest, most compact adapter of its kind on the market.
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| An adapter option available for replacing a broken OEM unit is Novak's kit #8624113. This strengthened replica can be used with your factory power transfer spline sleeve. Read more... |