Adapting the
Turbo Hydramatic 400 Transmission
to the
Jeep Models 207, 231, 242, 249, 241J & 241OR Transfer Cases
Our TH400 to the Jeep New Process (or New Venture) transfer cases conversion makes for an absolutely strong and capable assembly both on-road and off-road, and it remains one of our most requested automatic transmission to transfer case combinations.
The adapter kit is designed for compatibility with the Jeep NP207, NP231, NP242, NP249, NP241 & NP241OR "RockTrac" transfer cases.
 Choose a seal spacer sleeve if your transfer case has a short input gear (discussed below). If you're not sure of the length of your input gear, you may order the spacer and then return it for full credit if not used in your conversion. |
The Adapter
The Novak TH400 to Jeep NP / NVG transfer cases adapter is very compact at only 1.4" in length. This adapter assembly is the shortest Turbo 400 adapter in the industry, providing a combined transmission and adapter length of only 25.9" (equivalent to a 18.9" 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.
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 #141 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 directly into the existing 21 or 23 spline NP / NVG input shaft for the strongest combination. Note our 21 vs. 23 spline discussion below and its warranty caveat.
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 transmission master rebuild and update.
Transfer Case Input Gears
The included Turbo 400 output shaft splines in directly into the OEM Jeep input gears, and transfer case disassembly is not typically required.
Transfer case input gear styles vary. Order your adapter assembly accordingly.
The New Process / New Venture transfer cases in these Jeeps may feature a long or short style input. We provide an optional seal spacer (above) that will make sure your seal reaches your short input gear. Otherwise, leakage from your transmission into your transfer case is assured.
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| New Process input gears come in varying spline counts and lengths. Whichever you have, we've got you covered. |
231 Rebuild Components
It is not requisite that you rebuild your Jeep 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 NP 231 rebuild components available for this purpose.
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Transmission Rear Mount
This transfer case adapter's design is based on Novak's Universal Series adapters that feature a modular mounting and support system. Included with the adapter kit is a steel base that is a highly configurable mounting foot, and is ready to accept our industry standard urethane isolator mount, available for purchase (shown). This is our preferred rear mount unit. Our #141 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.
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.
Transfer Case Clocking
Our adapter is clocked optimally for slightly better ground clearance than factory locations, which is actually quite good. The Jeep transfer case should hang no lower than the transmission pan itself.
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| The Novak #SK2X shifter assembly as attached to a GM automatic (TH350 shown. TH400 similar). |
Transfer Case Shifter
This transmission conversion will allow for the retention of the factory shifters, or allow you to upgrade to the Novak #SK2X shifter assembly for a simpler conversion and a clean shifting, direct linkage system.
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
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.
| AMC Jeep style transmissions from 1976-1979 featured a tilted transmission case casting, and for this we make a variation of our #141 kit. This adapter configuration is unique on the market in that it levels the adapter's base foot for square mouting on the rear isolator mount and crossmember. This is useful in that the adapter allows installation without clocking your transfer case down, or having your transmission mount sit crooked. Customers should choose this version only for AMC TH400s of the above era. Earlier Jeep units featured "universal" cases that were level as conventional. These earlier cases are identified by thier use of a factory 400 transmission to Buick or AMC engine block adapter. |
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.
Since driveshaft modifications may be indicated for the conversion, it may be a good time to convert the transfer case to a heavy duty mainshaft and ultra-short tailhousing assembly. The #231XS Slip-Yoke Eliminator is the ideal solution to capture 7" of driveshaft length, significantly reducing u-joint strain and vibration.
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| Our RadLock radiators offer an available integrated transmission cooler, to greatly add to the life of your HydraMatic transmission. |
Transmission Cooling
For every ten 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.
We will suggest the use of our RadLock high-performance conversion radiators, which can include an integrated transmission fluid cooler. You may also use a standalone / supplemental transmission cooler depending on your driving style and climate conditions.
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 NP / NVG transfer case both considered.
AMC versions of the TH400 (usu. 1976-1979) can be mated to AMC V8, I6 (4.0L or 258). The Chevy TH400 can be married to the above engines as well with our #437AMC adapter assembly.
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| Advanced computer modeling methods were integral to the design of our #141 adapter assembly. We were able to push the bounds of product and process well beyond the status quo. |
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.
Summary
The TH400 is a prime building block to an indestructable Jeep powertrain, offering also GM or AMC engine compatibility and good parts and service availability.
| 21 vs. 23 Splines & Warranty Exemption We're often asked about whether the 21 spline style connection is strong enough, vis a vis the 23 spline version. In most situtations, 21 splines will be adequate. This is due to the fact that we don't use friction welding on our shafts, and that much of the punishing torque multiplication takes place within the transfer case low range, which occurs after the input portion of the transfer case. However, we have seen limited instances where 21 spline shafts have failed where 23 would not have. For this reason, we do not warranty the 21 spline version of our output shafts. If you require maximum strength, then you may purchase from us a 23 spline input gear for your NP231 in conjunction with the 23 spline version of our kit. |