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Adapting the

Turbo Hydramatic 400 Transmission

to the

Jeep Models 207, 231, 242, 249 & 241OR Transfer Cases

TH400 to Jeep transfer case adapter

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. It remains one of our most requested automatic transmission to transfer case combinations.

The Novak TH400 to Jeep NP / NVG transfer cases adapter is 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 an 18.9" manual transmission / bellhousing combination). The adapter itself is precision machined of aluminum alloy. The adapter is drilled for multiple clocking options and features a modular mounting and support base, ready to accept our industry standard urethane isolator mount, available in Related Products, below.

There are a few kit options to choose from, depending on your configuration.

AMC vs GM TH400 Case Rotation

AMC Jeep style TH400 transmissions from 1976-1979 featured a tilted rear case casting for which we make an optional "A". Our unique adapter configuration corrects for this so it will sit level on the crossmember and not clock the transfer case unnecessarily low. Customers should choose this version only for AMC TH400s of the above era. 1966-1973 Jeep units featured "universal" cases that were level as conventional. These earlier cases are identified by their use of a factory 400 transmission to Buick or AMC engine block adapter.

Option I: Replacement TH400 Output Shaft, 23 Spline

Adapting TH400 transmissions to Jeep NP 231, 242

Kit 141-23, adapting GM TH400 to Jeep NP 207, 231, 241, 241OR, 242 & 249, 23 spline output shaft version

Kit contents:

  • Adapter
  • Transmission output shaft, 23 splines
  • Seal
  • Seal spacer
  • Gaskets
  • Steel base foot mount
  • Fastening hardware
  • Instructions

The 141-23 kit is compatible with all TH400 versions and the following scenarios:

  • 2wd or 4wd
  • you believe it's time for a rebuild
  • Unsure of which version TH400 you have
  • you have a fresh transfer case that you'd rather not service

The provided 23 spline TH400 output shaft splines directly into the OEM style input gear of a 23 spline transfer case. The 241OR Rubicon must use the 23 spline output version. This requires disassembly of the transmission, and we recommend the services of a transmission professional.

The basic installation is a bench job, often including anything from gasket, seal and filter replacements to a full master rebuild and update.

Jeep NP241 with Novak TH400 Shaft

Option II: Replacement 32 Spline Transfer Case Input Gear

Adapter kit for TH400 to Jeep New Process transfer cases, input gear version

Kit 141-32, adapting TH400 to Jeep NP 231, 241, 242, 249, 32 spline input gears


141-32 Kit Contents

  • Adapter housing
  • Transfer case input gear, 32 splines
  • Seal spacer
  • Seal
  • Adapter gasket
  • Seal ring
  • Fasteners
  • Steel foot for rear mount
  • Instructions

One 4wd version of the TH400 has the right, short NP208 & NP241 style, 32 spline output shaft (see graphic, below) for this kit version, so disassembly of the transmission is unnecessary. However, disassembly of the transfer case is necessary to install a new matching 32 spline NP input gear, and you will need to identify whether you need an early or late cut style gear.

TH400 NP208 style output shaft

It is not requisite that you do a full rebuild of your transfer case to successfully complete the conversion. However, it is certainly worth consideration if you are opening it up and it shows many years of service. For the gear, gaskets, chains and even full rebuild kits, see Related Products, below.

Early vs Late Input Gear Style

The NP/NVG input gears are divided into early and late styles around 1993-1994. There are no guarantees that this change was a clean break pre-1993 vs. post-1993, or that your Jeep's transfer case hasn't been swapped by a previous owner.

If in question, the input bearing width on these models is a good indicator. "E" Early uses a 24mm wide bearing and "L" late is 16mm wide. The adjacent photos show the differences in width of the bearing location and the stepped shoulder between the bearing and the gear.


New Process Wide Bearing Input Gear
Early Style
Input Gear
New Process Narrow Bearing Input Gear
Late Style
Input Gear

Strength Comparison of 23 vs 32 Spline Versions

The 32 spline shafts are, as you would guess, stronger than 23 spline shafts. Whether this has practical meaning in a Jeep depends mostly on the power of the engine, the weight of the Jeep, the tire size and the style of driving and terrain.

We've corresponded with tens of thousands of Jeeping customers, and 23 spline breakage vanishingly rare. 32 spline breakage? We've not seen it yet. But if you are strength-maxxing and it's a real concern, you can go for the 32 spline connection.

Applications & Compatibility

Compatible Transmissions

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.

TH400 transmission, adapted

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 NP & NVG Transfer Cases

Transfer case seal spacer
New Process input gears come in varying spline counts and lengths. Whichever you have, we've got you covered. A seal spacer sleeve is included and pre-installed in your Novak adapter in the event your transfer case has a short input gear.

The adapter kit is designed for compatibility with the Jeep NP207, NP231, NP242, NP249, NP241 & NP241OR "RockTrac" transfer cases. Below is a summary of spline counts and lengths usually encountered.

  • NP 231 - The NP231 may have 21 or 23 splines and a long or short style input gear
  • NP 242 - The Jeep version NP242 may have 21 splines for the 1987-1990 versions. When connected to the 4.0L HO engine (1991- ) the 242 will have 23 splines as will the Dodge and later V8 Grand Cherokee, AMG versions are 32 splines long
  • 249 - The NP249 always has 23 splines, but may have a long or short style input gear
  • 241OR - The RockTrac will always have a 23 spline, short style input gear. Its gear is unique and not interchangeable with any of the other versions due to the lower 4:1 ratio
  • NP207 - The NP207 may have 21 or 23 splines. Many versions feature extra-long input gears whose stick-out lengths (as measured from the front edge of the input gear to the transfer case front mounting face) is longer than 2.2". These will require shortening with a cut-off saw and/or lathe and finished with edge breaking chamfers

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 TH350.

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

Compatible Jeeps

TH400 transfer case adapter

This conversion is compatible with most Jeeps. These are strong transmissions native to 1/2 ton truck applications. Full Size Jeeps such as Wagoneers, J-Trucks, etc. are conversion candidates for them. There are numerous upgrades available for the TH350 should stock strength come into question.

Related Products


Urethane Rear Mount for Crossmember

Urethane Rear Mount, #RMU

This rear mount bolts direct to your Novak adapter, and can be used with a variety of crossmember styles.

Transfer Case Input Gear, 23 Spline

Gear, input, New Process 23x & 24x transfer cases, 23 spline, long input, wide bearing, early gear cut, 1.5” pocket bearing, #23LWES

If you have a 21 spline input gear in your early (1988-1993) New Process 231, 242 transfer cases, swap it for this gear. Read more about these gears, here.

Transfer Case Input Gear, 23 Spline

Gear, input, New Process 23x & 24x transfer cases, 23 spline, long input, narrow bearing, late gear cut, 1.5” pocket bearing, #23LNLS

If you have a 21 spline input gear in your late (1994-2006) New Process 231, 242 transfer cases, swap it for this gear. Read more about these gears, here.

NP207 Input Gear 23 Spline

Gear, input, New Process 207 transfer case, 23 spline, long input, #716096

This is a new 23 spline input gear for your NP207 transfer case. As many of these were 21 spline models from the factory, this permits the upgrade to the stronger 23 spline design for compatibility with the Novak adapter kit.

NP231 transfer case parts

NP231 rebuild, repair and upgrade parts

Some transfer cases are due for a re-seal or even full rebuild. We carry the world's most full-featured kits and components, here.

NP242 transfer case parts

NP242 rebuild, repair and upgrade parts

Some transfer cases are due for a re-seal or even full rebuild. We carry the world's most full-featured kits and components, here.

Auto transmission cooler

Cooler, auxillary, transmission, 29,200 BTU (HD recreational), 1 ½” NPT fitting, #OC-H7B

For every 10 degrees cooler you can keep your automatic, you add 25k miles to their life. This stacked plate style cooler is both rugged and efficient, and it's flanges make it easier to set up than other styles of cooler. See related cooler lines, adjacent.

Kit, automatic transmission cooler lines, braided stainless, 2 lines, 74” OAL ea, -6AN female thread fittings, #TR-FLX

Tough, flexible stainless lines with easy sealing AN fittings. We've installed these on so many Jeeps and they're an in-and-done type product and compatible with our #OC-H7B cooler, adjacent.

1976-1986 Jeep floor tunnel cover

1976-1995 Jeep CJ & YJ Floor Tunnel Cover

Start with a clean slate tunnel cover for your 1976-1995 Jeep CJ / YJ floor. Cut your shifter holes and install boots onto it as required. Material is aluminum. Can be painted. More info here.

Arm, pivot, shifter, Hydramatic 350, 400, 700, 4L60E & 4L80E Lokar, fully adjustable, #ACA-1802

This is the arm we reach for when setting up a shifter linkage to a GM automatic in a variety of Jeeps. The adjustable bellcrank gives the installer the freedom to set the proportion of throw to the same as the shifter in the Jeep, be it a column or floor shifter.

Shifter Kits for New Process Transfer Cases

As this combination will frequently be paired with Jeep New Process chain drive transfer cases (NP231, NP242), this shifter kit will make the conversion go not just smoother, but will improve your shifting action and reliability significantly over factory. Read about them here.

Slip Yoke Eliminator for NP231 Transfer Cases

Heavy Duty Slip Yoke Eliminators for 231 Transfer Cases

Upgrade and shorten your NP231 output shaft to handle the increased torque of your new gears, and improve the driveshaft angle, especially important for lifted Jeeps. View the SYE kit that will work best for your situation.

TurnKey TH400 Transmission Package

TH400 Transmission Package, Turn-Key

Sourced, remanufactured and built in-house to Levels I, II or III. We then dyno test them, and profesionally package and deliver it to your location, adapted and ready for installation. See and order one here.

Also available as part of our popular Turn-Key powertrain program.


Installation Requirements

Tools

TH400 in rebuild jig

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

Urethane transmission mount
Our #RMU urethane mount (Related Products, above) is the ticket for nearly every Jeep swap we've seen. Whatever you do, do not hard mount this nor any powertrain component. Flexibility is critical to prevent breaking your transmission case.

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.

Novak Gen III+ engine mounts for 1976-1986 Jeep CJ models will remove any guesswork from the engine installation location as they are predetermined for the best location and engine clearance margins.

Driveshafts

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.

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 passenger side view, adapted

Axles

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.

TH400 transmission, overhead view

Also See

TH400 drivers side adapted view