Adapting the Turbo Hydramatic 350 Transmission

to the

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

Our TH350 transmission to the Jeep New Process (or New Venture) transfer case conversion makes for a tough and capable assembly both on-road and off-road. It is one of the most persistently popular transmission swaps into Jeeps, providing a compact geartrain, compatibility with excellent engines, and ease of installation.

The Novak TH350 to Jeep New Process transfer cases adapter is compact at only 1.1" long for a combined transmission and adapter length of 22.9" (equivalent to a 16.4" manual transmission), especially important for short wheelbase YJ & TJ Wrangler Jeeps. The adapter is machined from billet aluminum alloy, featuring a heavy cross-section. It also features a modular mounting and support system and mounting base, ready to accept an industry standard urethane isolator mount, available in Related Products, below. Remember to never hard mount any powertrain part.

Any version (2wd, 4wd, etc) TH350 can be made to be compatible.

Option I: Replacement TH350 Output Shaft, 23 Spline

Adapter kit for TH350 to Jeep NP transfer cases

Kit 131-23, adapting TH350 transmissions to Jeep NP 207, 231, 241, 241OR, 242 and 249 transfer cases, 23 spline output shaft version

Novak 131 adapter

This version of our #131 kit comes with a new, 23 spline output shaft for your TH350. This kit works with all 23 spline input versions the Jeep New Process / NVG transfer cases such as: NP207, NP231, NP242, NP242 and NP241J and NP241OR. It works with both long and short input versions of these transfer cases. This kit is often chosen when the above conditions are met and the installer has a 2wd transmission and / or the installer believes it's time for a TH350 rebuild, or if it is unknown which version of the TH350 you may end up with at the time of your conversion.

The new TH350 output shaft is machined from 300M alloy steel which provides a substantial strength upgrade. Installing it requires disassembly of the transmission, and we recommend the services of a transmission professional.

If your transfer case has a 21 spline input gear, see Related Products below for a 23 spline replacement, or consider Option II below for some scenarios.

Kit Contents:

  • Adapter plate
  • TH350 Transmission Output Shaft, 23 spline
  • Seal
  • Gasket
  • Seal ring
  • Fastening hardware
  • Steel mounting foot base
  • Instructions

Option II: Replacement 27 Spline Input Gear

Adapter kit for TH350 to Jeep NP transfer cases

Kit 131-27, adapting the TH350 4x4 version transmission to the Jeep NP 231, 241, 242 and 249, 27 spline version

TH350 4wd shaft stickout length
The 4wd version of the TH350 will have a ~3/4" output shaft stickout length.

This version of our #131 kit works only with the short, factory 4wd output shaft version of the TH350, and works only with the NP231, NP242, and NP249 transfer cases for which we've included is a new 27 spine input gear. This version will not work with a 207 or 241OR transfer case. Transmission disassembly is unnecssary, but transfer case disassembly is required. The input gear installation is a 2-3 hour bench job and usually includes an RTV seal replacement.

A full rebuild of a tired transfer case includes new bearings, seals, chain. If you do not already have an SYE kit, now may be a good time to install one. 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.


131-27 Kit Contents

  • Adapter housing
  • Transfer case input gear, 27 splines
  • Seal spacer
  • Seal
  • Adapter gasket
  • Seal ring
  • Fasteners
  • Steel foot for rear mount
  • Instructions

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

Strength Comparison of 27 vs 23 Spline Versions

The 27 and 23 spline shaft sizes are quite comparable in minor diameter (the diameter of the shaft at the root of the splines) and therefore, strength. Diameters are very close and the finer 27 spline vs the coarser 23 spline has shown no significant strength variation. The 300M material used in production of the Novak shaft is of a superior strength both in PSI yield and torsional resistance - which is exactly the type of load these shafts see. Aside from that, fundamental design strengths are comparable.

GM 27 spline and Jeep / Dodge 23 spline shafts have a history of great strength in factory applications and Jeep aftermarket set-ups.

Applications & Compatibility

Compatible Transmissions

TH350 adapted to NP231

This adapter can be made compatible with all versions of the TH350 as discussed above. Also, Chevrolet, Buick / Oldsmobile / Pontiac (BOP) and Unicase versions all work equally well with the correct adapter option.

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

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
Auto transmission cooler
1976-1986 Jeep floor tunnel cover
Transfer Case Input Gear, 23 Spline
Transfer Case Input Gear, 23 Spline
NP207 Input Gear 23 Spline
NP231 transfer case parts
NP242 transfer case parts
Slip Yoke Eliminator for NP231 Transfer Cases
TurnKey TH350 Transmission Package

Installation Requirements

Tools

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

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.

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.

Axles

Use of factory axles is completely acceptable with this conversion. Axle upgrades are not necessary, but they may bechosen for reasons external to this transmissionupgrade.

Also See