Adapting GM 6L80E & 6L90E Transmissions

to the

Jeep Models 231, 242, 249 Transfer Cases

Adapter kit for 6L80E to Jeep transfer cases

Kit 6L81, adapting 6L80E transmissions to Jeep New Process transfer cases

Kit contains:

  • Adapter housing
  • Transfer Case Input Gear*
  • Transfer case to adapter gasket
  • Transmission to adapter sealing ring
  • Fastening hardware
  • Modular steel mounting base
  • Instructions
*External 26 spline version comes with coupler shaft instead

Novak's 6L80 & 6L90 transmissions to the Jeep New Process (or New Venture) transfer cases conversion makes for one of the strongest geartrains possible for both on-road and off-road, and has become an increasingly popular transmission to transfer case combination.

The Novak 6L80 / 6L90 to Jeep NP / NVG transfer cases adapter is compact at 3-1/4" long for a combined transmission and adapter length of 26-1/2” for the 6L80 and 27-3/4” for the 6L90 (equivalent to a ~19-1/2” or 20-3/4” manual transmission). The adapter itself is machined of aluminum alloy, featuring thick flanges and a heavy cross-section.

Note that with the exception of transmission length and output spline count / diameter, all of the information discussed herein applies to both the 6L80 and 6L90 transmissions. Some references may be made to "E" suffix transmissions in various places (e.g., 6L80E), but GM refers to them without it, as all their transmissions are now 'E'lectronically controlled.

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.

The 241JK transfer case is a late cut gear, requiring a specially machined shoulder, provided if you choose this option in the above kit.

Because of the exceptional complexity of the GM 6L80 output shaft assembly, we do not yet have an option for early female spline NP241OR input shafts. However, because of the impressive gear ratio combination of the 6L80E vs the other options available, the 241OR transfer case 4:1 low range may not be necessary when one considers the added torque of the V8 in front of it.

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

The NP/NVG input gears are divided early and late about 1993-1994. However there are no guarantees that things are original or that the change was made as a clean break 93 down and 94 up. 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 photos above show the differences in width of the bearing location and the stepped shoulder between the bearing and the gear. The 241JK transfer case requires special gears.

Applications & Compatibility

NP & NVG Transfer Cases

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 Transmissions

There are two versions of the 6L80E transmission output shaft that will work with the Novak adapter. The stock 4WD version will work without modification to the transmission. The stock 2wd truck version will work with by cutting the output shaft off with a separating disc. Other versions require the output shaft of the transmission be replaced and the rear of the unit sealed. All years of the 6L80E are compatible with the kits. More specific information on the modifications needed is provided below.

The 4wd version 6L80 is ready to bolt up to our 6L81 adapter kit. Disassembly of the transmission is unnecessary as the 32 spline 4wd style output shaft which protrudes about 3" from the rear face of the 4L80E transmission main case is correct for this kit version. Disassembly of the transfer case is necessary to install a new matching 32 spline input gear.

The 2wd style output shaft, shortened

The input gear installation is a 2-3 hour bench job and usually includes either a simple gasket and seal replacement or preferably a full rebuild of a tired transfer case, especially a chain replacement. It is not requisite that you rebuild your transfer case to successfully complete the conversion. However, it is certainly worth consideration especially if your transfer case leaks or has many years of service. You will be going all the way into the front of the transfer case to install in the input gear. Our superior quality New Process master rebuild kits along with a top quality chain are the perfect solution for the job and, available below.

Compatible Jeeps

This conversion is compatible with all longer wheelbase (CJ7 93.4" or longer) or stretched Jeeps. The TH350 is the only real automatic option practical for CJ5 and other short wheelbase Jeeps.

These are exceptionally strong transmissions native to 1/2 ton and 3/4 ton truck and SUV applications. Full Size Jeeps such as Wagoneers, J-Trucks, etc. are conversion candidates for them. There are numerous upgrades available for the 4L80E should stock strength come into question.

Compatible Engines

Any engine that can be made to work with the 6L80 will work with this adapter. Natively compatible engines such as the 2006+ Vortec / LS / Gen III+ engines are good choices. The nature of GM Generation IV communications to run the 6L80 is complex, given the networked CAN bus and keyed TCM (or TECM) and only an ECM and TCM which are specifically coded to each other are compatible. Therefore, the following three options are available:

  • Run a matched ECM & TCM set pulled from the same donor vehicle
  • Have a specialty programmer or GM recode a GM ECM & TCM pulled from different donors
  • Run an aftermarket 6L80 controller. Brands such as Zero Gravity, Holley, TCI & PCS may work for your situation.

Lastly, be aware of Gen III engine crank spacing and torque converter heights. This is not much of an issue where installers have pulled a factory matched engine / transmission combo as their donor, whereas those who are mixing and matching LS components should verify accordingly. We've written a helpful guide to GM crank spacing here.

Transfer Case Clocking & Rotation Options

Our adapter is drilled for multiple clocking locations ranging 23 degrees. However, most New Process chain drive transfer cases will favor near factory clocking.

Rear Mount Provision

This transfer case adapter's design features a modular mounting and support system and mounting base, ready to accept our industry standard urethane isolator mount, available in Related Products, below.

Related Products

Urethane Rear Mount for Crossmember
Auto transmission cooler
1976-1986 Jeep floor tunnel cover
NP231 transfer case parts
NP242 transfer case parts
6L80E cooling lines AN adapter
6L80 rear seal kit
Slip Yoke Eliminator for NP231 Transfer Cases
TurnKey 6L80E Transmission Package

Installation Requirements


The New Process transfer case requires hand tools, large snap ring pliers and RTV to reseal. Rebuilds are a little more involved but not much.

Engine Placement

Engine placement in these applications is an effort in compromise to find the best position overall as a package. Generally the engine will be about 1” or even more towards the left, (driver’s side in the USA) away from the front differential in a CJ application. This gives the best balance of weight and more importantly clearance for the driveshaft going to the front axle. Your tight spots will be steering to exhaust on the left (hold that as tight as you can) and clearance for the front driveshaft on the right. Fore and aft position will vary with the Jeep model and engine. Have a CJ5 and Gen I with a rear distributor? You’ll be better off a little forward for more rear driveshaft length and clearance for that HEI. If you are in a longer Jeep and using an LS engine with no distributor, you’ll have more fan clearance and better balance if you hold it to the rear. Usually for ground clearance tuck things up nicely for height to avoid damage to oil pans and other life giving parts in the Jeep. Common sense and taking a step back to look at things overall goes a long way.

Clocking Information & Driveshaft to Pan Clearance

Our adapter is drilled for multiple clocking locations and driver’s side drop transfer cases usually clear nicely in most all applications.

Engines & Engine Mounting

GM Engine Mounts for JeepsNovak engine mounts are available to ease the installation of your GM V6 or V8 engine. Novak has engines mounts for most Chevy, Buick and GM Gen III+ V8's.

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.

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 TJ Jeeps have higher hood clearance, clearance for classic Small Block and high Vortec (truck style) intakes and accessories is not a problem. However, XJ / MJ Jeeps have low hoods and will require the use of lower car intakes and accessory packages.

Novak Gen III+ engine mounts for TJ, XJ, & '76-'86 CJs will remove any guesswork from the engine installation location as they are predetermined for the best location and engine clearance margins.


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.


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.

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