Novak Conversions Adapter 4L61-23 Kit

Adapting the GM 4L60-E (later version) Transmission to the Jeep Models 207, 231, 242, 249 & 241OR Transfer Cases with 23 spline input.

Our 4L60-E (including the 4L65 & 4L70) to the Jeep New Process (or New Venture) transfer cases conversion makes for one of the most refined assemblies both on-road and off-road. This kit has become our most popular transmission to transfer case combinations.

 **Please allow 6-8 weeks for delivery as these shafts are backordered.

Product & Features

***Please read 21 spline notes below before purchase***

The Adapter

The Novak 4L60E to Jeep NP231J transfer case adapter is very compact at only 3.0" long for a combined transmission and adapter length of 25.5" (equivalent to a 17.5" manual transmission). The adapter itself is machined from billet 6061 T6 aluminum, featuring thick flanges, a heavy cross-section, and superior strength.

Advanced computer modeling methods were integral to the design and manufacture of our #4L61 adapter assembly. We were able to push the bounds of product and process well beyond the status quo.

Transfer Case Clocking & Rotation Options 

Our adapter is drilled for multiple clocking locations from near-factory (20 degrees down) to near-flat (3 degrees down). However, pan clearances and front axle width may inhibit the flatter options.

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 for purchase below.

Contents 

Kit contains billet adapter housing, transfer case input, transfer case input spacer with seal, transfer case to adapter gasket, transmission to adapter sealing ring, fastening hardware, steel mounting base, and instructions.

Applications & Compatibility

Transmissions

The 4L61 adapter is compatible with all versions (2wd or 4wd) and years of the 4L60E, 4L65E & 4L70E transmission. We’ll refer to all these as 4L60E from now on but compatibility is all the same. For differences in these variations refer to our knowledge page linked below. There are no inherent advantages to the 2wd vs. 4wd transmission as the output shaft will be replaced. These kits are for the hexagonal six-bolt rear pattern case made starting in 1996 and include a detachable bellhousing. Some 1999 and earlier versions have a four-bolt rear pattern and non-removable bellhousing. See our Kit #171-E if this describes your transmission.

The new 23 (or 21) spline 4L60E output shaft is machined from 300M alloy steel which provides a substantial strength upgrade even over the stock 4L70E version. The new Novak shaft splines directly into the OEM style input gear of an NP transfer case. This requires disassembly of the transmission, and we recommend the services of a transmission professional.

The basic installation is a 1.5-2 hour bench job and will include at least simple gasket and seal replacements. This is a good time for a more thorough master rebuild and update of the transmission. Many consider one of our complete Novak remanufactured and upgraded transmission packages. The advantage of a superior, professionally built transmission, tested and ready to bolt in delivered to you is definitely worthy of consideration.

Strength of 21 spline versions and warranty exclusion

The 21 is substantially weaker than the 23 spline version. 21 spline versions were only used in the 4 cylinder applications and some very early automatic 6 cylinder versions. WE DO NOT OFFER A WARRANTY ON ANY OF THE 21 SPLINE SHAFTS because of this weakness. Our recommendation is to change the input to a 23 spline version as sold below and don’t put a drivetrain “fuse” in a bad location. On lower powered V6 applications in a lightweight vehicle driven conservatively you may get away with it.

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.

Output Shaft Strength

We’re avoiding the problem of a broken shaft like this one by going through the trouble and expense of making our output shafts from a solid bar of exceptionally strong 300M alloy steel, instead of lower grade alloys or friction welding salvage GM shafts. A second, sometimes less obvious but pernicious problem is the issue of warpage during friction welding and heat treating - which cannot be corrected when the shaft journals are already ground and worn at or below factory specifications. Novak shafts receive a final grind of bearing journals after the heat treat process to ensure the ultimate finish. The Novak shaft is produced to factory specifications with stress risers removed. Our 4-axis CNC cut of spiral gear, oil feed passages and other features are no small tasks. This attention to detail, maximized strength through design and material along with precise tolerances provide the ultimate parts for your project.

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 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 4L60E should stock strength come into question. 

Engines

Any engine that can be made to work with the 4L60E will work with this adapter. All of the above mentioned native engines along with others that can be adapted via a front adapter (such as our 437 series kits) open up numerous possibilities. 

There were also a lot of other combinations available to the 4L60E that may not be perfect candidates for a Jeep swap. The Atlas series inline engines, 3.8L in the 90’s era Camaro, a rear configuration for the Corvette, there is even a Bellhousing available in Australia for the Holden that will allow a High Feature DOHC V6 to mate to the 4L60E. Sometimes just because it’s possible doesn’t make it a good idea, but we’ll leave that part up to you.

Keep in mind that if you stray from a native configuration on the engine, you will need an aftermarket controller for the transmission as these are electronically controlled both in the shifting and pressure. Without a controller you’ll have reverse, third and max line pressure, probably not the best solution for a Jeep.