The Heavy Duty SYE (Slip Yoke Eliminator) kit is a highly recommended upgrade to all of the Jeep 231 transfer cases. By replacing the problematic OEM tailhousing and mainshaft with a fixed yoke design 32 spline HD output and Yoke you create a significantly shorter and stronger New Process (later New Venture Gear) 231 transfer case.
Top: 231XS Ultra-Short Shaft
Middle: 231SYE Standard Short Shaft
The Novak Designed 231XS kit, at 7” shorter than stock, is our shortest and strongest design. If you can accommodate the electronic speedometer output this kit is our first recommendation. 1992 and newer Jeeps have this type speedometer. This kit includes the reluctor and sending unit. On earlier Jeeps you will want the optional connector, later TJ versions plug in. If yours is different than shown below, you will need the plug.
This SYE kit is 4” shorter than stock and allows the use of a mechanical speedometer or the stock electronic speed sender.
Top to bottom, 231XS, 231SYE, stock.
A comparison of the three output shafts showing the difference in length. Notice the massive diameter difference on the output side of the shafts. This translates exponentially into strength. The factory NP231 output shaft exhibits a very weak area just behind the oil pump splines. It is not uncommon to see these shafts shear at any one of three stress risers in the section that needs strength most.
All kits include replacement output shaft, tailhousing, rear seal, bearing, 1310 CV style yoke, hardware, and instructions.
There are several areas worth noting on this topic. Our SYE kits solve the first three and parts are available below for the others as necessary or desired. Our recommendation is in italics:
These kits are designed for the NP/NV 231 AMC/Jeep transfer case. All kits will fit all transfer cases with the differences noted above which are:
The New Process (or later New Venture Gear) transfer cases are unusually strong for their mass and size. As long as your chain isn’t stretched and you keep up with fluid levels they will take a surprising amount of power and abuse. Like anything else, time and use takes its toll and these rebuild kits consist of the best available components including top quality chains and bearings. The rebuild process is not overly difficult and if you are in there anyway to put in an input gear our advice is to rebuild while you are there, especially considering the chains can stretch with use.
The 231 kits 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. Early uses a 24mm wide bearing and late is 16mm wide.
We also have the 1-1/4” wide chain (stock is 1”) upgrade rebuild and sprocket kits available
The necessary wide drive and driven sprocket set is required to run the wider chain
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 is a special case requiring special gears. It is a late cut with additional machining required.
This box allows your speed signal to be adjusted for tire size and gear ratio for accurate speedometer reading. It also allows the Jeep 3 wire square wave signal to be simultaneously sent as a 2 wire sine wave for your GM engine PCM on a manual transmission application. The GM PCM will need to be programmed to accept a 8,000 pulse per mile signal.
The stock Jeep 231 comes with a 3 planetary gear set. The HD units have 6 planetary gears which in essence doubles the strength of the low range gear set. They are also divided early and late the same as the input gears and rebuild kits. See the reference on early and late for proper selection for your transfer case.
The Late Gearset
The Early Gearset
Driveshaft length changes are often required. Most conversions to these transmissions will require that the rear driveshaft be modified to be shorter and front driveshaft longer. 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, usually to the detriment of the project. Our recommendation is to always value the correct position of drivetrain components over saving a few dollars which is usually regretted in the long run with compromised positioning. Sometimes the application allows use of the existing shafts without modification. While that is perfectly acceptable, it should not be the first priority. Driveshaft modifications and rebalancing can be affordable when performed by driveline, RV or tractor implement specialists. New driveshafts are an option, but not necessarily a requirement in regards to the actual successful conversion if your existing driveshafts are in good condition. 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. Novak can also accommodate your needs on this if provided with dimensions for the shafts at a reasonable rate.
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.
All versions require removal of the old tailhousing and output shaft and installation of the new components. Normal mechanical tools are required as well as RTV to re-seal the case halves. Often a rebuild is performed at the same time with new chain and bearings as you are already halfway there.
A new rear driveshaft with Double Cardan/CV style upper joint is required and will be longer than the previous shaft and at a lower angle. Often if you have dropped your transfer case in an effort to minimize angle of the rear shaft, the lowering kit can now be removed due to decreased angularity from both length and the fact that the upper angle is now split between two joints. Proper installation is to have the rear pinion yoke a few degrees below pointed directly at the transfer case. You will want to rotate your rear axle accordingly. If you provide us the center of yoke to center of yoke distance we can supply the rear shaft for you as shown above.