NP / NVG 231 Slip-Yoke Eliminator Assemblies
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.
Versions
Installers should be aware that neither of these kits have a provision for the vacuum switching feature as used in early YJ or Cherokee applications. However, most Jeep builders have usually eliminated that system at this point by either installing a cable or permanent front axle mechanism.
Super Short SYE
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 need the optional pigtail connector, shown in Related Products, below. Most TJ versions do not require this.
Standard SYE
This SYE kit is 4” shorter than stock and allows the use of a mechanical speedometer or the stock electronic speed sender.
Contents
All kits include replacement output shaft, tailhousing, rear seal, bearing, 1310 CV style yoke, hardware, and instructions.
Driveshafts Comparison
Top to bottom: 231XS, 231SYE, stock.
A comparison
of three driveshafts and the differences in length and corresponding angles. Reduced angles
means reduced vibration and risk of breakage.
Output Shaft Comparison
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.
Applications & Compatibility
231 Transfer Case Upgrades
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:
- Vibration: long factory slip-yoke allows excessive u-joint angles which are the root cause of damaging vibration in lifted Jeeps, or Jeeps that have had conversion transmission upgrades. - Essential
- Breakage: the reduced diameter of the rear portion of the factory shaft has stress risers and breakage under load is not uncommon.- Essential
- Driveshaft Disconnection: the slip-yoke design can slip right out the back of the transfer case at rear axle droop, disconnecting the driveshaft and allowing the transfer case to quickly lose its lubricant out the output assembly.- Essential
- Wear in the chain and bearings. For such a compact and lightweight unit, the 231 with SYE in place will handle a substantial amount of power as long as the chain is not stretched and bearings are in good condition. If your transfer case has more than 100K miles on it, is of questionable origin, has been subjected to hard use or you just want to be sure, a rebuild with a new chain is recommended. - This is highly recommended unless you are certain of the current condition of the chain in particular.
- A wide chain and sprockets kit adds 25% more strength by increasing the width of the chain from 1” to 1-1/4”. - This is usually overkill.
- If you want to go above and beyond on the strength the low range planetary can be upgraded from 3 to 6 gears. This doubles the contact area and substantially increases the strength of the gears when in low range. - This is usually overkill.
Compatible Jeeps
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:
- 231XS is electronic speedometer output only. It is so short there is not room for a speedometer drive gear assembly. It will work on all year 231 transfer cases but will only output a 3 pulse per rotation, 3 wire square wave speed signal which is what the 1992 and newer Jeep uses. Any speed adjustment will need to be done with an external adjuster box as listed below or in the speedometer itself.
- 231SYE uses the stock speedometer sending unit on all year Jeeps. It will work with any Jeep 231 transfer case not requiring the vacuum disconnect switch commonly used on early XJ Jeeps. you can use this kit on those Jeeps when eliminating the vacuum disconnect switch.
Related Products
Installation Requirements
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.
The #231XS, as installed.
Driveshafts
Driveshaft length changes are required.
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 be chosen for reasons external to this transmission upgrade.