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Installing the Tera 4:1 Kit
In a Jeep NP231 Transfer Case
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By: Randy L. Wheeler- 4/2001

Tera's 4:1 Gear Reduction Kit for the NP231 Transfer Case


One common question I hear people ask is "I just installed larger tires and need to know if I should change my gears?". The answer, of course is, it depends. Changing the gears inside your differentials can help get your engine running back within its proper power band, but often times, changing to lower gears (numerically higher) will only marginally increase your crawl ratio. For example, say you've got an NP231 in your Jeep. The low range for this transfer case is 2.72:1. To calculate your crawl ratio, use the following equation:

(transmission 1st gear) x (transfer case low range) x (gear ratio)

With the AX-5 transmission, NP231 transfer case, and stock gears, the calculation for crawl ratio would be:

(3.93) x (2.72) x (4.10) = 44:1

 

Photo courtesy of Teraflex
The TeraLow231 Kit from Tera Manufacturing comes complete with a new front case, oil seal, silicone and locktight.

Installing a set of 4.56 gears would only change your crawl ratio to 49:1. That's only about an 11% reduction. Changing the differential gears to 4.88's would net a crawl ratio of 52:1 (a 15% reduction). While these numbers are an improvement, they're only slightly better than the stock configuration. So how can we improve our crawl ratio without sacrificing too much performance on the street? The answer, install a transfer case gear reduction kit.

The TeraLow 231 4:1 Conversion Kit is an economical solution for providing low crawl ratios while preserving your highway fuel economy. The TeraLow 231 can be used by all vehicles with 231 transfer cases such as 1988-99 Wranglers, Cherokees, and Comanche's. It provides true 4:1 reduction without the noise of older systems. The TeraLow 231 affects only the 4WD low range and does not affect the 4WD high or 2WD high ranges.

The 4:1 kit comes complete with a brand new front case half with compound planetary gears, front output yoke oil seal, a tube of silicone and some red locktight for the bolts.

Disassembly of the stock case

Installing the TeraLow 231 kit requires disassembly of the transfer case once the case has been removed from the vehicle.

The installation instructions included in the kit start at the point where the transfer case has already been removed from the vehicle.  However, I thought it would be helpful to review the steps required to remove the transfer and separate the case halves and install the new parts. These steps should only be used as a reference. Always follow the instructions included in the kit and the directions in the service manual.  As a side note, the Haynes manual I have for my Jeep does not provide any detail on the removal or service of the NP231 transfer case.  A Factory Service Manual would a better reference.
View showing the transfer case separated

Drain the transfer case and transmission fluids and then support the transmission and the transfer case with jack stands. Shift the transfer case into 4WD low range.  Remove the transfer case skid plate, front driveshaft and rear driveshaft.  Disconnect the speedometer cable, electrical lines, vacuum lines and 4WD shift linkage from the transfer case.

Remove the six nuts that attach the NP231 transfer case to the transmission. Once the nuts are off, slide the transfer case off the transmission and remove it from the vehicle.  Once the case is on the bench, there are a couple of things we need to do to prepare the new front case half for re-assembly.  

The studs from the old case will have to be removed and installed in the new case.  Using the double-nut method, remove the studs.  Once the studs are out, remove the 4 bolts that hold the front bearing retainer cap on and remove it. Save the bearing retainer cap.

View showing the output shaft and main shaft removed. The sector, range fork and shift hub are still visible.

Remove the speedometer gear unit and remove all the bolts that hold the tail cone to the tail housing, and remove the tail cone. Remove the snap ring that retains the rear output shaft and save it, you'll reuse it. Next remove all the bolts that hold the tail housing to the case and remove the tail housing. Next, remove all the bolts that hold the transfer case together and pry the case apart. Once the case has been separated, remove the top half of the case and set aside.  This half of the case will be reused.  Remove the mode spring and gently pull the front output shaft and drive chain out as a unit. Slide the main shaft, mode fork and shift rail from the front case.  Set these all aside on a clean surface.  Remove the range fork and shift hub as a unit and also remove the shift sector.

Once the innards have been removed from the old case and set aside, remove the sector shaft bushing and O-ring from the old case and set aside.  These will be reused in the new case.  Remove the shift detent pin, spring and plug and set aside.  The front output shaft bearing will be reused in the new case.  To remove the output shaft bearing, remove the shaft oil seal from the front case and remove the bearing snap ring.  The new case comes with a new oil seal, so don't worry about messing the old one up trying to get it out. Remove the vacuum switch from the old case and set aside.

Reassembly of the new case

To reassemble the new case, start by installing the front output shaft bearing, snap ring and new oil seal.  Install the vacuum switch, shift detent spring/pin/plug, and sector shaft bushing and O-ring.  If you haven't already done so, install the case studs and bearing retainer cap.

Install the output bearing and the snap ring. Install the output bearing and the snap ring.
Install the studs into the new case. Be sure to use Locktight! Install the front output bearing and vacuum switch Install the snap ring and oil seal.

Once all the external components have been installed, we can start assembling the new case. Turn the case over onto the front side and install the shift sector, then the range fork and shift hub as an assembly.  Next, install the main shaft, mode fork and shift rail as a unit.  The main shaft will engage the splines in the new case.  Be sure the main shaft fully engages the splines and seats properly.   

Install the main shaft, mode fork and shift rail

Install the front ouutput shaft and drive chain Slap the case halves together using silcon sealer between the case halves.
Install the main shaft into the new case. Install the front output shaft and drive chain along with the mode spring. Run a bead of silicon around the cleaned surfaces of both case halves and mate the two together.

Once the main shaft has been installed, install the front output shaft and drive chain and mode spring.  Double check the operation of the output shafts, bearings, shift forks, etc before mating the two case halves together.  Make sure the two alignment dowels from the old case are reused in the new case.  Once your satisfied that everything is working properly, apply a bead of silicon around the one case surface.  Mate the two cases together, making sure the oil pickup tube and pump are positioned correctly. Torque the bolts to specifications.

Install the rear tail cone housing and tail cone using a small bead of silicon between surfaces.  Install the front output shaft yoke and retaining nut using an impact wrench.  With this step, you're just about finished with the swap.  Install the front bearing retainer cap and torque to specifications. Double check all bolts and re-install the transfer case back into the vehicle.  Fill the transfer case with ATF fluid (DEXRON II or III) and top off the transmission with gear oil and you're all set.  Note that Tera recommends running synthetic ATF fluid in their 4:1 kit.

Install front output shaft yoke and retaining nut. Install the front bearing retainer cap
Using an impact wrench, install the front output yoke and nut. Install the front bearing retainer cap. The competed unit.

 

Install the front bearing retainer cap
Front view of stock case on the left and the new unit on the right. Top view of stock case on the left and the new unit on the right.

 

Conclusions

Installing a low range transfer case kit has been on my To Do list for about as long as I've owned my Jeep.  Swapping in lower gears (numerically higher) helped quite a bit, but I still needed a lower crawl ratio to creepy-crawl up and over rocks.  By reducing the transfer case gears, you put a lot less stress on your clutch and other drivetrain components.  The 4:1 transfer case reduction kit from Tera Manufacturing is an easy and economical way to maximize your crawl ratio and to help minimize stress on other components.  If you can go slower and smoother on the trail, you can keep your rig under control and reduce the risk of breakage.  This kit is easy to install, and using standard hand tools, can typically be done in a day.  So if you're considering improving your trail rig, consider the Tera 4:1 kit.  You won't be disappointed. In my case, with the 4:1 kit and 4.88 gears, my overall crawl ratio is now a respectable 77:1!  That's a 43% reduction! 

(4.00) x (3.93) x (4.88) = 77:1

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