Assembling Strong Box DIY Kits

Thanks for purchasing Behemoth Drivetrain’s Strong Box!
You are only a few steps away from transforming your off-road machine into a “Torque Monster”!
Fig. A

     First, you will need a planetary gear set to install into the Strong Box housing. We make several different versions of the Strong Box that can mate various transmissions to many transfer cases. Make sure you have chosen a gear set that has an input shaft that is compatible with your transmission’s output shaft; i.e. TH350 – 27 spline input, TH400 – 32 spline input, etc.. Bolt pattern adapters are available, if needed.
A basic transfer case planetary gear set for the Strong Box consists of 7 pieces:
✓ Input bearing.
✓ Larger snap ring to retain input bearing in gear housing.
✓ Smaller snap ring to retain input shaft in input bearing.
✓ Input shaft. (must be compatible with your transmission’s output)
✓ Planetary gears. (can be 3 to 6 pinion)
✓ Ring gear (a.k.a. Annulus Gear)
✓ Range Hub (a.k.a. Slider Gear)

Fig. B

Above we see a NP231 planetary gear set. Other gear sets will be similar, but may vary slightly. Contact us if you have any questions about your parts. Also, see our Guide for Preparing Planetary Gears.

Fig. C

Kit Contents

✓ Front Section (gear housing) with bolts and seal
✓ Rear Section (shifter housing) with bolts and plugs
✓ 4340 thru hardened output shaft with bearing support
✓ T-Case Clocking Ring Adapter (for non-Shorty kits only)

   Shorty kits replace the input shaft of the transfer case behind them. Shorty kits bolt directly to the transfer case behind them, and do not use a Clocking Ring Adapter.
1. If your kit is a non-Shorty kit, and came with a Clocking Ring Adapter, install the Clocking Ring Adapter onto the transfer case you will be
using behind the Strong Box. You may choose to use either RTV sealant or a paper gasket.
We do not provide bolts to fasten the Clocking Ring Adapter to your transfer case. This is because some modern transfer cases will use M10 metric bolts, while others that are usually older will use 3\8”-16 standard threads. The user must determine which they have.
NOTE: The 3/8” bolts will thread into the M10 holes, but they will be a very loose fit and will strip out easily. This can cause the transfer case to detach, damaging components and creating a potential safety hazard.
2. Most ring gears will have “splines” on their outer diameter. Some Borg Warner Planetary gears will use “dogs”. Both are to keep the ring gear from rotating. One side of the ring gear will be smooth, without splines or dogs. This is the side that you will use as a pilot, when pressing the ring gear into the Strong Box gear housing.

Fig. 2A


Take care that you do not rest the gear housing on the input seal lip while pressing, or it could damage the housing. Press gear in until it bottoms out in housing. Should be ~ 0.010” gap for splined gears.
3. There are 2 ways that input bearings are held in place in Strong Box kits. For internal snap rings, the bearing is installed into the housing, then a snap ring is installed into a groove and retains the bearing. For external snap rings, the bearing with snap ring is installed into the housing, then bolts are installed which clamp snap ring in place.

Fig. 3A

Fig. 3B



Fig. 3C

Some users will need to convert their external snap ring to an internal. This is easily done with a grinder by cutting off about 1” of the snap ring, and grinding it at an angle to a point, so that it can be easily removed for servicing.

Fig. 3D

Be sure to clean any debris from housing as you go through each step. Even a small particle can disrupt rotation, and can result in a “thumping” sound as you operate your vehicle.

4. Proceed to install the planetary gears in the same manner which they were removed from the donor transfer case.

Fig. 4A

Fig. 4B

Then, flip the gear housing over, with the input shaft pointing upward. Install the snap ring to retain the input shaft in the input bearing, then install the input shaft seal.

Fig. 4C

Fig. 4D

Once the input shaft snap ring and seal have been installed, the gear housing is ready for installation.

Set the assembled gear housing aside, and turn your attention toward the rear section, which is the shifter housing.

5. There are a few different types and variations of Range Hubs (slider gear).

Fig. 5A

*NP231/241 and similar use the hubs pictured above. The hubs can vary slightly in overall length, so we like to measure from the middle of the shift fork groove to the middle of the external gear teeth.
*The long hub measures about 1.6”, and is usually associated with a manual shift transfer case.
*The short hub measures about 1.3”, and is usually associated with an electric shift transfer case.
*NV271/273 and similar cases use a hub that measures about 1.4”.

*Borg Warner Cases will generally use a shift hub with a flange, instead of a groove, and the shift fork pads will have a groove to accept the flange. However, some Borg Warner cases also use grooved hubs, similar to the NP cases.
Once you’ve determined which hub you have, you can proceed.
Alternatively, if you can let us know at the time of order which hub you have, or if your donor transfer case is manual shift or electric shift, we can configure it for you, and you can skip step 5. Also, if you have a kit for gears other than the NP231 and similar gears, you may skip step 5.
For NP231/241 and similar gears, you will need to configure the shift fork according to your hub.

Fig. 5B

Fig. 5C

Fig. 5D

Once you have the shift fork correctly assembled, you can install it into the Strong Box shifter housing.

Fig. 5D

*Use short hub setting for NV271/273, then“Fine Tune Shift Position”. *
✓ Configure shift fork & shift plate/detent shaft to match your hub.
Be sure to tighten bolt holding the shift fork onto detent shaft just enough to take out any slop, but not so much that it cannot rotate.
✓ Install detent shaft through bearing; tap into place.
✓ Install 2nd O-ring onto detent shaft. Then, install tube spacer and shift lever arm.
✓ Install detent ball, spring, then detent screw. Detent screw allows you to adjust the shift pressure to suit your needs.

Fig. 5E

Fig. 5F

6. Install the Range Hub (slider gear) onto the shift fork pads. Shifting the kit into High or Low, can sometimes make this easier.

Fig. 6A

Borg Warner gears use 2 fork pads that swivel on a “C” shaped carrier. The retainers for the fork pads are offset, and should be installed with the offset toward the rear of the Strong Box.

Fig. 6B

Fig. 6C

7. With the Range Hub in place, you can now slide the output shaft through the Range Hub and output bearing. You may need to tap the shaft with a mallet to be sure it is properly seated in the bearing.

Fig. 7A

Fig. 7B

8. You are ready to bolt the gear housing to the shifter housing. Take note or mark where the top of the gear housing is, when bolted to your transmission. Position the shifter housing so that your Strong Box shifter mechanism, and your fill / drain / vent plugs will be where you require for your build. Once you have it where you want it, take note or mark it against the gear housing.
NOTE: It is a good idea to do step 8 as a dry fit without sealant first. Then, use your marks to align it after step 9. Once you are sure of the optimum clocking position for your build reassemble with sealant.

9. Bolt the assembled Strong Box to the Clocking Ring Adapter (or to the transfer case, if it is a Shorty kit).
When using a Clocking Ring adapter, you have another set of bolt patterns for clocking the transfer case to the optimum position for your build. Take note and mark the clocking ring position against the shifter housing.