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5. Install input seal. Set gear housing aside. 

​​​  4340 alloy steel is mainly used for manufacturing landing gear for heavy aircraft. Our 4340 thru hardened shafts come with a Limited Lifetime Warranty* against breakage. 

   We use a rear bearing to support the output shaft, and keep all components concentric.

2. Install input bearing and snap ring, then install ring gear. 

Strong Box Underdrive For Full Size Ford Applications

3. Install input shaft with planetary gears. Some tapping may be required. 

1. Using a rod or shaft that is bottomed out, mark a 1/2" from the end of input shaft, and tap down into the shaft to your mark. 

BW Strong Box Underdrive For Full Size Ford Application 31 spline input / 31 spline output - DIY Kit $699

Pre-Assembly / Good, Used Gears $300.00

4. Flip gear housing over, and install input shaft snap ring. Ring gear may fall out. 

​   This 2.7:1 Strong Box can be ordered to use either BW1356 gears, or BW4406 / BW4407 gears. Either version is 8.2" long X 7.9" diameter, and weighs about 40 lbs. It is argued that the BW gears are just as strong, or stronger than the NP and NV planetary gears. 

   The advantage to using the BW gears mainly applies to those who are building a Full Size Ford drivetrain. While the NV271 / NV273 can work great in our 2.72:1 Strong Box, they can be expensive or difficult to locate. We find that many full size Ford enthusiasts can locate BW cases 15:1 over the NV cases with the 31 spline Ford input shaft option. 

   With all of the same great clocking, shifting, and strength qualities as our other Strong Box versions, it's an obvious choice for someone with access to BW gears. 

6. Shift fork pad retainers will be offset toward rear of housing. 

9. Now you are ready to decide which of 12 positions is suitable for you to bolt the gear housing to the shifter housing. You can use the clocking on the front of the housing to fine tune the position of the shifter mechanism, once bolted to the transmission. Be sure to add sealant to prevent leaks. 

10. Bolt your clocking ring adapter to your transfer case. Be sure to add sealant to prevent leaks. 

11. Now you can bolt your clocking ring adapter to the rear of the Strong Box. You have 360 degrees of clocking in 5 degree increments. If you need a position between the 5 degrees positions, you can mount the gear housing to the transmission in one of the next holes, and it will divide the 5 degrees. Essentially, The Strong Box can be clocked 360 degrees in about 3 degree increments when you combine the series of multiple indexed bolt patterns. 

7. Set shift mechanism to the high range setting. Angle shift fork pad retainers for installing shift hub slider. 


8. Set shift mechanism to neutral or low range setting. Then, install the 4340 output shaft. Tapping with a mallet may be required to properly seat the shaft.