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NP205 Gearing Options From Offroad Design

Posted in How To: Transmission Drivetrain on August 15, 2017
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Big tires and big obstacles can rob you of torque, traction, and possibly wheel speed. Changing axle gear ratios can help and is most often done to restore reasonable gearing for on-road driving with tire sizes that are much larger than stock. While this does help on the low end for wheeling as well, sometimes it is beneficial to have far deeper gearing when playing in dirt and boulders. Super-low gearing can provide you greater control over obstacles, prolong the life of clutches and transmission parts, and give you more slow-speed traction since the drivetrain can provide a more consistent power delivery to help tires resist breaking traction while crawling.

Offroad Design (ORD) has developed several solutions to give people running a New Process NP205 T-case the ability to run lower T-case gearing. The venerable NP205 is generally considered to be bulletproof and was installed in a variety of fullsize vehicles from Ford, Chevy, Dodge, and International Harvester ranging up to 1-ton trucks.

The ORD kits essentially add a second high/low gear reduction placed between the transmission and the NP205 T-case to provide additional low gear ranges. With the added front gear reduction, you can now run in 2WD low range with that ratio. The kits can be added to almost any domestic transmission, either directly or with an adapter. Another benefit is the ability to clock, or rotate, the T-case as needed for ground or chassis clearance. The company’s two kit choices are the Magnum Box and the Doubler.

The Magnum box proves a relatively compact, add-on planetary reduction unit in a cast aluminum housing that effectively makes your T-case a four-speed unit. A six-pinion planetary provides a 2.72:1 gear reduction to combine with the factory 1.96:1 low-range ratio of the NP205. In the end you get standard 1:1 high-range gearing, the two individual midrange reduction ratios (2:72:1 and 1.96:1), and a compound 5.33:1 crawling gear when you run both reductions in low range.

The components in the Magnum Box are designed to be stout enough to use the T-case in any gear mode without worry of breakage downstream of the gear reduction in the planetary. The intermediate shaft (new NP205 input shaft) used is oversized to accommodate the torque that additional gear reduction adds. The box bolts directly to GM/Dodge Figure 8, GM round-pattern, or Ford NP205 T-cases with no additional adapters.

The Doubler kit places a gear reduction unit (front range box) from an NP203 between the transmission and the NP205 using a billet aluminum adapter plate and a custom output shaft. It is a bulkier solution than the Magnum Box but also more economical. In the case of the Doubler setup, both reductions are nearly 2:1 so you don't get quite as much gearing flexibility as you do with the Magnum. In fact, ORD recommends you not use low range in the NP203 while using high range in the NP205 to avoid the possibility of breaking the intermediate shaft. In effect, the Doubler can be considered a three-speed T-case system.

In either case, ORD can provide T-case parts and kits for the DIYer or build a complete T-case system to your specifications to include shifters, heavy-duty shaft and yoke components, and other upgrades. The work of installing a kit can be done in a home garage with common mechanic’s tools plus a high-quality set of snap-ring pliers.

Installing a twin T-case setup is not without further modifications to your rig. Generally, modifications will need to be done to driveshafts, crossmembers, shifters, and mounting when installing the overall longer T-case assembly. However, for those drivers who crave deeper gearing and more control over their ability to tackle gnarly obstacles, the upgrade will prove to be worth it. A T-case with a wider range of gear options allows you to effectively use your rig to travel down the highway, run those easier dirt sections, and then drop into your lowest gearset for the really challenging stuff.

ORD starts with a raw aluminum casting and machines it in-house to build the Magnum Box housing. Multiple threaded holes are machined into the face of the housing to allow the T-case to be rotated as desired in the vehicle.
Inside the Magnum Box housing, ORD uses a New Process six-pinion planetary unit with a 2.72:1 gear ratio. Here you can also see the sliding shift hub and shift fork used to change between high and low range.
Over the years, New Process manufactured NP205 cases with a small (80mm, shown here) and large (90mm) input bearing case diameter. The Magnum kit needs to mate in a large bearing bore, so you will need that style of case or else have the bore enlarged by a machine shop.
We visited Offroad Design to follow one of the assembly experts as he installed a Magnum Box and later a Doubler kit. He began by working on an NP205 T-case to replace the stock input shaft with the new Magnum unit. He removed the rear output assembly, stock input gear, and bearing from the housing.
The new Magnum input gear shaft was placed into the NP205 housing, followed by the clutch ring and shift fork.
What makes the Magnum the strongest solution? The chromoly Fat Shaft (bottom) maintains the large diameter of the planetary output throughout its length. It can be compared here to a typical 31-spline input (intermediate) shaft such as is used in the Doubler. The strength of the Fat Shaft allows you to safely run any gear reduction combination upstream or downstream in the T-case system down to 5.33:1.
The assembler placed an input bearing on the Magnum shaft followed by a snap-ring to hold them in place.
Needle bearings are used in the end of the rear output shaft. It is necessary to make sure they stay in place for reassembly; thick grease helps with this task.
The original rear output assembly was placed back onto the housing, leaving the NP205 ready to accept the Magnum Box.
The Magnum Box housing comes in two halves. The back half was bolted to the front of the NP205 using six socket-head cap screws and secured with thread-locking compound.
Next, the front half of the housing was bolted in place. ORD recommends using antiseize compound on the bolts that join the aluminum case halves.
ORD uses silicone sealer to seal the machined surfaces. Once assembled, the shift fork was worked back and forth to confirm that the unit spun freely in both high- and low-range modes. The added Magnum Box increases the weight of the T-case by only 34 pounds.
A seal was pressed into the Magnum Box where the input coupler resides. The unit is sealed at both ends and uses its own oil supply.
A tee was plumbed to the top of the Magnum Box. One end will later be connected to a vent tube. The other end was connected to a clear fluid sight tube that also connects to the drain hole at the bottom of the housing. The brass fill plug can also be seen here. ORD recommends filling the housing with just under a quart of either high-quality automatic transmission fluid or gear oil.
The supplied shifter plate was bolted on last. The shifter arm can be flipped either direction to accommodate various shifter locations. A spring-loaded detent ball in the Magnum box keeps the shift fork held snuggly in the desired position.
ORD has a number of shifter assemblies available to adapt to most any vehicle setup. Incorporating the ORD twin-stick conversion in the NP205 will allow independent control of front- and rear-wheel drive, and high or low range.
Here are the two versions of the back half of the Magnum Box. Each comes with a steel mount plate that can be used for support from a crossmember, typically through a polyurethane bushing assembly.
If you choose to go with the ORD Doubler kit, you will need a donor NP203 gear reduction portion. You can source a used one yourself or buy one from ORD. Here the front range box is separated from the chain case on a salvage NP203.
The factory low-speed gear and input bearing from the NP203 are also reused in the Doubler setup, so they need to be harvested from the input shaft assembly.
Here is a look at the components included in the Doubler kit. A billet 6061 T-6 aluminum adapter plate is used to mate the range box of the NP203 to the front of the NP205 housing. A custom, heat-treated chromoly input shaft is included.
The original NP203 low-speed gear was placed onto the new input shaft.
The original input shaft bearing was pressed into the new adapter plate and secured with a snap ring. The adapter plate can be machined by customer request to allow rotation of the mating NP205 for ground clearance or chassis fitting.
The input shaft assembly with thrust washer was mated to the bearing in the Doubler adapter. A snap ring secures the shaft in the adapter, and a seal is used where the input shaft exits.
With needle bearings placed in the tail of the input shaft, the man joined the adapter assembly to the range box, then bolted them together.
Here is the completed Doubler unit ready to mate to the NP205. The gear reduction in the NP203 range box has a ratio of 2.01:1, and combined with the 1.96:1 low range of the NP205 this means a low-low range of 3.94:1.
The total weight of this T-case combination is about 240 pounds, so ORD recommends installing the NP203 portion in your rig first, before the NP205. The Doubler assembly is heavier and bulkier compared to the more compact Magnum Box.
A shifter is added to the vehicle to control the NP203 range box, the same as on the Magnum Box.
Multiple T-case gearing options allow you to choose the proper gearing for running fire roads, mud, or dunes, and for rockcrawling.

Sources

Offroad Design
970-945-7777
www.offroaddesign.com

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