Here is the finished product...
Here is the finished product in all its low-gear glory: an NP203/205 Doubler built for serious trail abuse.
For more than 11 years now, Off Road Design (ORD) has offered a NP203/205 Doubler kit designed to multiply the low-range gear ratio of fullsize trucks and SUVs. The kit enables consumers to couple the front half of an NP203 to an NP205, making it possible to run a choice of low-range gear ratios that simply would not be possible with any other factory arrangement. We like the ORD setup because it was designed to handle high torque levels, such as that of the GM Performance crate 383HT found under the hood of our SuperBurb.
Looking at the details, you can break down the Doubler system into three distinct sections: The front half or "gearbox" from an NP203, the billet aluminum ORD adapter plate, and a complete NP205 transfer case. This system provides an additional 2:1 reduction gear, which, when added to the existing NP205 low-range ratio of 1.98:1, multiplies out to a final crawl ratio of nearly 4:1. This combination of gearing vastly improves crawlability and torque multiplication without adversely affecting high-range cruising speed and engine rpm. Thanks to the popularity of the stout NP205 transfer case, this setup will work in a variety of different applications.
The system uses a combination of trail-proven, readily available factory parts, along with a handful of custom components manufactured exclusively by ORD. When compared to other aftermarket transfer cases, the ORD Doubler kit offers unparallel value thanks to the low initial cost of the used factory parts. In terms of strength-to-cost ratio, the Doubler setup is in a league all its own. Check this out: altogether, this project would have cost us just $1,927 to complete (not including labor). And speaking of labor, our capable friends at Right Gear and Axle of Salinas, California, completed the build in 16 hours, which may seem like a long time to some, but Right Gear and Axle pays considerable attention to detail. Follow along as we highlight their work during the conversion process
| Adding Up The Costs |
| Doubler kit |
$770 |
| Rebuild kit for NP205 |
$169 |
| Used NP203 |
$300 |
| ORD Triple Stick shifter |
$289 |
| Advance Adapters fixed output |
$399 |
| Total |
$1,927 |
We started this project by...
We started this project by locating a used NP205 transfer case found behind a TH400 transmission. These are available in most wrecking yards across the country. We picked this version because it features a stout 32-spline female input shaft. We could have used the version of the NP205 found behind the popular SM465 manual transmission found in 1968-78 GM pickups, but with only 10 splines on the input shaft, we would have had to swap it out for either a 1979-91 (32-spline) version or the TH400 type mentioned above. If you do not have or cannot find the proper type of NP205, don't fret. Off Road Design has everything you need to convert your NP205 into the version that fits your transmission.
The first thing Right Gear...
The first thing Right Gear and Axle did in preparation for this build was clean the exterior surfaces of each of our donor transfer cases. Here, you can see the NP205 as it was loaded into the parts washer.
The kit requires the front...
The kit requires the front gearbox portion (the first six inches of the case) of an NP203 transfer case. Luckily, these cases are readily available at just about any wrecking yard in the country. We paid $300 for this complete unit. It came out of a '70s-era Chevy 3/4-ton pickup. Off Road Design recommends getting a 203 from a '73-'77 pickup equipped with the Turbo 400 automatic transmission. This is preferred because it has the strongest input shaft available (32-spline) on NP203s.
With each of the cases cleaned,...
With each of the cases cleaned, the technician from Right Gear and Axle began disassembling the NP203 to separate the front half of the unit from the back. Here you can see one of the main gear clusters of the NP205 as it was being removed from the case. These gears are what make the NP205 desirable to four wheelers-no chain to stretch or break, just stout helical-cut gears with massive roller bearings and hardened support shafts.
With both cases completely...
With both cases completely gutted of gears and other internal parts, Right Gear and Axle sent the parts back into the parts washer for a second round of cleaning. Once each was washed, Right Gear's technician performed a crack check on every internal part in a 3,000-amp wet-method Magaflux station. Next, each case was baked in a 750-degree oven for three hours to remove impurities from the castings. From there, each case was allowed to cool before another technician media-blasted each case to provide an as-new finish for sealant and paint to adhere to. This process ensures that each case, as well as every internal part, is free of debris and ready for reassembly. Here you can see the rear cluster gear as it was re-installed in the freshly premiered NP205 case shell.
This is the basic ORD Doubler...
This is the basic ORD Doubler kit as we received it. The conversion includes everything you need to mate a NP203 low-range gearbox to a complete NP205 transfer case. These kits are available for GM, Ford, and Dodge applications.
Here you can see all the shifter...
Here you can see all the shifter parts ORD sent us to use with the Doubler kit. This particular kit provides consumers with three individual gear selection sticks. One shifter is designed to control the NP203 gearbox, and the other two handle the NP205, allowing individual control of the front and rear drive shift rails. This allows easy access to front-wheel drive and rear-wheel low-range should the scenario call for it.
With each component ready...
With each component ready to install, the technician started by fitting the roller bearing to the billet aluminum adapter plate. This required a torch to heat up the aluminum adapter plate so that the bearing opening could expand enough to allow the bearing to be tapped into position.
This was the toughest part...
This was the toughest part of the installation. It involved assembling the NP203 range box main gear cluster. The technician had to take great care as several small needle bearings were installed with grease to help them cling to interior of the shaft. Once all the bearings were installed, the Doubler adapter and the NP203 range box were mated together.
Next, the low-range gear and...
Next, the low-range gear and output shaft were installed into the appropriate opening of the NP203 range box. Once installed, the NP203 range box was attached to the billet aluminum Doubler adapter plate. The first step was to apply a good bead of RTV sealant to each side of the adapter gasket. The technician told us that it is best to do one side at a time, as shown here.
With the gasket prepped and...
With the gasket prepped and ready, the ORD Doubler adapter plate was secured to the NP203 range box.
The rear main shaft of the...
The rear main shaft of the NP205 also uses the same type of needle roller bearings as the NP203 range box, which again required a thick coating of grease for assembly.
Two modified factory PTO cover...
Two modified factory PTO cover plates come with the ORD Doubler kit. These covers feature drain plugs to simplify lubrication services on each of the cases. A layer of RTV silicon was applied to ensure a leak-proof seal.
This is the complete and freshly-painted...
This is the complete and freshly-painted NP203 range box after it was mated to the ORD billet adapter plate. Next, this unit was coupled to the input side of the NP205 transfer case.
A few weeks after our Doubler...
A few weeks after our Doubler project was completed, we got word of a brand-new rear output shaft upgrade from Advance Adapters. Shown in this computer rendering, the kit vastly improves the strength of the rear output shaft of the NP205 and includes a trick-looking billet-aluminum rear seal collar.
Here is one of the final reassembly...
Here is one of the final reassembly steps: the rear output bearing installed in the rebuilt NP205 case.
This photo shows how the triple...
This photo shows how the triple stick shifter setup was mounted to the side of the finished unit. We plan to have these steel brackets zinc plated in the future to prevent rust.
This is the NP205 HD rear...
This is the NP205 HD rear output shaft from Advance Adapters. This kit features a 32-spline fixed-yoke output shaft for the 205 transfer cases that originally came with a slip-yoke rear output. The kit utilizes Advance Adapters' exclusive 400M output shaft material and an updated shaft design, making it the strongest NP205 output shaft available today. Advance Adapters also offers this kit for NP205 transfer cases with fixed yokes for just $276. As we went to print, they told us that these kits would be available in mid-summer of 2010.