GM 203 to GM 205 Flat Rotation on 203 and 205
GM NP203 to GM NP205 Doubler kit, flat rotation
4-to-1 (4:1) transfer case gearing in an NP205/203 package strong enough for big block power and full size trucks.... Or any other really abusive application you can come up with - Chevy, GM, Dodge, Ford or custom!
Click here for our Doubler™ quick facts and answers to most frequent questions.
The Gen2 Doubler is stronger, more compact and more versatile than ever.
First, the Basics: The Doubler is an all gear, dual transfer case system uniquely suited to full size trucks or other heavy duty, high output/abusive applications because of the durability of its components (all massive helical gears and large input/output shafts). Basically, the Doubler consists of the gearbox section from an NP203 transfer case coupled to a complete 205 transfer case. This gives you an additional low range gear of 2:1 plus your existing NP205 low gear of 2:1 for an overall low gear of 4:1.
Adding lower transfer case gearing gives you huge improvements in off road control and torque without affecting your highway gear ratios (4.10's with 37-38" tires can work well). Adding a gearbox also allows you to run in your normal 2:1 reduction or in super low 4:1 reduction for best performance in any situation. We use a combination of bulletproof and easy to find factory parts with a minimum number of modified components to give you easy bolt together gearing options. Our baseline kit is just the parts to adapt YOUR 203 to YOUR 205, we can also ship complete 203 and 205 units if you like.
Click here for NP203 and NP205 transfer case ID guide
The Doubler is only available in 31 or 32 spline which eliminates any weak link in the transfer case system. Very early on we experimented with 10 and 27 spline input gears in the NP205 and they simply didn't hold up. We don't want you to have to buy the kit again when you find the limits of the smaller input gears!
We also have the option of doing custom rotations of the 205 and 203, and can mix and match Ford or GM 203's with Ford or GM 205's. More on this later under "Custom Rotations and Hybrid applications".
Our new units use a one piece billet 6061-T6 aluminum adapter housing instead of the multi piece welded steel unit of the past. This allows us to build a stronger housing and better keep up with demand. We also converted to using studs in the front face of the NP205 which makes the 'case easier to install and more secure than a bolted connection. If you have doubts about the strength of our billet adapter housing click here!
We now use a new billet chromoly steel shaft. No re-splined stock shafts. As always, our Doubler shafts are fully heat treated so spline wear is not a concern.
The basic Doubler kit is intended for the "do it yourself" user and includes all the parts necessary to mate your 203 gearbox and your NP205 transfer case. These parts are a splined and heat treated adapter shaft to install in your 203 range box, a new heavy duty adapter housing, and the necessary gaskets, seals and hardware. We provide an oil fill and the breather port for the 203 range box.
Offroad Design now offers complete rebuilt transfer cases so you can purchase a Doubler transfer case system totally ready for you to bolt in to your truck. Price varies with transmission type, but it's typically about $2500 for everything you need, including our triple stick Doubler shifter.
Kit Strength: The Doubler has proven to be an extremely tough system. The all gear construction means that most vehicle's weak point is not the transfer case. If anything is going to fail in the transfer case, it's most likely going to be an NP205 output shaft, and those sure don't break easily.
In the real world, we have customers using all versions of the Doubler in pretty brutal conditions and other drivetrain parts are the first things to go. After seeing and using our units in competition fourwheeling and desert racing, on heavy vehicles with up to 66" tires, and seeing numerous driveshafts, 14 bolt and D60 axleshafts break we're not worried about anyone breaking our Doubler!
About the 203 rangebox:
The NP203 chain drive transfer case is widely regarded as junk. It does have its problems but by using only the "range box" gear reduction unit, you eliminate those problems. Our kit only uses the front 6" of the NP203, the chain drive and the original differential unit are thrown away. The reduction unit uses large 2 to 1 ratio, helical gears like the famous NP205 and is packaged in a compact housing that bolts easily to a variety of transmissions.
First things first
Since the Doubler™ adapter is meant to bolt behind an NP203 range box, the first thing you need to do is get a 203 that will bolt up to your transmission. We're going to walk through the steps to mate the NP203 to the most common transmissions in use today. In general you have to look at three things when mating a transmission and NP203. The first is the transmission output shaft, the second is the NP203 input gear, the third is the adapter housing between the two.
GM TH350 Automatic Transmission:
The TH350 transmission is very common and was coupled to the NP203 in MANY GM trucks. The TH350 transmission will need a 27 spline output shaft that sticks out of the case by about 7/8". This output configuration was used with NP203 and NP205 transfer cases. The NP203 will need the 27 spline input gear to match the 27 spline shaft. This input is only found on 203's that came with a TH350 originally. The adapter between the transmission and NP203 is a 1" thick plate with a mounting foot to secure it to the vehicle's crossmember. This adapter plate was only used on factory TH350 to NP203 applications.
A less popular auxillary method of attaching the NP203 to the TH350 is to use the TH350 with a longer output that was originally used with an NP208. You will use the 27 spline input in the NP203 to mate to this shaft and use the TH350 to NP208 adapter housing. This requires drilling and tapping new holes in the front of the NP203 to bolt it up in place of the NP208, and may require very minor machine work on the NP203 input bearing retainer. The drawback of the second technique mentioned here is that the factory TH350 to NP208 adapter housing is about 2.5" thick so your overall drivetrain length will be longer than using the factory 1" plate.
GM TH700 R4 / 4L60E Automatic Transmission:
The TH700 has the same bolt pattern on the back of the transmission as the TH350 and uses a 27 spline output shaft. Since the TH700 only came with the NP208 or later alum t-cases, it's shaft stickout is a little over 2" in factory form. One difference to be aware of is that the TH700 uses 4 metric bolts to secure the adapter to the transmission case.
There are three ways to bolt to the NP203. They are very similar to the TH350 mounting mentioned above.
1: replace the output shaft in the TH700 with an aftermarket output that sticks out of the case by about 7/8" (click here). This makes the output identical to the TH350 and you use a TH350 version NP203 with the 27 spline input and a TH350 to NP203 factory 1" thick adapter plate. This is the shortest method and is the preferred way if you are working in a transmission rebuild at the same time.
2: Use a spacer (click here) with a factory TH350 to NP203 adapter. This method is longer and has one more spot to leak but if you aren't tearing apart the transmission and don't need the absolute shortest setup this is a good way to go.
3: use the factory TH700 output shaft and the factory TH700 to NP208 adapter housing with a TH350 version NP203. You will have to re-drill and tap holes in the NP203 gearbox to be able to bolt it to the original NP208 adapter housing. You may also have to do minor machine work on the input bearing retainer on the NP203.
GM TH400 Automatic Transmission:
If you have a TH400, there are three options for mating the 203 to your trans.
1: Use a factory configuration. The TH400 was available with the NP203 in heavier duty trucks (3/4 and 1-ton generally) in approximately '78-'79. The factory configuration uses a 1 3/8" stickout 32 spline shaft in the TH400, a 32 spline input gear in the NP203 and a 1.1" adapter plate between the two, somewhat similar to the TH350 adapter plate. This is the shortest way to mate the TH400 and NP203.
2: Use a TH400 that originally had an NP208 mated to it. This TH400 uses a 32 spline shaft that sticks out of the case about 2.5" and has an adapter housing that's about 2.5" thick. You'll use a TH400 version 32 spline input gear in the NP203 and re-drill and tap the face of the NP203 with the round bolt pattern to match up to the NP208 adapter housing. Benefits of this kit are the parts are quite common since the NP208 was very widely used with TH400 trannies. The drawback is the adapter is about 1.5" longer than the shorter factory configuration.
3: Use a TH400 that originally had a NP205 behind it. This TH400 uses a 32 spline shaft that sticks out about 4.5". You'll need to use a factory TH400 to round pattern NP205 adapter housing that is 4" thick and once again, re-drill the front face of the NP203 to match the round pattern. The input gear will be the TH400 version NP203 32 spline input. Drawbacks to this system are the overall length with the 4" housing between the two and the expense of buying the factory housing. They're not common as used parts and typically must be purchased new. ORD can supply this adapter.
GM 4L80E 4 speed Automatic transmission:
The 4L80 is similar to the TH400. The most common output is the 2.5-2.875" stickout 32 spline shaft shaft that was used with various aluminum transfer cases. This case uses a 2.5" adapter and is mated to the NP203 by using the factory output shaft, adapter housing and a 32 spline input in the NP203. The front face of the 203 will need to be drilled to the GM round bolt pattern to allow it to bolt to the factory 4L80 adapter housing.
GM SM465 4 speed Manual Transmission:
If you have an SM465 transmission there are 3 ways to mate it to the NP203.
1: Use a factory configuration. GM mated the NP203 to the SM465 in 1973-1975 using the 10 spline output in the 465, a 10 spline female input in the NP203 and a 4.25" adapter housing between the two. This configuration was used in a wide range of GM trucks but is somewhat rare. Problems are the 10 spline connection tends to wear in the long term and it's difficult to find. Positives would be the short length and the ability to use the common 10 spline output in the transmission.
2: Use a SM465 that was originally mated to an NP208 transfer case. This 465 will have a 32 spline output shaft with a 8 3/8" adapter housing. You'll use a NP203 gearbox with a 32 spline input gear and the face of the 203 drilled and tapped with the GM round bolt pattern to match up to the factory 208 adapter housing. This works very well if you have the available length in the vehicle since the parts are commonly available and the 32 spline connection is very strong and durable, we have brand new adapter housings.
3. Use the factory SM465 to NP203 adapter housing or a brand new adapter housing that we can provide with ORD's new 32 spline SM465 output shaft and a 32 spline input gear in the NP203 (click here). This allows the shortest overall length with the highest strength, most durable spline connection.
NV4500 5 speed Manual Transmission:
The NV4500 came in Dodge and GM output configurations and the one that's used to mate to the NP203 is the GM 4wd NV4500. This uses a 32 spline output shaft and an adapter housing that's approximately 7" long. The 203 will need a 32 spline input gear and the front face will be re-drilled to match the GM round bolt pattern on the factory adapter housing. This makes maximum use of commonly available parts and uses the desirable 32 spline shaft connection.
Ford Transmissions (most types):
Conveniently for us, Ford used an identical round bolt pattern and 31 spline output shaft for all of the truck type transmission and transfer case combos. This means you can take your Ford NP203 and bolt it directly in place of most Ford transfer case without adapter and shaft changes. This also seems to carry into late model trannies also. If only GM used this system!
There are a couple of different output housings available for the back of the C6 that you should be aware of, info is a little sketchy but we know of at least 2 lengths, one is about 7" long and the other is about 5" long. The output shaft length must match the output housing length. Otherwise you can bolt a Ford NP203 gearbox directly in place of the Ford NP205, NP208, or even the newer aluminum cases.
Dodge Transmissions (most types):
Dodge used the NP203 with the NP435 and the 727 auto tranny through the '70's. These combinations are still relatively easy to come by as used parts. All Dodge 203s use a 23 spline input gear. Later model Dodge transfer cases use a round bolt pattern similar to the GM round pattern. This pattern is adaptable to the front of the Dodge NP203 with re-drill work similar to the work on the GM 203 bolting to a GM round pattern.
Jeep applications:
We have had limited dealings with bolting the NP203 to Jeep transmissions but in general Jeep used a round bolt pattern and once again the NP203 is usually able to accept the round bolt pattern to mate to the transmission. On applications that use the Jeep TH400, you'll basically use the same methods of adapting the 203 as if you had a GM TH400.
ORD can supply many of the factory adapters, availability can be a problem with used adapters but we always keep our eye out for the parts to make a Doubler work. About all you can do is call us to check if we have what you need.
Once you have a 203 ready to bolt to your tranny, you can move on to choosing your adapter and transfer case.
NP205 Choices:
The only 205 choice for use with the Doubler™ in GM or Dodge applications is the TH400 version 205. This transfer case uses a 32 spline female input gear for maximum durability and strength. If you don't have or can't find a TH400 version 205, Offroad Design has the parts to convert your SM465 or TH350 version 205 to a TH400 type. If you have any other GM or Dodge NP205, see our "NP205 Tech info" section for more information on converting your NP205 to work with the Doubler™. Ford Doubler™ kits use any married Ford NP205. All the married Ford 205's came with the correct input gear and bolt pattern to work with our kit. Click here for more conversion kit price info.
Adding them up
The overall length of the Doubler™ is now 21.5" for all versions. This is measured from the front face of the 203 to the rear yoke centerline on the 205. This is the same length as the original 203, and only 6" longer than original GM 205 applications. It's 9" longer than a 205 for the Ford guys. The new compact length allows the Gen2 Doubler™ to fit in many more applications while still leaving adequate rear driveshaft length. Keep in mind these measurements are not absolute, they can vary as much as 1/2" either way depending on the case length variations and yoke length. This does not include the adapter between the transmission and NP203.
Custom rotations and Hybrid applications This is some pretty cool stuff! The GEN2 Doubler™ allows us to mate different brand 203 and 205 transfer cases and to rotate them to some pretty crazy angles for more ground clearance on extreme vehicles. Check this link to see pictures and descriptions.
Installation Overview:
The installation of the Doubler™ can be broken down into steps. The first is disassembling your NP203 and installing the new adapter parts. The second is actually installing the gearboxes in the truck.
Removing the range box from the NP203 is not a difficult project at all. You need a 9/16 wrench or socket, a hammer and something to drive out a 3/16 roll pin. More exact instructions are included in the kit but the summary is to unbolt the aluminum differential housing, remove the differential shift fork, unbolt the chain case and take it off the range box. That’s all.
Installing our adapter shaft involves removing one external snap ring from the factory 203 shaft (snap ring pliers will be needed), installing the shaft bearing in our adapter housing, transferring the gear and shaft assembly to the new housing and re-installing the snap ring. At that point, you can bolt the adapter housing to the range box and it’s ready to go in the truck.
Installing the cases in the vehicle is just like installing factory parts. You bolt the 203 range box to your transmission and then bolt your 205 to the Doubler™ adapted 203 just like you would bolt it to a transmission adapter. That’s it!
Shifting
The Doubler™ is shifted just like your 205, you should be stopped and transmission in park or neutral. You can run with just the Doubler™in gear to give you a 2:1 reduction in 2 wheel drive or use any of the standard positions in the 205 with the Doubler™ in high or low range. Any combination of gears in the Doubler™ and 205 will work with the highest being high and high for a 1:1 road gear, and the lowest being low and low for a 4:1 offroad gear. We recommend that you use the low gear in the 205 any time you want a 2:1 low gear in 4wd. Using 4wd Hi in the 205 and Lo in the 203 gearbox is the only way possible to break the intermediate shaft. You should avoid this situation.
Shifters
Our NEW 3 stick shifter uses one stick to shift the 203 gearbox and 2 sticks on the NP205, allowing individual control of the front and rear drive shift rails. This gives smoother shifting characteristics and with the proper internal modifications to the NP205, will allow easy access to front wheel drive and rear wheel low range. This allows you to use the full potential of your transfer case for maximum maneuverability. Price for the 3 stick is $314 and includes floor boots and all the parts shown, we do a discounted price of $289 if you order our doubler kit as well.
Other Necessary Modifications:
The Doubler™ is not an afternoon bolt-in due to the number of details involved. Some things that will need to be addressed are:
Floorboard clearance
Floorboard clearance is rarely a major problem but you should be aware of the possiblities. You may need to modify your floor pan, install a short body lift, or lower the transfer case to get the proper floorboard clearance with this kit in some vehicles. A combination of the above may work also.
Crossmembers
Crossmember modification will be required due to various frame changes when the crossmember is moved. Also, while a single crossmember under the 205 is adequate, dual crossmembers are strongly recommended. The front crossmember will sometimes have to have a loop or tunnel built into it for clearing the front driveshaft. Custom fabrication is required to completely skidplate the transfer case setup. This is really recommended for heavy duty 'wheelers. Here is a photo of the crossmember setup we built for one of our trucks. It's a fairly simple combination of stock parts and custom fab work. You can see that the rear crossmember was widened (note welded in steel to widen it), and the front was modified with a tunnel for the front driveshaft.
With the GEN2 kit rotated 2" up, the front crossmember (the one under the NP203 mounting foot) will probably not need to be modified with suspension lifts under 4". For some more ideas on high clearance crossmember mounting, click here.
Speedometer Cable
You may need a new speedometer cable to reach the t-case in the new position. They are available from the dealership or a parts store. Ours is from an '82 blazer with the 700R4 and 208 t-case. It reaches with no difficulty.
Exhaust
Some exhaust systems run near the T-case or driveshafts and will need to be modified for clearance.
Driveshafts
You will probably need a longer front and shorter rear driveshaft with the Doubler™. If your shafts are in good condition, you can probably just have them retubed to minimize extra cost. This is a good time to look at u-joint and yoke upgrades also.
Driving with Double Low Gears
The deep final gear ratios available with the Doubler™ (usually over 100 to 1) will give you much greater control of your vehicle in tight off road situations and much greater torque available for climbing and crawling. Automatic tranny owners should also experience their first compression braking off road.
The increased torque can be harder on drivetrain parts but the best driving technique is slow and easy so this is usually not a problem. You’ll find that since you have the ability to select a 2 to 1 low gear you will still use it in situations that require a lot of wheelspin, like mud, snow and sand. When you get into slower speed, rougher terrain, you can shift into double low and crawl along smoothly. You may also find yourself using 2WD low range a lot. With a front locker, this gear selection lets you maneuver and steer much easier than in 4WD low.