- Article/photo's courtesy of:
- Steve Fox @ CK5.comIn the quest for lower gearing and drivetrain strength many of us start with the axles and then move up to the transmission, leaving the transfer case for last even though the transfer case provides perhaps the greatest range of options for achieving low gear ratios. There are a few other options out there as far as low geared t-cases go but in the end we decided to go with Off Road Design's Doubler for a number of reasons. The first being strength, with a near 6,000 pound vehicle we felt that using the venerable NP205 and the very stout range box portion of the NP203 would offer a good mix of strength and reliability as well as availability. Perhaps the biggest reason on choosing the Doubler was the great tech and service we have received over the past few years, if we had questions during the install we knew that they would be there to help us.
ORD Doubler Low Range Transfer Case
This section was pulled from the OffRoad Design website and has a lot of great information about their Doubler and some of the benefits of running two transfer cases.
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). Addinga 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. ORD uses 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. The baseline kit is just the parts to adapt your 203 to your 205, they can also ship complete 203 and 205 units if you like.
For those of you familiar with ORD's previous versions of the Doubler, the difference between the Gen1 (old ones) and the Gen2 (new kits) is that the Gen2 eliminates the factory adapter housing. This makes the overall length 3 inches shorter than the shortest Gen1, and also makes the unit much stronger since there is only one adapter housing between the 203 and 205. The Doubler™ is only available in 31 or 32 spline which also eliminates any weak link in the transfer case system. You 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".
ORD has also changed the material for their adapter housing, the 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 them to build a stronger housing and better keep up with demand. Lead times on the Doubler kit are now 2-3 weeks or less with many kits shipping within 2 days your order. ORD 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.
ORD now uses a new billet chromoly steel shaft, no more re-splined stock shafts. As always, their Doubler™ shafts are fully heat treated so spline wear is not a concern.
Doubler adapter starts from a block of Billet 6061-T6 aluminum
ORD Doubler Kit
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, an oil fill and the breather port for the 203 range box is also provided.
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 $2100.00 for everything you need, including our twin stick Doubler shifter.
Gathering the Parts:
Knowing that we were going with the Doubler setup we took our time locating the components that we were going to need, in fact we decided to change from a 700R4 transmission and NP241 t-case to a much stronger TH400 transmission, chances are you might already have an NP203 or NP205 under your rig already. One thing to keep in mind is that there are quite a few different combinations as far as transmission adapters, t-case output shaft spline counts and length etc., so be sure to do your homework first in order to locate the correct parts before you start this project.
To keep this article from becoming rather lengthy we are going to focus on our project using a TH400/NP203/NP205 setup, there are many more combinations that can be done, contacting ORD about your particular setup and taking advantage of their experience would be very beneficial and an excellent place to start. We located a complete TH400 transmission and NP203 transfer case setup from the parts for sale forum right here at ColoradoK5.com, the setup was complete and still bolted together pulled from a 1979 3/4 ton Suburban, all for $300.00. A short time later we took advantage of the CK5 for sale forum again, this time finding a completely rebuilt 32 spline short shaft NP205 for $400.00, with the needed t-cases in our possession we purchased the ORD Gen II Doubler kit and twin stick shifter.
Now that we had everything we needed it was time to start on our project, the first step we did was drop the transmission off at Shift Masters to be rebuilt so it would be ready when we had the Doubler setup together. The rebuild of the TH400 transmission included a few upgrades such as Kevlar clutches and other strength goodies but the biggest upgrade was going with a reverse manual valve body from Art Carr.
Disassembling the 203:
Before disassembly remember to drain the oil, there will be a lot of it even after it has been fully drained, best to do this in your neighbors driveway to avoid a mess. After the oil has been drained place the 203 on the input face and remove all the bolts holding the aluminum tail housing to the chain case (the part that bolts to the cast iron chain case), you will probably get a lot of needle bearings dropping out of the case but don't worry, they don't get reused. Next, lift the housing off the chaincase, a little persuasion from a hammer may be needed but the two should come apart fairly easy.
After the housing has been lifted off the chain case you should be able to see the differential shift fork and shaft, drive the pin that holds the fork on the shaft out, and pry the fork from the shaft.
The next step is to remove all the bolts holding the chaincase to the rangebox; remove the chaincase by lifting it straight up (you might have to move around the intermediate shaft while you do this) and apart from the range box, now you have the needed 203 range box with the shaft sticking out.
203 Removed from transmission
Remove tail housing from chaincase
Drive the pin out of the fork and shaft
Pry the fork from the shaft
Un-bolt range box from chaincase
***It is very important to remove the steel rod that attaches to the differential shift rail in the gear reduction box, if it falls out it could get between gears. Some of them are designed so they can not fall out which was our case but we removed it just to avoid any problems.
***The only needle bearings you need to keep are the 15 fat needles that are in the end of the intermediate shaft which is the shaft that gets changed with the Doubler kit, the 15 fat needles will fall into the case when you lift the shaft and gear assembly out of the range box.
Off Road Design can also supply or rebuild any transfer cases or parts that you may need for this project, if you need to rebuild your NP203 or NP205 give them a call.
Assembling the Doubler:
Freshly rebuilt NP203 rangebox
NP203 rangebox ready for assembly
Getting the Doubler fully assembled isn't to bad of a job but some parts can be a little tricky to deal with, a good idea before you start would be to pick up the biggest pair of snap ring pliers that you can get a hold of, it will make assembly a lot easier. Since we were assembling the Doubler kit at the ORD shop and our NP203 hadn't been rebuilt yet, they were able to set us up with a freshly rebuilt box, just needed to swap out the input shaft from a 10 spline on the rebuilt box to our 32 spline and we were good to go.
Start by separating the factory NP203 shaft from the low range gear, you can now start to use those big snap ring pliers.
New Doubler shaft on the left, factory assembly on the right
Removing the factory shaft from the low range gear
Parts that go on the new Doubler shaft
Remove bearing from aluminum dome, you will need to install the bearing in your new Doubler adapter.
Remove bearing from aluminum dome
Install bearing and snap ring in new adapter
Installing snap ring
The Doubler kit comes with all the hardware you need for assembly including a breather fitting, might as well install it now before it goes in your truck and it's a lot harder to get to.
Installing the breather fitting
Applying a little grease to the new shaft
Apply grease inside the low range gear
Apply a little grease to the new shaft and low range gear before assembly installing thrust washer and retainer pin.
Low range gear and shaft
Install thrust washer and retainer pin
Install shaft assembly through new adapter
After the shaft assembly is ready, it's time to mate it to the NP203 range box, the first step is to apply a quality RTV sealant to both sides of the adapter gasket.
Installing big snap ring
Installed shaft assembly
Install seal after snap ring is seated
Slide shaft assembly through bearing and housing, you may need some heat on the roller bearings - the shafts are a precise fit. Next, install the big snap ring on the new shaft, this is what holds the shaft assembly to the Doubler adapter. Install seal after snap ring is seated and checked.
** Seal must be 1/2" below gasket surface and no more than 5/8", be careful not to damage seal.
Apply RTV to both sides of gasket
Apply RTV to new adapter
Everything nice and sticky
Remember the 15 fat needle bearings that fell out of the NP203 when you took it apart, if you haven't already, install them into the end of the shaft using lots of grease to hold them into place. Next, place the adapter assembly onto the NP203 range box and install adapter bolts.
Use lots of grease to hold 15 fat needle bearings in place
Installing adapter to NP203 rangebox
Tighten adapter bolts
At this point the completed NP203 range box is ready to be mated to the NP205, use the supplied studs and gasket to mate the two together making sure to put RTV sealant on both sides of the gasket.
All ready to be bolted to the NP205
Studs installed into the NP205
NP205 shift linkage
When you order your Doubler kit you have several options with rotation, we opted to have our NP205 rotated up 2", this makes the t-cases just about flat at the bottom and provides 2 more inches of ground clearance.
NP205 is clocked up 2" to obtain additional ground clearance
Ready to be installed into our K5
We had planned on installing the Doubler setup for quite a while when our driveshaft got real friendly with a rock and looked more like a banana than a driveshaft, since we needed a new driveshaft anyway, this was about as good time as any because we were going from a slip yoke style to a fixed yoke style driveshaft and didn't want to replace it twice.
In order to drive our rig back home after the demise of the driveshaft a plastic Mountain Dew bottle was duct tapped to the output to keep the fluid in, we will not have this problem with an NP205 fixed yoke transfer case. The folks down at Shift Masters had the TH400 transmission all ready to go by this time, complete with Kevlar clutches and an Art Carr reverse manual valve body, we also had the Art Carr gate shifter that was going to be used.
NP241 ready for removal
With a much better tool selection then we have not to mention a lift we decided to let Shift Masters install the TH400 they built for us and install the Doubler. With a 1" body lift on the Blazer we had just enough room to clear the shift linkage etc., without it we might have had to feather the transmission tunnel a bit with a hammer.
Everything in place
New Drive shaft (1410)
With the Doubler installed temporarily using a factory NP203 cross-member we called High Angle Driveline to supply us with their one ton CV shafts, both with 1350 joints at the t-case side and 1410 at the axles, nice and beefy just the way we like them. With the front shaft installed and the suspension fully drooped we were worried that the shaft would contact the exhaust as it crosses under but it did end up clearing with out making contact.
Old exhaust was close but didn't rub
New VSS output
New Drive shaft (1350)
One other thing to note about this project is the speedometer hookup, GM changed from the older mechanical setup to an electronic version the VSS (Vehicle Speed Sensor) which is used on newer vehicles, they started this setup in 1990, what year is our project Blazer you ask, why a 1990 of course. After a lot of research we found that there were numerous solutions to connect a speedo cable (mechanical) to a newer VSS t-case but not any aftermarket solutions to connect an electronic speedo (wire) to a mechanical input which was our case.
Luckily Donovan from Ram grinding had a solution, what he did was use a tone ring from a NP241 t-case which is just a small gear that is used by the VSS sensor, the sensor merely counts the number of teeth on the tone ring and sends them back as pulses. The tone ring had to be machined down a little to fit perfect in the NP205 case, after this the tail section of the NP205 had to have a new entry location machined and tapped to accept the VSS sensor. The new setup has been working perfect for just over a year now with no problems and is more accurate than the old style mechanical speedo cable.
Drive shaft installed
ORD twin stick kit
ORD twin stick installed
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.
ORD's twin stick shifter locates both of the shifters in the same hole in the factory location. One shifter will operate the NP203 and have Hi-N-Lo, the other shifter will operate the 205 with the stock shift pattern, 4HI-2HI-N-4Lo. The twin stick shifter is heim jointed, uses urethane bushings for the pivot points, and comes with a floor boot and proper hold down rings.
We ran the twin stick for about a year when ORD came out with a triple stick setup, well we had to have that, the nice thing is that the third stick can be purchased as an add-on to the twin stick setup. The triple 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.
New ORD triple stick kit
ORD triple stick installed
Just a note here, we sent our NP205 shift rails into ORD to be modified, you can grind them yourself but they can be kind of tricky if you haven't done them before so we just opted to have them do it correctly the first time. This allows you to use the full potential of your transfer case for maximum maneuverability. Believe it or not, using three sticks for transfer case operation was actually easier to use than using only two, one stick operates 2lo and 4lo, one stick operates front wheel drive and the other rear wheel drive, that simple.
Bolting up the 3rd stick was pretty easy but we did have to cut out a good chunk of transmission tunnel to accommodate the extra width, there just isn't enough room between the stock tunnel. We ended up simply using black duct tape to cover the hole as a temporary solution, when time permits we will make a new top cover that can be unbolted to access the top of the t-cases should we ever need access.
Even though our stock exhaust did clear everything, we did notice that the bottom of the pipe where the passenger side crossed under the drive shaft was getting a little flat so we had a local shop reroute the exhaust. The passenger side exhaust runs forward up to the engine cross-member and then turns back to hook up with the drivers side and straight back via 3" pipe with a hi-flow cat and Magna Flow muffler, with this setup we no longer have any exhaust running under our front driveshaft.
With our GEN II Doubler kit rotated up 2" we could use a factory NP203 cross-member without modification, had it not been rotated we most likely would have had to modify it to clear the front driveshaft. Running just one cross-member is not recommended but it was adequate enough for us to drive it down to 4x4 Iron where Tim built a custom one piece cross-member with built in skid plate that gave us a few more inches of ground clearance with a flat belly pan.
Custom 4x4 Iron cross-member (side)
Custom 4x4 Iron cross-member (back)
One of the biggest problems for vehicles without a proper amount of gear reduction is having the motor wrap up against the torque converter (automatics) while trying to climb an obstacle. This happens because the gearing is to high, and those larger tires you just installed made it even worse. Think of getting on your 21 speed Mountain bike and starting out in 14th gear all the time, your drivetrain components (your legs) would get very tired, not to mention very difficult if starting up against a rock. Well, with the Doubler it is like getting an extra set of lower gears and being able to start out in 1st gear again.
We have been testing the Doubler for about a year now and have run several tough trails in Colorado and Utah and it has performed flawlessly, we love having compression braking with our automatic and decents are much more controlled without having to ride the brakes.
Climbing is much more controlled also and is a lot easier on drivetrain components when you can idle over as opposed to stepping on the skinny pedal. We all love having options and when it comes to gearing it's no different, with the Doubler the stock 2 to 1 gearing is still available for those times when you need a higher gear for added wheel spin, need lower gearing simply shift down into 4 to 1, the best of both worlds. Another thing we should mention to the automatic transmission guys is transmission heat, having the added lower gear selection of the Doubler will help keep it cooler and on the trail longer.
If your looking for super strong low range gearing give Off Road Design a call, they can set you up with everything from just the Doubler kit and extra parts you may need to rebuild your t-cases etc. to a complete bolt in Doubler assembly for many different applications.
- Off Road Design
- 484 County Road 113
Carbondale CO 81623
- Phone Number:
- (970) 945-7777