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How to Build an NP203/205 Doubler For A 4L80E Transmission

Posted in How To: Transmission Drivetrain on May 25, 2016
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Building a trail rig is a never-ending series of compromises. Balancing power and fuel economy, durability and weight, and dirt-ready suspension with civil road manners are trade-offs any builder faces. Occasionally, a part comes along that affords the best of both worlds. And Off Road Design’s (ORD) Doubler is one of those rare parts. By adapting the low-range gearbox section of an NP203 transfer case to the face of the ever-popular NP205, ORD is able to deliver more gearing without resorting to steeper axle gears. That corresponds to a rig with the guts to get you there, without winding out the engine and further killing fuel economy on the way to the trail.

“The big goal is to have deep enough gearing to get around effectively off-road,” says Stephen Watson of ORD. “Lower gearing offers more control, it’s easier on the transmission—especially automatics—and you are not having to slip a clutch off-road.”

It’s no secret the NP205 is a seriously stout box. However, its Achilles’ heel is its lackluster low range, which checks in at only 1.96:1. Adapting the range-box portion of a 203, which carries 2.01:1 gears, effectively “doubles” the 205’s gearing without compromising any of its famed strength.

“A lot of people look at 4:1 gearing as being for super deep rockcrawling only,” Watson says. “They think, ‘I don’t need that, I just do trail rides,’ but it can be something that lets you fudge your transmission and axle gearing to work better on the highway and still be able to handle moderate trail situations.”

Another aspect Watson is quick to point out is suspension technology and how modern aftermarket suspension components allow certain vehicles to carry more momentum at an obstacle. But if you don’t have the latest and greatest coilover shocks on all four corners, a more sane approach is needed. “When you don’t have a foot and a half of suspension travel, [the Doubler] gives you the ability to slow down and not bang your vehicle up,” Watson says.

The ’Burb
When the Jolly Green Giant that is this ’70 Suburban came to the shop, it was in sorry shape. But one of the few things it had going for it was that someone had installed an NP203 gearbox already, and the previous owner left an NP205 in the back—no doubt planning for a swap. It was a sign from the gods of off-roading that we needed to build a Doubler. We already had both transfer cases. How hard could it be? Well, the answer is not quite as easy as we initially thought. While the process of assembling the Doubler is very simple, the shopping list of parts to make it work in our application was much longer than initially anticipated. With sage-like guidance from ORD’s owner, Stephen Watson, we were able to acquire all the right pieces to build an affordable, bulletproof transfer case setup with three speeds and our choice of a 1.96:1, 2.0:1, or 3.92:1 low.

Picking The Right Transfer Case
The most important wisdom we can impart—which will save you both dollars and time—is to start off with the right NP205 box. That being said, most people building a Doubler will be in the opposite situation as we were. Rather than adding an NP205, they will likely be adding an NP203: This is much easier to do because your NP205 should already be equipped with a lot of the components we had to buy. While all the 205 transfer case variants feature massive, helical-cut gears and a rugged, cast-iron case, over its long production run, it was offered in several configurations that can become a mid-build surprise.

Input Bearing Size
In the case of Chevy applications, such as ours, one of the most important variables to identify is whether the NP205 is a TH400 variant or not—ours wasn’t. ORD bases its Doubler on a TH400-style NP205, which utilizes a larger/stronger 90mm input bearing. NP205s with the small bearing will require case machining in order to build a Doubler. This is an easy job for most machine shops, but it does represent an additional expense. Once the case is modified for the larger bearing, a new 32-spline TH400 input gear needs to be ordered. ORD offers a TH400 input bearing conversion kit (PN GU4033) for exactly this situation.

Mounting Pattern
NP205 transfer cases were offered in both figure eight and round patterns, depending on the year of the case. Both styles work fine, and ORD can machine a Doubler adapter for either, as long you properly identify which you have.

Fixed Yoke vs. Slip Yoke
Certain NP205s came with a slip-yoke tailshaft, which is not the best setup for wheeling. Under extreme suspension droop, the driveshaft can slip out of the transfer case. If you break a rear driveshaft or U-joint, you can’t easily remove the rear driveshaft and limp off the trail without puking fluid. Our NP205 was so afflicted, so an ORD slip-yoke eliminator kit was a necessary precaution.

Matching The Doubler To Your Transmission of Choice
Finding An Adapter
Adapting the Doubler to any transmission that could have come from the factory with an NP203 is a walk in the park. Simply find an original or reproduction-style adapter and everything bolts up. If you’re moving to a modern overdrive transmission, like we were with a 4L80E, things become a little more complicated. ORD is a great resource for this kind of information, offering a guide to different transmission-to-transfer-case solutions on its Doubler page. To mate our 4L80E to the NP203, we changed the NP203’s input gear to a 32-spline TH400 unit, ordered a factory 4L80E transfer case adapter on eBay, and redrilled the face of the 203 to a new mounting pattern. Having access to a quality drill press, die grinder, and a sharp set of taps is all you’ll need, as the register diameter on the face of the NP203 and the factory 4L80E adapter are the same. One thing we noticed was the factory had clearance issues with the bolts of the NP205’s front bearing retainer (shown in captions). We milled the bolt bosses down, but grinding the bolt heads a few tenths would work just the same. Also note that the 4L80E does need to have a four-wheel-drive output shaft, though this can be swapped in.

Mounting Your Doubler
Adding a second cast-iron transfer case to the mix does add a significant amount of weight. For that reason, properly mounting the setup is paramount to a long and healthy life off-road. The aluminum Doubler adapter has a mounting foot that splits the difference between the two boxes, balancing them. Watson recommends using this “foot” as the primary mounting point for the transfer cases.

“People tend to over-constrain the transfer case, and you can end up breaking transmissions,” Watson says. As the engine and frame twist, over-mounting can put the transmission in an unfortunate rotational battle and cause the case to crack. Ideally, Watson likes to see the factory crossmember, with rubber C-bushings, mounted underneath the Doubler foot.

Doubler Or Magnum: Which Box to Choose?
After working through two complete transfer case builds (an NP205 and an NP203), replacing nearly every shaft in both boxes, and pestering Watson with plenty of tech calls, it became clear that in some instances, ORD’s Magnum box, which retails for $1,990, is actually a pretty competitive alternative. The Magnum, which is a planetary-style underdrive, provides four reduction gears, the lowest being 5.33:1, and is a lighter, more compact unit. It also has a massive 1 5/8-inch intermediate shaft, dubbed the “Fat Shaft,” that is incredibly strong.

“By the time you buy a 203 and make everything work, it can be easier to buy a whole bolt-on system,” Watson says.

We did the math on our transfer case build and realized we were into it for the sum of $1,840, with $1,020 spent on the Doubler components (ORD Doubler kit, NP203 rebuild kit, and NP203 input gear). The other $840 went to necessary components for the NP205, such as the new input gear, rebuild kit, slip-yoke eliminator, and 1350 CV output yoke. If you already have a functional NP205 in your rig, the only thing you’ll likely have to do (in addition to buying the Doubler kit and finding an NP203) is upgrade the input gear to TH400 spec. Watson tells us TH400 boxes are hard to find and this is a very common modification.

To save even more coin, avoid starting off with a slip-yoke NP205 if possible, as that tacked an additional $250 onto the build for the slip-yoke eliminator kit (PN GU4085). Things add up quickly when you don’t begin with the ideal parts combination. So, if you are purchasing them, keep a sharp eye out for the best option.

At the end of the day, $1,020 for a dual transfer case setup and 4:1 gearing is pretty reasonable. But, if the concept of spending an additional $970 and having a bolt-on, true four-speed gearbox is more appealing, look to the Magnum.

Time to Wheel, Almost
After a few months of collecting parts, sandblasting, and painting, our NP205/NP203 Doubler is ready to be installed in the ’Burb. Follow along in the captions to see just how this beast of a transfer case came together and how to mate it to GM’s baddest overdrive transmission. Learn from our teething issues and, if you have a question, give the guys at ORD a call.

“This stuff is all a giant puzzle,” Watson says. “Don’t be afraid to give us a call or spend some time on our site. We can help you figure it out.”

Part Cost
*ORD Doubler Kit $770
Slip-Yoke Eliminator $250
*NP203 Input Gear $155
NP205 Input Gear $135
*NP203 Rebuild Kit $95
NP205 Rebuild Kit $190
Factory 4L80E Adapter (eBay) $150
1350 CV Yoke $95

Note: Parts with * are Doubler-specific components

We disassembled our NP203, saving only the range box (shown), then cleaned, painted, and replaced the needle bearings in preparation for the Doubler build. Right: TH400 input shaft. Left: ORD’s proprietary NP203 output/adapter shaft.

A TH400 gear and subsequent 90mm NP205 input bearing needs to be used with ORD’s Doubler kit. Our NP205 was a TH350-spec unit and needed to have the input-bearing bore opened up. We stripped it down and headed to the machine shop.

Here is a comparison between the 80mm NP205 (left) and 90mm input bearing (right).

PhotosView Slideshow

Ken Gilispie uses a dial indicator mounted to the spindle of a vertical mill to center the bearing bore on the table. A boring bar is then used to enlarge the bore a couple tenths at a time until the final size is reached.

With the case machined, the NP205 was reassembled with this new TH400 input gear. The box is now a TH400-spec unit and ready for Doubler duty.

ORD’s Doubler kit includes everything necessary to join an NP203 and NP205. Shown is the adapter output shaft, billet-aluminum adapter, gaskets, seals, and hardware.

Because the 4L80E uses a 32-spline output shaft, just like a TH400, our NP203 needed a new input gear. If you’re using a TH350 or 700R4 transmission, you will likely not have to disassemble the front of the NP203 to change gears.

Now, it’s time to start building the Doubler. With the NP203 range box on the bench, remove the rear output shaft and gear. Next, prep the new output adapter shaft from ORD.

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Slide the gear onto the ORD output shaft, making sure the inner teeth (which engage the shift collar) face the teeth on the bottom of the shaft. Next, slide the spacer washer onto the shaft, on top of the gear.

If the NP203 rear output bearing is in good shape (if not, now is the time to replace it) slide it into the ORD aluminum adapter.

Next, slide the output shaft adapter through the bearing.

A snap ring will slip down the splined side of the shaft and lock the gear-and-shaft assembly into the aluminum adapter.

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The output shaft of the NP203 uses uncaged needle bearings. In order to keep them in position, coat them liberally with petroleum jelly before installing them one by one.

Laziness would suggest lifting the aluminum adapter onto the back of the NP203 simply because it’s lighter. However, doing so could cause one of the needle bearings to fall out of place. You know, gravity and such. The best way to join the case and adapter is to lower the NP203 onto the adapter from above. This may require an extra set of hands.

With the adapter installed, tighten the supplied hardware and install the oil seal into the front of the adapter face (on the figure-eight portion of the adapter).

Next, install the included studs in the face of the NP205. Slide the adapter and NP203 onto the face of the NP205; and, Houston, we have a Doubler.

If there is a commonly available adapter to mate your NP203 to your transmission of choice, this is all the work necessary to build a Doubler. However, if you are installing it behind a 4L80E, like we are, here are some supplemental instructions just for you. We scored this factory 4L80E adapter on eBay, and it is a fairly easy job to make it fit the NP203.

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We set the new transfer case adapter onto the face of the NP203 (before assembly) and clocked it to the desired rotation. The NP203’s front bearing retainer indexes the adapter. Use a transfer punch to make the location of the new mounting holes, then drill and tap them.

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There was some interference between the bolt heads of the front bearing retainer and the transmission adapter. We ground on the front bearing retainer (arrow), so that the bolts sat a few tenths of an inch lower, then the adapter bolted right on.

If you are starting off with all the wrong parts—like we were—or no parts at all, ORD’s Magnum box may be a quicker or even more cost-effective alternative. It is a planetary-style underdrive that gives the NP205 four distinct gears.

No Slip Yokes Allowed
Our NP205 came with a slip-yoke output, which is a no-go for serious off-roading. We used this slip-yoke eliminator kit from ORD along with a 1350-series CV output flange. Starting with a fixed-yoke NP205 will save you some coin.

Sources

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

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