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NP205: Beefy Transfer Case Made Better

Posted in How To: Transmission Drivetrain on October 24, 2017
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The NP205 transfer case is a heavy-duty, gear-driven unit manufactured by New Process Gear in Syracuse, New York. It was introduced in 1969 and used until about 1993. The part-time, manual-shift T-case was found in GM, Ford, Dodge, and International Harvester vehicles, and in some construction/industrial equipment.

This transfer case is considered by most people to be nearly indestructible in most applications. The cast iron housing combined with beefy internals puts this case weight at nearly 140 pounds dry. High-range gearing is 1:1, and the NP205 has a low-range ratio of 1.96:1. It was offered in both driver-side and passenger-side front output versions. There were two input mounting configurations: figure-eight racetrack (as shown in the lead image) and a circular six-bolt pattern. Some versions of the NP205 have also been equipped with a gear-driven PTO output.

Multiple input spline counts were available to mate with a variety of factory manual and automatic transmissions. GM cases were always passenger-side front output. The GM SM465 manual versions of the NP205 had a male 10-spline input, and the TH350 automatic mated to a 27-spline male input, both using drive sleeves to mate the transfer case shaft to the transmission shaft. The TH400 automatic versions used a female 32-spline input. Beginning in 1985, GM swapped over to the circular six-bolt pattern with a longer 32-spline input used for both the TH400 and the SM465.

Ford used driver-side front outputs and a divorced NP205 in its trucks from about 1972 to 1977, then changed to a six-bolt pattern married NP205 with a 31-spline female input through 1979. Passenger-side-drop divorced cases were used on Dodge trucks in the early 1970s until a married version with the figure-eight mount pattern came along. Inputs used drive sleeves and were male 23-spline in most cases or male 29-spline when used in the diesel trucks behind the five-speed Getrag manual transmission. International Harvester also used a passenger-drop divorced version of the NP205 from about 1969 to 1975. Note that there were some other weird combinations and custom-order variations with the NP205 that are hard to fully describe, so it's a good idea to fully understand what you have in your transfer case before ordering parts.

Rear output shafts on all versions mate to a 32-spline yoke. The 1979 and older versions used a fixed rear-output yoke, while some later versions often used a slip-style yoke at the tail. Front output shafts were typically 10-spline (early) or 30-spline (late) pieces, with the crossover occurring around 1978. Some Ford and Dodge diesel NP205 cases were optioned with a 32-spline front output.

The NP205 was often found in heavy-duty pickups with ratings up to 1-ton. The transfer case is a good match in fullsize trucks running a healthy engine and big rubber, but in stock form is limited to a 1.96:1 low-range ratio.

We spoke with Stephen Watson, an NP205 expert from Offroad Design who mentioned a few items to watch for when looking over an NP205 for use or during a rebuild. For starters, the needle bearing stack between the input gear and output shaft can wear grooves in both surfaces and it’s easy to miss inside the cavity of the output shaft. Check the condition of this surface. You'll often find this wear on trucks with substantial road mileage, but they were seldom or never put in low range. High-mileage transfer cases may have the problem of popping out of low range. This could be due to worn needles under the low gears or wear on the shaft and/or low gear. Cases that have been forcibly shifted into gear may show wear issues on the sliding collar teeth and corresponding teeth on the gears themselves. Note also that drive sleeves on male-input NP205 versions can wear and exhibit a sloppy fit. Eventually, the splines can strip out and fail if used under this condition.

Upgrades to the NP205 are readily available today. They include stronger shafts and yokes, shifter enhancements, and lower gearing options. Another big benefit of the NP205 is the fact that it can now be fitted to many more powertrains. There are the OEM adapter components, but aftermarket vendors have added new adapter components to put the transfer case behind an even wider range of transmissions. The NP205 was manufactured as a stout drivetrain component and, with aftermarket support, can handle most anything an off-roader can throw at it.

Many of the later versions of the NP205 came with a slip-yoke-style rear output as shown here. In other words, the rear driveshaft has its splines on the end where they mate into the transfer case. This configuration adds transfer case length and is generally weaker than a standard fixed-output yoke. Aftermarket kits can eliminate the added length and convert to a standard fixed-yoke output.
Here is a divorced NP205 from a 1973 Dodge 4WD truck. The divorced versions typically used 1310-series input and front output yokes, and a 1330-series rear output yoke. This one has been upgraded to accept 1410 CV driveshafts front and rear.
The NP205 has an identification tag on the rear of the housing confirming the model number, low-range gear ratio, and date of manufacture, but these tags are frequently missing. The quick junkyard spotter’s way of identification is to look for the small, round, three-bolt cover in the middle of the back of the case between the output shaft and front output gear cover plate.
The helical gears inside are huge, and one can quickly see why this transfer case is so robust.
Here's the rear output shaft assembly. The shaft rides in the housing on a large ball bearing and a set of roller needle bearings. A rubber seal sits behind the yoke to seal oil inside. All NP205 transfer cases were built with a 32-spline rear output shaft.
The front output shaft has a splined end which mates to a front driveshaft yoke or flange. This one is a GM 10-spline front shaft with limited mating yoke variety. The GM 30-spline with Saginaw 3R flange is a bit beefier. Upgrading to a factory 32-spline or aftermarket front shaft offers both increased strength and the most options for driveshaft yokes.
The middle gear in the case is the idler gear. It spins on a heavy-duty shaft between two large tapered-roller bearings. A middle spacer and shims between the bearings set the end play of the assembly.
The housings were offered with two input bearing sizes. This case has the smaller, 80mm diameter used with male-spline input shafts, while the larger (90mm) bearing can be found in others using the female input shafts. The NP205 cast-iron case thickness averages about 3/8 inch in most places and 5/8 inch in reinforced areas.
This is an NP205 32-spline, fixed-yoke rear output shaft. The earlier rear output shafts have a larger hole and a larger oiling slot and tend to break, often predictably, through this slot. Later-model shafts have a smaller hole and a less aggressive cutout on the outside diameter. They seem to be stronger enough that they sometimes break at the yoke splines and sometimes at the oil cuts. Generally, only the most aggressive users will snap either shaft.
Typically the NP205 came with a gear-drive, mechanical output to drive a speedometer cable. Advance Adapters reports some 1991 Chevy trucks equipped with the NP205/4L80E transmission were provided with an electronic vehicle speed sensor (VSS) to send speed information to the engine computer. Several aftermarket companies provide tail kits or machining services to install a VSS in place of the stock speedometer drive gear in the rear output housing.
Inside the NP205 there is a front-wheel shift fork (left) and a rear-wheel shift fork (right). They are each pinned to a shift rail, and the shift rails are connected externally with a shift rail link that limits the movement of the shift rails to specific combinations. Here the transfer case is sitting in 4-Hi range. Power enters the case at the input shaft at the lower right, which is coupled to the rear output shaft with a sliding collar. Power also transfers through the idler gear (center) to the front-wheel high gear (lower left), which is coupled to the front output shaft.
With the rear output (upper right) still engaged with the high-range gear, a click of the shift lever pushes the front-wheel shift rail and sliding collar to neutral position. This engages standard 2-Hi range.
A second click pushes the rear-wheel shift rail and sliding collar to the center position, disabling both outputs and putting the transfer case in neutral.
A final click moves both shift rails fully rearward. Power enters the case at the input shaft at the lower right and is transferred to the idler gear. The smaller gear on the idler turns the two larger low-range gears (top) that are connected to their respective output shafts using the sliding collars. This enables low-range mode in 4WD modes.
Like most factory transfer cases, the NP205 is controlled using a single shift lever in the cab that manipulates both the high/low range and 2WD/4WD functions within the transfer case, but 2WD low range is not permitted in stock form. Aftermarket companies offer the ability to split the two shift functions and make them independent. In the case of the NP205, one lever would control the rear drive and one would control the front drive. It is also possible to engage front drive only for doing “front digs,” but this requires internal modification to the shift rails or new rails, such as ones from Offroad Design. Stephen Watson at Offroad Design recommends running a front Dana 60 axle if you want this action.
Offroad Design manufactures a number of aftermarket shifters to accommodate custom installs, twin-stick conversions, and control of multiple transfer case reductions using one, two, or three shifter levers.
Many times when disc brakes are swapped onto the rear axle, the factory parking brake is eliminated. Northwest Fabworks offers a parking brake kit for the NP205 that bolts to the rear of the housing. It uses a cable-actuated caliper and rotor mounted to an output flange.
As mentioned, the NP205 is one heavy chunk of iron. Northwest Fabworks has the means to provide additional mounting support to the rear of the transfer case with a billet bearing retainer and mounting foot.
This clocking ring from Offroad Design allows an owner to rotate, or clock, any circular six-bolt pattern NP205 in increments including 16, 24, 32 or 40 degrees. Different versions allow for clocking passenger-side cases up and driver-side transfer cases either up or down.
Along with being a reliable, heavy-duty transfer case, the NP205 is immensely adaptable thanks to a number of factory drivetrain options and with aftermarket support. These kits allow mating the very common NP205 models behind transmissions in configurations that were uncommon or nonexistent from the factory.
Several aftermarket companies offer upgrades for output shafts, including the Titan Series shafts from Northwest Fabworks. With the various options available, one can build an even stronger NP205 to handle both high horsepower and huge tires in the most extreme terrain.
The NP205 came from the factory in a handful of flange and yoke sizes. Now, between stock and aftermarket offerings, you can use 1310, 1330, 1350, 1410, 1480, 1410 or 1310 CV, or several flat driveshaft flanges.
The stock low-range ratio on the NP205 is 1.96:1 which is not much of a creeper gear. JB Conversions offers its LoMax kit that provides a heavy-duty replacement GM-style housing that is more compact and provides improved mounting options. Beefier replacement gears bring the low range down to 3:1. JB provides both gear sets and complete transfer cases.
If you crave even deeper gearing, Offroad Design offers several dual transfer case kits and complete transfer cases that allow you to “double” your low-range gearing. These setups can provide three or four transfer case gear ratios to provide maximum versatility.

Sources

Offroad Design
970-945-7777
www.offroaddesign.com
JB Conversions
Sulphur, LA 70664
337-625-2379
www.jbconversions.com
Northwest Fabworks
Parksville, AK
250-248-6024
http://www.northwestfab.com

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