NP205 Tech INFO

We've created this section to help our customers learn the ins and outs of the NP205 transfer case. Please keep in mind that this information is presented to be accurate to the best of our knowledge. Don't treat it as absolute truth, the factory did some strange things from time to time.

click here for: NP203/NP205 transfer case ID guide

Input Gear configurations:

Here are the 4 available GM input gear configurations. From left to right: 32 spline "short", 32 spline "long", 10 spline with a worn drivesleeve, 27 spline with a new drivesleeve.

GM used 4 different input gears in the 205.  The first is the 10 spline male "coarse" input which came with the SM465 manual transmission.  This sticks out from the case about 2". The factory used a fully splined "drive sleeve" to mate the male transmission output shaft to the male t-case input gear.  This input gear routinely wears, most commonly in the drivesleeve, but often on the input splines directly.  Note the pointed profile on the teeth in the above picture.  This drivesleeve has lost about half of its tooth to wear and has a good bit of slop on the shaft.  This is the most common 205 setup there is.

The next input is the 27 spline male "fine" input which came with the TH350 transmission. It also used a female drive sleeve between the transmission output shaft and the male t-case input. This shaft sticks out of the case by about 2" also. These gears wear well, but have been know to strip splines out of the drive sleeve, but usually after a long and happy life. The proper shaft stickout from the back of the TH350 case for this 205 configuration is about 7/8". This is the same as the stickout for use with a NP203 transfer case also.

Next up is the biggest and best input, the short 32 spline female. This came with TH400 transmissions and is as strong and as durable as anything can be on a truck. It wears very well and does not use a drive sleeve, so there is one less splined connection to create "slop" (backlash) in the drivetrain. This shaft sticks out of the 205 case by about 1 1/4" and the male transmission shaft plugs directly into it. The proper shaft stickout from the back of the TH400 for this type of 205 input gear is about 4 1/2".  This gear configuration is the one necessary to run our GEN2 Doubler kit.

Last is the "long" 32 spline female input gear. This is far less common than the "short" 32 spline input outlined above, but is as strong and durable. This gear will stick out of the 205 case by about 3 1/2". This was used behind later model SM465 trannies and later TH400 trannies, from about '85 to '91 in straight axle 1-ton trucks. Proper shaft stickout from the TH400 is 2 1/2", and the proper SM465 output is the later model 32 spline output shaft with the round bolt pattern adapter housing. This gear will not work with the GEN2 Doubler, but is easily replaced with the short 32 spline input gear.

Converting input shafts in NP205's:

Click here for our catalog page for transfer case conversion parts

Convering the input of the NP205 to 32 spline has become very important with the GEN2 Doubler™ kit since the Doubler™ requires a 32 spline input. This details the process:
First the basics, the 205 cast iron case came with 2 different input bearing diameters, one is about 3 1/8" diameter, the other is about 3 1/2" diameter. The "small hole" 205 case is used with male input gears, examples being the 10 spline and 27 spline. The "big hole" case is used with female input gears, examples being the GM 32 spline and the Ford 31 spline. The photo below shows the 32 spline input with it's input bearing and the 10 spline male input with it's smaller input bearing. There's a pretty noticible difference between the two, they measure about 3/8" difference.

The reason for the different size bearings is the difference in sizes between the male inputs and the female inputs. The outside diameter of the female input gear is much bigger than the male input gear and requires a corresponding larger bearing.
If you want to convert a male input (small hole) case to a female input, you will have to bore the input bearing hole in the cast iron case out to accept the larger bearing. This is not a complicated job for a qualified machinist. It's a straight bore, no tapers or steps, and should be a tight slip fit on the bearing at room temperature. It's best to have the bearing in hand to measure and test fit, therefore we're not going to specify a dimension, beyond saying it's about 3 1/2".
The kicker is, to machine the case, you have to totally disassemble the 205. This isn't a hard job, but it is a couple of hours of work/bonding to take it apart and put it together. We supply input gears and bearings, and can machine your case in our shop. We sell the input gear, input bearing and a seal and gasket kit for $200.00.

Front output shafts:

GM used two front output shaft configurations, one had a spline count of 10, the other was a 30 spline. 10 splines came before 1977 (roughly) 30 spline shafts came after 1977 (again, roughly).
One quick way to tell them apart is by the fact that 10 splines came with a 1310 series yoke output, capable of accepting a CV or yoke driveshaft. 30 spline shafts came with the flat flange on the output, capable of accepting the GM 3R series CV joint.

Strength:
Strength wise, the 30 spline is the way to go.  The 10 spline shafts have been known to break, usually in abusive applications (big blocks, 40"+ tires, etc).  30 spline shafts seem to hold up well, especially since there isn't anything in a front axle that can take the torque required to break the finer spline t-case shaft.  Also, the 3R CV joint should be a little stronger unit overall due to the wider crosses.  Practically speaking, it's nice to have the fine spline, but if you're running a 1/2 or 3/4 ton front axle, the front will break long before the NP205 10 spline output.  At that point, it comes down to ease of service, yoke choice, and other such factors.

Yoke Choices:
For this one we consulted Tom Wood of Tom Wood's Custom Driveshafts. The 10 spline output will accept a 1310, 1330, or 1350 ujoint yokes, and of course the 1310 CV joint.
The 30 spline output will accept the 1330 or 1350 CV joints.
This info is incomplete, we were unable to reach Tom at the time this was written but these are the most common driveshaft configurations in 4wd vehicles.

Ease of use:
We prefer the flat flange output on the 30 spline output shaft due to the fact that you can actually put a socket on the bolts that hold it together from the front. The yoke style CV joint is a real pain to install since it uses fine thread bolt installed from the rear of the yoke. Nothing like turning a 5/16" fine thread bolt 1/8" of a turn at a time with an end wrench. GRRR. Other than that, there may be details that a drive shaft shop would be best suited to helping you with.

Heavy duty upgrades:

There are 2 heavy duty upgrades that you may be interested in. One is the current availability of 1350 series CV jointed drive shafts. The most common configuration of these shafts is a flange connection. There are 2 ways to get these to connect to the NP205. One is to use a lightly modified standard 30 spline front output flange. The mods must be done on a lathe, but it's not much work once it's chucked up.
The other way to connect the 1350 CV is to use a Ford 32 spline NP205 front output shaft and use a specialty flange type output.

The other HD upgrade is using the Ford front output shaft itself. This output shaft configuration is the same as the rear of the NP203 and NP205, so the yokes interchange. You can get about any driveshaft connection you can imagine for this output configuration. And with the 32 spline, you're basically guaranteed it's not going to break. If you have a 30 spline front output already, it's probably not worth swapping the 32 spline shaft in unless you need the yoke options.

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