Building a Bulletproof NP205Posted in How To: Transmission Drivetrain on May 5, 2005 0) (
The venerable NP205 transfer case has a reputation for being as tough as a rock, but the truth is--it's even tougher. Just let this cast-iron beauty slam down on a trail boulder and watch the chips scatter.
If that isn't a tough enough reason for using this unit on your wheeler, we don't know what is. Small and relatively compact, the stock NP205's only serious drawback is the menial 1.96 low-range reduction, but future mods and aftermarket doublers can take care of that issue. We've seen this geardriven case used and abused by massive amounts of big-block torque and monster tires and run to death with water in the oil, and we've been constantly amazed at its durability. In fact, we doubt this unit, properly built and maintained, could be seriously hurt.
The 205 came in GM, Ford, Dodge, and International trucks as far back as 1969, and it was produced in GM trucks until the mid '80s when the NP208 finally replaced it in all applications except for cab and chassis trucks, which kept the 205 until '91. The venerable 205 was made in lefthand drops for Ford and righthand drops for the rest, as well as remote and married versions, and late-GM applications even had a slip-yoke rear housing. Different input and output shafts were used through the years, but the core case design and center cluster gear remained the same on all units. Identifying an NP205 is relatively easy: It has a cast-iron case and center idler shaft with a nut on the front and a three-bolt cover on the rear, is about 12 inches long, has a center rear output, and usually has an ID tag above the front output shaft.
The super-tough NP205 case is found on many '70s and '80s vehicles from all manufacturers. We chose a GM unit with the drop on the right side, and Performance Gear & Axle cleaned and checked the bores on the case before we started. This casting number (C-99404) on the left identified that we had a '70-'77 GM case with direct mounting, which is almost exactly what we wanted. The case to the right is a mid-'70s Ford with direct mounting and a lefthand drop. Notice the eight-bolt racetrack type pattern on the GM case, which was used from '71 to '85, before the common six-bolt round pattern was introduced.
We went to Performance Gear & Axle in Mississippi to build a unit to our unique specifications, since we have no intentions of leaving our 205 stock. In fact, we had an NP208, so Performance Gear & Axle pulled a 205 out of stock just for this rebuild. Performance Gear & Axle has a great reputation for unique driveline combos, even mating a Clark five-speed to a Dana 300 transfer case or an NP205 with its own line of adapters. In fact, the company even offers reproduced factory style adapters for the hard-to-find TH400- and TH350-to-NP205, and even one for the never-produced 700R4. The company even has a special bearing available to run a lefthand drop behind Chevy and Dodge trannies for those of you with your front diff on the left side. Owner Ed Hotard sat down with us to go over the many variations found in the 205 over the years, and came up with a killer recipe just for our uses.
Since our application for this bulletproof 205 project is our '85 GMC 2500 3/4-ton with a TH400 tranny and an NP208, we decided to go for maximum strength and adaptability. Our present TH400 had a cracked case, and we knew we could sell the functional 208, so we decided to rebuild the TH400 with a short output shaft for a stock TH400-205 application. This is the shortest and strongest combo and required a minimum of modifications. However, we felt that the 1.96 gear ratio in Low was a bit on the wussy side, so an Off Road Design Doubler was thrown into the mix. The Doubler is essentially just an NP203 low-range reduction unit, and is designed to bolt between the transmission and transfer case. This doubles the gear reduction and gives us many more options for wheeling. We also wanted the twin-stick conversion for shifting the 205, which Off Road Design offers as well. Our new plan was to use the factory TH400-to-NP203 output shaft in the tranny so that the Doubler would bolt right in. Then the adapter from Off Road Design would bolt into the NP205 from Performance Gear & Axle. Strong, simple, and usable is what we wanted, and this would work great. We even upgraded the front output shaft for maximum durability. Check out how we put the transfer case together with this assortment of parts, and wait for a future issue to see the trick TH400 and Doubler install.
The NP205 has numerous variations, only a few of which we show here. Different input-shaft diameters in male or female varieties, left or right drops, adapter bolt patterns or remote styles, and strange stuff we've never seen--yet all combine to provide a mystical aura for the hallowed 205. The most common varieties are the early-model GM (A) with eight-bolt racetrack pattern (round six-bolt front mounting pattern shown here), the Ford remote mounted with a fixed yoke on the input shaft (B), and the late-style GM with a slip-yoke rear output (C). Even input bearing diameters vary, so measure to see which one you have.
For super strength and durability the TH400-to-NP205 package is ideal. The long female input shaft and six-bolt circular mounting surface make for a great package, but we decided to do things differently. Since we wanted to use an Off Road Design Doubler between the two, this unit shown wouldn't work. It has a long 32-spline female input, and we wanted the shorter one with the eight-bolt racetrack pattern mounting for the most compact package.
The more common 205 case for GM trucks is the one with the TH350 bolted to it, which has a smaller input-bearing diameter than the TH400 application. Fortunately, you can simply bore the opening out to the larger size, if you have the right equipment. Ed Hotard of Performance Gear & Axle has done plenty of these conversions, and dials in his monster mill for precision machining.
Once we figured out all of our components we wanted to use, we laid them out for this photo. Input, low range gear, rear output, cluster, front output, low range gear. Notice that both clutch sliders and low range gears are identical, just on opposite sides of the case.
The center cluster or idler gear is supported on both ends by tapered roller bearings. This is far better than needle bearings due to the helical gears' inherent thrust loading. The bearings are adjusted by shims, and the oil flow is through the holes in the spacer between the bearings and the gear and shaft. Make sure all holes are free of obstructions, since this is the most important area for oil flow.
Front Output Beef
Most early NP205 units came with the 10-spline, 1.25-inch front output shaft. Later ones came with a 30-spline, 1.25-inch shaft which is stronger, but it still isn't the best available. For serious beef we chose the 32-spline, 1.401-inch shaft, which was found in many Ford and Dodge versions.
The 10-spliner is used for a 1310 series U-joint or CV, or a 1350 U-joint. The 30-spline worked with a 1310 CV or the 3R CV, or Saginaw type. The burly 32-spline can use a yoke for a 1310, 1330, or 1350 CV or U-joint, and even the 1450 size joint and a few other weird options. If you need the strength of the biggest, this is the one for you.
The input gears available for the 205 are many, but the main differences are male (external splines) and female (internal splines). The NP205 used these three female input gears. From left to right: Long 32-spline for the TH400 and some late SM465 trannies, short 32-spline for the TH400, and the 31-spline for Ford applications. The transmission output shaft fits directly into the input gear without a sleeve for the best arrangement, with no extra slop to develop from a sleeve. We chose the short 32-spline for our bulletproof 205.
GM also used external-spline input gears, which required the use of a sleeve to couple to the transmission. From left to right: Fixed yoke for remote use, 10-spline for the early SM465 manual transmission, 27-spline for the TH350, and 29-spline for the early Dodge diesel manual applications. Not shown here is the 23-spline for Dodge gas engine and diesel automatic applications.
The standard 205 shifter is a single lever running two shift rails to provide high 2WD, high 4WD, neutral, and low 4WD. These shifters aren't available anymore, but Performance Gear & Axle makes its own stock-style replacements for Ford, Chevy, and Dodge applications. In a stock 205, the interlock pin between the shift rails prevents any other combo of positions, which is both good and bad. We like to have independent control of the rails so we can have the same positions as stock, but also have low rear-wheel drive only, and low front-wheel drive only. But simply removing the interlock pin will allow the case to be put into high and low range at the same time, which is not a good thing.
By modifying the rails properly you can get high, neutral, and low out of both the front and/or rear output, without having one in low and the other in high. Both range and mode shafts should be modified, but we decided a set of modified rails from Off Road Design was best, as the detents and reliefs are machined to precision, rather than just guessing at what approximate amount to grind off. With ORD's twin-stick conversion, we'll have every option we need. If you must do it yourself, check out "Twin-Stick 205", an article about homemade twin-stick mods to do the same thing.