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You are here: Home Tech Articles & Tutorials Engine/Transmission 4WD Transfer Case Basics
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4WD Transfer Case Basics

Transfer Case Terminology
  • Married/Divorced - 'Married' means the transfer case is bolted directly to the back of the transmission via an adaptor, whereas 'divorced' means the transfer case is separate from the transmission and is connected via a short driveshaft.

  • Full-time vs. Part-time - A full-time 4WD system provides engine power to both the front and rear axles on all surfaces at all times. This usually requires a transfer case with a center differential, a viscous coupling, or both. Full-time 4WD improves traction and handling on paved surfaces in inclement weather, but may reduce fuel economy.
         A part-time 4WD system is designed to be operated only on reduced-traction surfaces. The transfer case lacks any mechanism to allow front-to-rear axle speed differentiation. Thus, any accumulated driveline bind must be released via tire scrub. Extended use of part-time 4WD on a high-traction surface, such as dry pavement, can adversely affect handling and damage the driveline.
    Some, but not all, part-time 4WD systems allow the operator to shift from two- to four-wheel drive "on-the fly."

  • Hubs, automatic vs. manual - Automatic-locking hubs automatically engage, or "lock," the hub and tire and wheel assembly to the front drive axle's axle shaft when the operator engages a four-wheel drive mode. When released, or "unlocked," the axle shaft is disengaged from the hub body assembly and the wheel can rotate freely on the spindle. To unlock most automatic-locking hubs, the operator must select a two-wheel drive mode and drive the vehicle straight backwards at least 10 feet. The hubs of any part-time four-wheel drive system should always be unlocked before driving on dry, hard-surfaced roads.
         Manual-locking hubs perform the same function as automatic-locking hubs. However, the hubs must be manually locked or unlocked, usually by twisting a part of the hub from a "free" position to a "locked" position. Advantages of manual hubs include greater flexibility of operation, durability, and they provide the option of flat towing of the vehicle without a trailer.
 Ford F-series Transfer Cases

NOTE: The info provided below was gleaned from a variety of internet messageboards and websites while doing personal research. I cannot verify the accuracy of the data presented, so please do some additional research prior to spending time/money on a conversion. If you would like to add/correct any info in this section, please e-mail me.

Transfer Case Basic Description

Shift Pattern

Dana 21 - All '62-'72 F100 4x4s 4-spds (usually an NP435) and '73 6-cyl./4-speed trucks featured this married one-speed power divider instead of a two-speed (hi-lo) transfer case. It's a cast-iron gear-driven unit, and all it does is engage and disengage the front driveshaft. There is no neutral, only '2WD' and '4WD'. 1:1 ratio


Dana 24 - 'divorced' part-time 2-speed (hi-lo), used on '59-'72 F250-F350s 4x4s and were also pretty common in the '59-'66 F100s. They were also used in 6-cylinder '73-'75 F100/F150 trucks. Cast iron, gear drive, 1.86:1 low ratio. It's a tough case but lacks a low range, and parts are getting hard to find for this.


NP-203 - (1973-1979, F100/F250 4x4) The NP-203 is a "full-time" medium-duty transfer case. On F100s it was a 'married' setup thru '79. It was a 'divorced' setup on F250s through mid-'77 and then became 'married' thru '79. It has a set of differential gears which allow for full-time operation; the differential action can be manually locked out. Kits are available to eliminate the action and convert the 'case to part-time. An NP-203 can be distinguished from a part-time NP-205 by its single-piece rear output housing and shift rail coming out the side of the case. 31-spline input shaft, chain drive. 2.00:1 low ratio

NP-205 - (1973-1979, F100/F250 4x4). On F100s it was only offered as a 'married' setup. It was a 'divorced' setup on F250s through mid-'77 and then became 'married' up to '79. The NP-205 was a heavy-duty part-time, gear drive transfer case with a 1.96 low ratio (although another source reports 1.98:1 low ratio). It's smaller than the NP203 and about 40-50 pounds lighter. This is the choice of most hard-core off-roaders, due to it's strength.





The following links are .jpg scans of the factory shop manual service and repair pages pertaining to the Dana 21.

Description and Operation; Removal and Installation
(p. 01)

Major Repair Operations
(p. 02)
Manual Transmission F100 4x4 Transfer Case Torque Specifications (in ft./lbs.)
Transfer Case to Transmission Extension Bolts 20-30
Gear Transfer Case to Transmission Output Shaft Nut 60-80
Front Output Shaft Rear Cover Bolts 25-32
Front Output Shaft Bearing Retainer Bolts 25-32
Idler Shaft Cover Bolts 25-32
Rear Output Shaft Bearing Retainer Bolts 25-32
Shift Fork Set Screw 25-32
Transfer Case Cover Bolts 13-17
Front Output Shaft Flange Retaining Nut 250-300
Rear Output Shaft Flange Retaining Nut 250-300
SOURCE: 1967 Ford Truck Service Specifications - Form 7202T-67

Major Repair Operations, continued
(p. 03)

Major Repair Operations, continued
(p. 04)

The Dana 24 was installed in F-250 4x4's from 1960 through 1972. The transfer case accepts power from the transmission output shaft and transfers it to the front and rear axles. This model is a stand-alone unit sometimes referred to as a divorce-mounted transfer case. It has two-speeds: High and Low. The gear ratios are 1.86 and 1.00 to 1. There are four shifting positions: 4L-N-2H-4H (and an undocumented and not-recommended 5th position between 4L and N which will give you 2wd low). Defective transfer case bearings are a common source of drive-train vibrations. It is well worth the time and effort to have the transfer case rebuilt.

Dana 24's are PTO-capable. The Dana 23/24 PTO's are very rare and highly prized by early Napco conversion truck owners. There were 2 styles: The early style Dana PTO that was a big cast unit that bolted directly to the case and the later style that used the standard 6 bolt PTO pattern with a thick adapter plate to fit the 23/24 cases. The early style are worth their weight in gold.

If you are using 80 or 90W gear lube in your transmission or transfer case then it will not operate as smoothly as it should. Ford service and operating manuals both recommend using 50W engine oil in both the 4-speed manual trans and the transfer case when the operating temperatures are over 10 degrees Fahrenheit (30W engine oil is recommended when the operating temperatures are below 10 degrees Fahrenheit).

A big shortcoming of the Dana 24 case is it's lack of a strut rod or really any way to easily adapt one to it. Under heavy loading (such as towing conditions), a divorced transfer case tends to jerk forward-and-aft and makes for a rough ride and the truck's drivetrain takes a good beating from it.

Many, but not all, 73-77 F-250 4x4's with the NP205 case came with a factory strut rod that consisted of a plate that bolted to the NP205's lowest rear cover, a tube that bolts to one of several adjustment holes in the lower bracket and an angled bracket with rubber shock bushings that's riveted to a frame crossmember.

This strut rod (pictured at right, click to enlarge) is very easy to adapt to any '67-'72 F-250 4x4 after swapping in an NP205 case. The strut rod will have to be shortened several inches due to the difference in wheelbase from '67-'72 trucks vs. the '73-later trucks, but works very well and appears factory.


The New Process 203 was used in GM, Ford, and Dodge through the 70's behind auto and manual transmissions. They were originally a full time 4WD case, meaning all 4 tires receive power at all times. This was made possible through use of a differential in the back of the 203 that let the front and rear driveshafts turn at different speeds as just as a differential in an axle lets each axleshaft turn at a different speed. For off-road use, the 203 had a "lock" position in which the differential was locked making the front and rear outputs spin at the same speed. Shift positions are: High, Lo, High Lock, Low Lock and Neutral.

A popular modification to the 203 was to install a "part-time" kit to eliminate the differential. In this configuration the 203 operates like more conventional transfer cases in that it's either in 2WD or 4WD and when in 4WD the power is evenly split from front to rear.

The NP203 is identified by several features.

1. The transfer case is made up of 4 sections bolted together: a range box (used by various companies to make a 't-case doubler' set up to get even lower low-range for rock crawling), the chain case which houses the chain drive to the front driveshaft output, the differential housing, and the output housing. The front 2 sections are cast iron, the back 2 sections are typically aluminum.
2. The overall length is about 22" - 23" from the face of the case to the rear output yoke centerline.
3. The shifter is a somewhat complicated box that actuates the 2 levers on a single shaft on the side of the range box section of the 203.
4. The model tag (if it still exists) will be found on the front of the chain case above the front output shaft. It will list the model number, the manufacture date and the gear ratio.

Type: Chain-driven, cast-iron
Low-range ratio: 2.00:1
Weight (lbs.): 165
Lubricant: 10W-30/10W-40
Length (in.): 21.5
Width (in.): 19.0
Height (in.): 15.0
Used by: Ford, Chevy, and Dodge in all 1971-80 full-size vehicles. Use depended on the year, model, engine, and transmission. It is suitable for V-8 power in trucks with GVWRs of up to 10,000 pounds.


The New Process 205 was also used extensively in GM, Ford, and Dodge. GM and Dodge used the 205 in some trucks into the early 90's, Ford stopped using the 205 in 1979. The 205 is a part-time 4WD case, meaning when it's in 2WD the front driveshaft will not receive power. It has 4 stock shift positions, 2WD Hi, 4WD Hi, Neutral, and 4WD Lo.

NP205 cases had several different configurations with Ford and Dodge using both married and divorced models and GM using many different input spline types and 2 different bolt patterns.

The NP205 is identified by several features:

1. The transfer case is one piece of cast iron. There is a short bearing retainer/output housing at the tail of the case, and aluminum bearing retainers/seal retainers at the front and rear outputs but the working parts are all in a single iron housing.
2. There is a small idler shaft cover on the back of the 205 with 3 small bolts holding it on. This is somewhat unique among transfer cases.
3. The overall length of the NP205 (in fixed output yoke versions) is about 12-13" from the front of the case to the center of the output yoke.
4. The shifter is a very simple lever operating a crossbar between the two shift rails that plunge in and out of the front of the case.
5. The ID tag is found above the front driveshaft output, and will list model, manufacture date and gear ratio.

Type: Gear-driven, cast-iron
Low-range ratio: 1.98:1
(WEBMASTER'S NOTE: another source reports 1.96:1 low ratio)
Weight (lb.): 138
Lubricant: 80- to 90-weight
Length (in.): 13.0 (GM); 16.0 (IH)
Width (in.): 18.0
Height (in.): 12.0
Used by: 1971-1980 Broncos, Blazers, and Ramchargers (and corresponding full-size pickups); the NP 205 is still used on heavy Dodges. Some Dodge and IH models were longer, "divorce-mounted'' versions. The NP 205 has left- or right-side front outputs and a center rear output. Caution is advised: there were many varieties in NP 205 shaft splines and so forth. A PTO plate can be found on the left side of the case.

Divorced ford units are 32 spline input/output front and rear. They are very tough and cheap to rebuild when needed. Common upgrades for the Ford divorced NP205 are the late-'80's Dodge fixed-yoke output shaft that has smaller drilled oiling holes to replace the weaker early-style output shaft with slotted oiling passages. 32-spline 1350 and 1410 series yokes are also available. The stock Ford divorced NP205 came with a 1310 input and front output and a 1330 series rear output yoke.

Case Variations
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.





Transfer Case Swapping Notes

NOTE: The info provided below was gleaned from a variety of internet messageboards while doing personal research. I cannot verify the accuracy of the data presented, and just added them to this page for easy reference while continuing the research. Therefore, please do some additional research prior to spending time/money on a conversion. If you would like to add/correct any info in this section, please e-mail me.

  • The NP435 in an F-100 with a Dana 21 transfer case is a 2WD unit. To convert to a married NP205 you'll need to get an NP435 from a 4x4 F250 which matches your bellhousing, or at least change the 2WD NP435's 10-spline output shaft to the 4WD NP435's 31-spline output shaft.

  • ALL married NP205's and NP203's for Fords are identical. They bolt to an adapter that then bolts to the tranny. The adapters come in different lengths but that's the only real variable. The C6 has an adapter and the manual tranny has a little different adapter. What engine powers it makes no difference. If you go from a divorced unit to a married unit you'll have to have both driveshafts rebuilt and make your own crossmembers and junk like that.

  • Divorced Dana 24s and divorced NP205's share the same 4-stud mounting pattern and will swap with each other easily. However, the speedometer cable is a screw-on on the  Dana 24's and bolt-on on Ford NP205's.

Transfer Case Notes / Misc. Info

NOTE: The info provided below was gleaned from a variety of internet messageboards while doing personal research. I cannot verify the accuracy of the data presented, so please do some additional research prior to spending time/money on a conversion. If you would like to add/correct any info in this section, please e-mail me.

  • The biggest difference between the Dana 24 and the NP-205 (aside from the Dana 24 being a married unit and the NP205 a divorced unit) is that the latter uses helical-cut gears for the low range as well as the high range. Low-range gears on a Dana 24 are straight-cut spur gears, hi-range uses helical cut. Also, the gearset in the NP-205 is thicker, allowing more contact between the gears to spread the load.


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