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.
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
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.
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
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
- '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
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
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 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.
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).
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
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.
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
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
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
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.
Low-range ratio: 2.00:1
Weight (lbs.): 165
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.
NP-205 TRANSFER CASE
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.
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
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
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
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.
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.
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
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
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.
Notes / Misc. Info
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
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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