Article and photos courtesy of Eric Hummel, AKA
I really liked Full-time 4wd on my 1975 Jimmy. Full-time 4wd worked
great in poor weather conditions such as ice, snow, or heavy rain. This
truck always felt solidly planted on the street but Full-time does have
draw backs. Your driving a lot more parts to make the truck go down the
road. You also tend to get accelerated tire wear and less fuel economy.
If you made the mistake of not replacing all four tires at the same time
or did not rotate regularly, these problems would be very pronounced.
These are the reasons that the NP203 had such a bad reputation of being
a power hungry, fuel sucking beast.
I kept my tires rotated, I always changed all four tires at the same
time. I didn't have any big issues with the above problems. Yeah, I did
have a little less fuel mileage and it was a little rough on the front
tires but it was not bad. The truck is not my daily driver so I was not
going to see many gains by converting to part time the first couple of
years I owned the truck. Things changed and with the addition of lockers
front and rear, I had to part time convert my NP203 transfer case if I
wanted to be able to steer the truck on the street.
The stock 79 Blazer my wife drives was converted before we purchased the
truck. I'm not sure of the brand kit that it had but I had problems with
it. While attempting to stop a leak on the tail shaft housing, the kit
began making a noise. I went to my local shop that works on transfer
cases and manual transmissions and was lucky enough to find a
knowledgeable person to get some information from. After he heard what
kit I had, he recommended that I replace it with the Mile Marker 501
kit. This kit solved my problems and was a much better design. That was
almost 5 years ago and this truck still happily serves as my wife's
|Mile Marker 501 Part-Time Kit.
With my previous experience I decided to use the 501 kit in the 75
before I installed the lockers. A few weeks later I re-geared my truck
and put a locker in both axles. The kit was working fine for almost a
year. I happily crawled with new found traction at my local trails. I
really enjoyed the fact that once converted I could run in 2wd low. When
your running lockers in both axles the truck tends to want to go
straight when in 4wd. I could shift back into 2wd while rolling when I
didn't need the extra traction of 4wd and get the truck to turn easier.
The next upgrade I performed on the truck was going from 32 inch tires
to 35's. I started to notice a little extra play in the drive line. I
would get a good "Bang" when I put the truck in gear after
about 6 months of running the 35 inch tires. After checking everything I
found the slop in the transfer case was more that the slop that I had in
I pulled the tail shaft off the 203 and found one of the thrust washers
used to adjust the end play were in pretty bad shape and had allowed the
tail shaft end play to increase. This allowed the 501 part time kit to
get more slop in it where it mates with the tail shaft crown gear.
I'm not blaming the kit here. It was a far better design than the other
kit I had dealt with but I was putting the kit under some heavy stress
with the 35's and front and rear lockers. This combined with a crown
gear with 250,000+ miles of full time wear and tear was not helping. The
locking tabs of the 501 kit were rocking sideways under hard torque and
pushing the tail shaft hard against the shims. This wore out the shims
allowing more movement in the part time kit and creating even more drive
After some looking around trying to find a solution, that didn't include
replacing the transfer case, I found out about the other Part-Time Kit
Mile Marker manufactures. This kit was the answer to my problems. This
kit replaces the rear output shaft completely. It removes all remnants
of the differential in the NP203 and that was the main source of
problems I had experienced with both the Mile Marker 501 kit and the
To understand why this kit is the ultimate way to convert a NP203 to
part time operation you need to understand how this transfer case
accomplishes "Full-Time" operation. Once you understand how
the case was designed then you can understand how each of the part time
The Guts of a 203
The picture above is the disemboweled guts of the NP203, except for the
gear box that is located in the front section. The way it works is the
output shaft of the reduction box runs through the center of the top
chain gear and drives the "cross" on the differential in the
center. This differential works no different than a differential in a
axle. If both the front output and rear output of the case are traveling
the same speed then the whole assembly from the tail shaft to the top
chain gear spin together. When you turn you need to allow the two
outputs a way to travel at different speeds, just like an axle will let
the wheels travel at different speeds, so the drive line will not bind.
501 kit installed in Differential cage.
What the Mile Marker 501 Kit does is remove the differential gears
replacing them with four tabs that locks the rear output shaft to the
cross shaft of the differential and the gearbox output shaft as a
result. The lack of differential gears to the crown gear on the top
chain gear disables the full time function of the case.
Picture of the locking Cog.
The way the NP203 performs "LOC" or Locking to drive both
axles with equal power was with a sliding collar on the top chain gear.
The collar would slide over the crown gear and engage teeth on the
differential case. This locked the top chain gear to the differential
and would not allow the gears to spin giving a 50/50 power split. Once
the transfer case is converted then the sliding collar becomes the only
drive to the front drive shaft. Since the 203 can be part time in both
low and high range you now are able to have two wheel drive low. The
shift pattern from all the way forward as you are seated in the truck in
stock form is the following:
After a part time conversion the shift pattern becomes the following:
2x4 High (Normal Street use)
The Shaft replacement kit (top) replaces the stock output and
differential assembly (bottom)
Here is the Mile Marker shaft replacement kit. Notice it replaces not
only the rear output shaft, it replaces the complete differential
assembly. Here is why this is the best way to convert the case to part
time four wheel drive.
As I pointed out above, the output shaft from the range box engaged a
set of splines in the middle of the differential. That drove the
differential cross and gears or the tabs for 501 part-time kit. The
gears or the part-time kit then drove the crown gear on the rear output
shaft. You start adding up all the slop in all those pieces by the time
you get to the rear yoke your getting a lot of play. The shaft
replacement kit drives the rear output directly off the range box output
shaft. The resulting reduction of drive line slop is astounding. What
little slop that was left after installing this kit was all in the range
box or transmission. A BIG improvement in not only removing the play but
it should also be a much stronger arrangement.
Installing these kits is not very difficult. As always, start with
safety items. CHOCK THE WHEELS! You are going to remove the drive shaft
so park is not an option and I don't know about you but I don't trust
the parking brake with my life. You might also want to pick up a new
rear output seal and a yoke gasket if yours are old. It's a few dollars
well spent to prevent having to come back and fix any problems that may
My truck has 6 inches of lift on it so jacking was not needed. If your
truck is still stock height, you might want to put the back wheels on
ramps or jack stands to get some room to work. This job is best done
with the rear of the truck higher than the front if possible.
First thing is to unbolt the rear drive shaft from the transfer case
side. You can tie the shaft out of the way so no need to fully remove. I
would also take a stiff brush and knock any loose dirt off the transfer
case where the tail shaft cone meets the main housing and where the two
halves of the cone join.
203 ready for disassembly.
Now I am straying from Mile Markers included instructions. I have MANY
reasons for doing so after having worked on the 203 transfer case a few
times. The first one is that nut is easier to get off while the output
shaft is still on in the truck. I will get to the other 25 reason in a
couple more paragraphs. Make sure the transmission is in park and remove
the 1 5/16 inch nut that retains the rear output yoke. Have a catch pan
ready, you may have some oil drain when the yoke is removed. Be careful
not to lose the flat washer and seal that's under it. If the seal is in
bad shape it should be replaced.
Mile Marker also sells replacement chains. If you feel your chain is
loose then this is the time to replace it. The removal steps needed to
install a part time kit is half the work you will need to do to replace
the chain. Might as well get it all done in one shot. If you are going
to replace the chain at the same time as you install the part time kit
you also need to drain the case and loosen the bolt for the front yoke.
If you shift the case into LOC this will let you get the bolt loose on
the front yoke.
|Remove the Yoke before removing tail housing.
Remove the speedometer cable and if you have extended your transfer case
vent you will need to remove the vent line at the top of the tail shaft
housing. Remove the 6 bolts that retain the smaller tail shaft housing
to the large piece. This section has an O-Ring seal and you will have
oil pour out so have a catch pan ready. Sometimes you may need to remove
the speedometer drive from the side if your having troubles removing the
cone. The cone should come off with very little force once the seal is
|Remove small section of tail cone. Notice location of
end play shims.
Once the cone is off you will see the end play shims. Keep track of
these shims because some of these will be reused. Remove the speedometer
drive gear. Now you can remove the bolts that are holding the rest of
the tail shaft housing to the rear of the case.
Carefully break the seal where the housing joins the case. You will have
some oil drain so have the drain pan ready. While pushing the output
shaft into the case work the larger section of the tail housing off the
output shaft and set it aside. The output shaft should not fall off
unless it's knocked off. You don't want to knock it off.
Stock configuration of differential.
This is where I come to the other 25 reasons to disassemble the case
this way as opposed to the way Mile Markers instructions indicate. In
the middle of the output shaft are 25 loose roller bearings. When you
pull the output shaft off these bearings are going to fall out. Put your
hand under there to catch them. If you are going to use the 501 kit you
will need to reuse these bearings. If you're using the shaft replacement
kit then you will not need to use these bearings. You just want to make
sure the bearings don't fall into the chain housing and get wedged
between the chain and the lower gear.
Once the shaft is off you can slide the differential assembly off the
splined range box output shaft. It will be very tempting at this point
to spin the front output shaft if you have already installed the locking
hubs. That top chain gear rides on 120+ roller bearings. Once the
differential is out there is nothing to prevent those bearings from
coming out if you spin the gear. DO NOT spin the front drive shaft.
If you are also installing a chain at this point then put a radiator
clamp on the shaft right against the face of the crown gear to prevent
those bearings from coming out. Replacing the chain from this point is
just a matter of removing the rear bearing support for the lower gear.
Remove the front yoke and bearing cover. The front bearing is then
removed. This will let you get the lower gear out of the case. Remove
the roll pin that retains the shift fork for the locking cog. Remove the
bolts that hold the rear case section to the front case section and the
shift linkage to the rear housing. The chains outer links form a lip
that prevents the chain from walking. The chain needs to be lifted off
the upper gear to clear the lip and the rear housing and chain are slid
off the upper gear. This usually involves a bunch of cussing. Even more
cussing when you put it back together.
This completes the disassembly you will need to perform for both the
Mile Marker kits.
Clean and inspect the parts and unpack the part time kit your going to
use. Be sure to thoroughly clean the mating surface where the tail
housing meets the chain housing. Gaskets do not form a good seal if the
surface is dirty or has a film of oil. Some brake cleaner sprayed on a
paper towel will get the oil film off. If you have some gasket glue then
you could tack the supplied gasket to the tail shaft housing and have it
ready for installation.
Use grease to hold the 25 bearings in place during assembly.
||Assembling the 501 output assembly.
If you are going to use the 501 kit then you will need to take some
grease and put it into the crown gear end of the output shaft where the
25 bearings belong. The grease will hold the roller bearing in place
during the installation process. Installing the 25 roller bearings into
the shaft at this point is the best. It's difficult to install the
bearings once you bolt the rest of the part time kit together. Don't
bang the shaft around once the bearings are in there or they may become
Remove the four bolts that hold the differential assembly together. Pull
the cross out and remove the 4 gears. Install two of the locking tabs
onto the cross shaft. Take the output shaft and mate it to the two
installed tabs. Put the last two tabs on the cross shaft and slide the
cage over the assembly. Bolt the cog side back on. Double check that the
bearing stayed in place and then you're ready for the next installation
Pump gear installed on shaft replacement kit
Here is the procedure for the Shaft replacement kit.
If you look closely at the above picture, you will see a gear that
resembles a speedometer drive below the splines and next to the bearing
surface. This is the pump that feeds oil to the rear most bearing. On
the older cases this pump is metal. The pump either needs to be replaced
with the newer plastic pump or you need to press the old pump off and
press it back on to the new shaft. I did the press route and after the
hassle it took to remove that without damaging it, I highly recommend
you go with the plastic replacement gear like the Mile Marker
instructions recommend. My 75 had the steel one. I do not know the exact
year these parts changed but even with a newer truck that probably has
the plastic pump it might not be a bad idea to install a new pump. This
will be a dealer item and the Mile Marker instruction have the part
number. This is the only extra preparation that is required for the
shaft replacement kit. The bearings are not needed like they are with
the 501 kit.
A bearing is used to keep the spacing correct for the output shaft.
Both kits require the use of a thrust bearing (Pictured above). Grease
the bearing and sandwich it between the flat washers that form the
bearing races. Slide this over the range box output shaft up against the
chain side crown gear. This bearing keeps the spacing correct for the
output shaft. It also prevents the roller bearings for the top chain
gear from coming out.
Stock assembly (left) 501 assembly (right) and Shaft kit (center)
||Both kits install in the same sequence from
501 kit pictured
From here the assembly is the same for both kits.
Slide the output shaft onto the range box shaft. Make sure that the
output shaft is fully seated against the bearing. Carefully slide the
larger section of the tail shaft housing and it's new gasket into place.
Install the 3/8s bolts with some thread sealent and torque to about 20
pound feet of torque. Over tightening will damage the gasket and
possibly warp the housing so don't over do it. Reinstall the speedometer
drive gear on the output shaft.
Installing the end play adjustment shims.
Now the critical part of the install.
The end play on the tail shaft is very important. Too little and you can
damage the bearings. To much and you will develop a lot of drive line
slop with the 501 kit. The Mile Marker kit does not give a clear
indication of what the recommended end play is. If you have a dial gage
I would say somewhere around 5 thousandths is about right. If you don't
have a dial gage then this amount is enough that you should just
"feel" the shaft move in and out a very slight amount but not
really enough to see. I cannot stress how important it is not to have
this too tight. If it were too tight then as the parts expand as they
heat the tolerance will decrease and you can damage the bearings or
worse. Too loose would be the side to error on but that may allow
excessive play in the 501 kit.
With both kits a good starting point would be half the original stack of
shims. Place those over the shaft and install the small section of the
tail housing with about 4 bolts and check the end play. Make sure you
don't have too many shims on. You should be able to push the tail cone
all the way on and have it seat without forcing it. This step usually
requires that you pull the tail cone on and off a couple times before
you get it right so have patience.
Mile Marker includes extra shims on the 501 kit. Both my previous
installs I needed to add a shim to the original stack. The shaft
replacement kit did not come with any shims. I had to remove about half
the shims in the original stack and no extra's were required. I think
the kit was engineered to be a little longer than the stock assembly it
Install the yoke and speedometer cable. Top off the oil in the case.
Hold off on installing the rear drive shaft. For the time being until
after you install the locking hubs.
Mile Marker Supreme Hubs
Both kits can be ordered with or without locking hubs. I decided to
order mine with the Mile Marker Supreme Hubs. Over all the Hubs seemed
to be pretty good quality. I have run them on a couple of rides and so
far I have not had any trouble with their operation. I would say this
locking hub is on par with the rest of the locking hubs on the market
for this application. One of the thing I really did like about these
hubs is Mile Marker uses a larger Allen bolt for holding the hub
together than most of the other hubs I have dealt with. The Hubs that
were replaced with the Mile Markers I had 2-3 of the bolts stripped out
because of the such fine threads that come with a smaller bolt. That was
a big plus right there. I take hubs off after every trail ride, where
deep mud or water is involved, to inspect for water contamination.
Because of the larger bolts I believe I'm going to have less problems
with stripping out one of these bolts.
The finger grips are also nice and deep, this makes engaging the hubs a
little easier on the fingers, that was nice to find. It might even be
deep enough to work while wearing gloves in the winter.
The kit comes with instructions on installation of the hubs so I'm not
going to get into that. The removal of your existing hub should be
pretty well the reverse of the instructions on installing the hubs.
If you're not using the Mile Marker hubs my only warning is NEVER
install auto locking hubs on a truck with a part time converted 203.
There are some regular maintenance steps that will need to be performed
and auto hubs will interfere with these. I discuss this further down.
Now the final thing I would recommend doing before your test drive. When
you installed the part time kit you have lost more oil than you could
top off. With the front hubs unlocked and the rear drive shaft off you
can run the case for a couple minutes to refill the reservoir in the
tail shaft before you put any load on the bearing, that at this point
have no oil in them to lubricate them.
Start by putting the transmission in neutral and the transfer case in HI
LOC ( Pull all the way back on the shifter). This will engage the front
output but since the hubs are unlocked the truck will not move.
HOLD YOUR FOOT ON THE BRAKE and start the truck. WHILE HOLDING THE BRAKE
put the truck in "Drive" and let it idle. Let The truck run
for 2 minutes or so and listen for any unusual noises. You will hear a
little noise that going down the road would be drowned out by tire noise
and the engine. Listen for noises like screeching and grinding. That's
the bad noises.
Turn the ignition off after a couple minutes but LEAVE THE TRUCK IN GEAR
until the transmission and transfer case stop spinning on their own. If
you try to put the truck into park before the transmission stops
spinning your going to hear a lot of grinding from a very angry parking
The Reservoir will now be full and the case will take about half a quart
or so of oil to top off. I really recommend that you top the truck off
with the rear wheels on ramps. This will slightly over fill the transfer
case and help overcome the chain not spinning. Over filling will allow
some oil to slosh into the reservoir in normal driving and keep that
tail shaft bearing lubricated. It is also recommended that you regularly
drive the truck in High Loc (hubs unlocked) about every 200 miles. This
will make sure the reservoir has oil in it to prevent the tail shaft
bearings from running dry and being damaged. Most people make a point of
putting the case in Loc every time they stop for fuel. Drive for 2-3
miles at normal surface street speeds and then you can shift back into
Hook up your drive shaft and take it for a spin. Listen for bad noises.
If everything is quiet with no unusual noises your ready to hit the
Well I'm a happy camper!
The kit has done a great deal to removing the excessive drive line slop
I previously had. This slop was a result from the stress the lockers had
put on the 501 kit. The trucks drive line feels very tight now. It feels
better then it did when the truck was still full time in fact. There is
always slop in full time due to the chain, differential and numerous
parts involved to make full time work. I'm seriously considering a shaft
kit for my wife's 79 simply for that reason alone. It's not a defect in
the 501 kit it's just the limitations of what can be done with the low
price kit and the parts in a transfer case with 200,000 miles on them.
The shaft replacement kit gets all those high mile parts out of the mix
and the result is most of the banging and clunking the truck was doing
when dropping into gear and during hard stops is gone.
Both the Mile Marker kits have their place. If your truck is stock with
stock size tires then the 501 kit is a good choice for you. Your really
not going to have a problem with the kit. My 79 has run that kit for
many years and many miles with no noticeable problems. There is also
several hundred miles of trail use on that kit and with a stock truck it
has held up just fine.
If you're truck is modified or you plan on modifying it and adding
larger tires lockers, then step up to the Shaft replacement kit. This
kit will handle hard off road abuse MUCH better. It is a much stronger
design with fewer parts. There is less to fail as a result and less to
wear when the truck is under the extreme loads.
The only thing that can surpass the strength of this kit and the 203 is
to replace the transfer case with a NP205 or another all gear transfer
case. That was out of my budget even if I could find the elusive 205
that would bolt to my TH350 transmission.
I would recommend these kits to anybody that is in need of part time
operation of their NP203 equipped truck. The products are of good
quality and do what they promise. Just make sure to buy the kit that
best suits your application.