By Gordon Pritchard
Inspired by reports from people using lower gearing, including Marlin's gearsets
for 'Cruisers, I too began to dream of going slower on the rocks. My FJ-45 pickup
has a long wheelbase, which let me contemplate an option other than new gears:
a dual-transfer-case setup.
I already had a 700R-4 overdrive automatic in my 'Cruiser, so a new Advance Adapter
shaft quickly made that "look" like a TH-350. The gear reduction box from a GM NP203
transfer case plugged on using factory parts. My own adapter, which made the backside
of the NP203 "look" like a TH-350 also, allowed a GM NP205 transfer case to plug
right on! The combination offers 1.96:1 (2WD) Lo range, a 2:1 (2WD or 4WD)
Lo range, and a 3.92:1 (4WD) double-low range. These two mated 'cases
feature cast-iron housings and all-gear internals.
Dual Transfer Cases!!
NP205 and stock GM TH-350 input adapter (left), my adapter plate,
and NP203 reduction box (right)
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The Recipe
Basically, you need a plate with the two different bolt patterns of the
transfer cases to be joined. You also need a shaft to carry torque through
from one box to the next.
Assessing the Reduction Box
There are many other donor transfer cases for the reduction box,
including Toyota mini-truck units, other New Process units, or Borg-Warner units.
The low cost and ready availability of the NP203 made it my choice.
If you're contemplating using an NP203, or other built-up transfer case,
you need to break that 'case down to obtain the reduction box alone. Use this
"quality time" to get a feel for how the original designers put the thing
together, and note other important features. For the NP203, it becomes
apparent that the bearing retainer on the back of the reduction box will
need to be held by the adapter plate. Also, oil coming through the open
bearing needs to be addressed, as do the various shift-rail holes and other
openings.
Other reduction boxes may have other needs, including seals or plugs.
An original-equipment overhaul manual can help you spot these considerations.
The Adapter Plate
If the 'case bolt patterns are non-overlapping, you'll have a much easier
time than if there is overlap. The NP203 reduction box pattern does not
overlap the NP205 input adapter pattern. This consideration will affect
the complexity and size of the adapter, and how it can be assembled. A
look through the Advance Adapters catalogue will show quite a variety
of approaches when faced with overlapping patterns. You'll develop a real
appreciation for the home-garage guy making use of non-overlapping patterns!
Once the bolt patterns have been assessed, have a look at how you can
carry the torque from one 'case to the other. If a Toyota transfer case were
used as the second unit, this dictates a male splined shaft to skewer through
the Toyota 'case. An NP203 main shaft could likely be cut down and resplined
to fit. For mating to the NP205, which features a male splined shaft on the
input, a double-female coupler inside a factory adapter housing is used.
Carefully measure all the necessary extensions from the NP205, and determine
how much you require from the NP203. You should then arrive at your minimum
adapter thickness.
The most important aspect of the adapter plate is maintaining shaft
alignment between the 'cases. With this in mind, look at how this alignment
can be achieved. The NP203 backside has a machined circular bearing retainer
- machining a tight-fitting recess on the adapter to fit the exterior of this
bearing retainer will accomplish alignment. It turns out that this recess
can also hold the bearing retainer in place. Measure and sketch the recess
needed on the adapter, to fit the bearing retainer and hold it in place.
Don't forget to allow for gasket thickness. The adapter plate will join
to the NP203 by using the holes found around the perimeter of the reduction box.
The NP203 reduction box's bearing retainer sticks out 0.890 inches from the
surface of the box, so we note that for later.
The other side of the adapter plate: The GM NP205 automatic transmission
input adapter has a circular snout, which will provide excellent shaft alignment.
There are four bolt holes surrounding this snout, which will join the NP205 to
the adapter plate. There is no need for anything fancier than a simple hole of
the correct size in our adapter, unless you wish to provide a recess for an
OEM-style O-ring. Since we're not sealing against ATF in this application, a
film of RTV will provide adequate sealing during assembly.
The NP205's snout protrudes 1.0 inches into the adapter. Combined with the
NP203 bearing retainer stickout (0.890 noted above), we need 1.890 inches minimum
adapter thickness. If we start with a 2.0 inch thick plate, this will give us a
little bit that can be ground off each face of the adapter plate, and some internal
clearance. In fact, it almost looks like the factory designed it to work this way!
Slide the NP205's input coupler onto the NP205 input shaft, with the GM input
adapter in place. Check the available depth in the coupler, from the mating
surface of the GM input adapter. From this measurement, and the adapter plate
thickness, we have now determined what the mating shaft length must be, relative
to the rear face of the NP203 reduction box.
For other transfer cases, check and re-check all your measurements. The specifics
above won't apply, but the general approach should work well.
Sketch out the cross-section of the main hole through the plate and check
everything. I found it useful to make some cardboard mock-ups that the simulated
adapter thickness, just to double-check. Below are a couple of links to Acrobat files
that contain two of my sketches. I DO NOT guarantee their accuracy. I am only
providing them to illustrate the general design of my adapter.
Off to the machine shop. Take your sketches, take the NP203 reduction box,
and take the NP205 input adapter - basically, give them the pieces that your
adapter plate must accommodate. My local machine shop then traced the NP203
reduction box outline, and used an optically-controlled cutting torch to quickly
cut some 2 inch steel plate. Steel was chosen simply because of the weight of
the transfer cases, the torque that would be carried, and my strength concerns.
The main hole was cut, using a lathe. In this case, all the necessary main-hole
steps can be cut with the plate set up only once in the lathe - this is almost
essential to make sure the two 'case shafts will be in alignment.
With the plate now cut, and the main hole bored, the NP203 reduction box (with
the rear bearing retainer fitted) could be aligned on the adapter plate, and
transfer punches marked the hole locations. The bottom-most two holes were drilled
through (because the box has the threads for these bolts), and the remaining six
holes were threaded (3/8" UNC).
The same process was duplicated for the NP205 adapter side of the adapter -
mark the holes, and drill and tap. On my adapter plate, these four holes are blind
(not all the way through), although this really doesn't matter.
Now, you should have an adapter plate. Mine took about 10 days (of evenings),
and cost me CAD$400.
NP203 side
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NP205 side
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The Shaft
From the adapter plate design, we have a shaft stickout length allowance from
the rear of the NP203 case. Careful use of calipers let's us translate this into a
length beyond the defined shoulder seen on the NP203 main shaft. As a sanity check,
we can insert the shaft into the NP203 output bearing, place the adapter plate on
the NP203 reduction housing, and measure for a stickout from the back of the adapter plate. The standard stickout for a 4WD TH350 (which we're mimicking) is 0.875 inches. If we duplicate this, the NP205 (originally used with a TH-350) will plug on perfectly.
A drawing of the desired shaft length, the NP203 main shaft, and the NP205
double-female coupler were sent off to Moser, for cutting down and resplining.
Because they use a rotary cutter for the splines, they cannot spline right up
to the "wall". Consequently, the double-female coupler must be bored larger
to allow for the unsplined portion of the shaft. Here is a link to an
Acrobat image of my shaft drawing. Again, it is only provided for illustration, I DO NOT guarantee its accuracy:
Moser only required a day or two for this shaft work, and charged about U$85.
Miscellaneous
The NP203 reduction box doesn't have provision for oil-draining, oil-filling
or breathing. In my case, I had a machine shop add these holes to the NP203 housing;
other approaches might incorporate some of these requirements on the adapter or
even the PTO cover. After examining a few different brands of transfer case, it
seemed reasonable to place the fill hole such that "full" was just below the idler
shaft. The drain hole was drilled and tapped on the bottom boss (which appears
to have been cast there for just such a hole!), and a 1/4" NPT vent hole was
drilled and tapped on the top. This fill-hole placement results in about 1 quart
of oil in the NP203 reduction box.
The original GM shifter mechanism fits right onto the NP205 input adapter, as
it came from the factory. Depending upon your vehicle, and the seats that are fitted,
you may need to make shifter or floor hole modifications. In my particular vehicle
(FJ-45 pickup), the bench seat obstructed any shift-lever. Consequently, I used Morse
marine controls, operating one cable each for the NP203 and the NP205. The NP203
transfer case shifter simply has three positions - H N L - selected by a rotating
shaft. The NP205 has four linear positions - 4H 2H N 4L. Some thought and fabrication
will likely be required to achieve a shift linkage you're happy with.
Modify your driveshafts or treat yourself to new ones, and E N J O Y!!!
January 4, 2001
Since writing this article, Gord has found the marine controls to be unsuitable for use as transfer case shifters. He has since switched to an aftermarket hard-linkage shifter which required some customization to work with his custom adapter plate.
