first ... here you see the input face of the Ford NP205 unit. It
is a 31-spline female input shaft with a mirror-image Atlas II 6-bolt
pattern. This is important to note because This conversion
utilizes a stock GM adapter that is absurdly similar to the Atlas II
This is the
stock Ford C6 automatic trans adapter that I made sure to get along with
my NP205. I'm glad that I did, too because I used it as a stencil
later on for a spacer that ended up being necessary for the adapter.
Now for the
ribcage of the whole conversion. I call it the rib cage because
the splined coupler, or spud shaft as some refer to it as, is really the
heart. What you see here is a stock GM TH350 auto trans to NP208
t-case adapter. I purchased this used from someone on the internet
for $25 shipped. They are very common and not very highly
desired. You see one hole wallowed out on the lower left because
it is the only hole that slightly differs from the stock Dana 300 /
Atlas II bolt pattern. This adapter lived previously in front of
both. (See my previous article on the
TH350 to Atlas II conversion
for more details on that.) But as it turns out, this is also
the very hole that requires the most modification for the current
conversion in question.
adapters side by side with the C6 - Ford NP205 on the left, Chevy TH350
- NP208 on the right. The Ford adapter looks a lot bigger, I know,
but believe me, the bolt patterns are practically the same. I
guess the Ford adapter just has a bunch of extra meat, probably needed
due to the aluminum construction vs. the cast-iron of the Chevy.
is deceiving, but just trust me when I say that it is trick-photography.
The adapter that you want is 3.5" long from mounting face to mounting
face. People have told me that there were three different TH350 -
NP208 adapters made by Chevy, all with different lengths. I
have only ever seen three in my life and all of them were this length,
so I am slightly skeptical on that "fact," especially since no one has
been able to produce pics of any other than this one. (Watch a
million photos get sent to me now for saying that.) But just to be
safe, when you buy yours from someone on the net, have them measure it
before they send it. You're looking for right at 3.5".
reference, this is the mounting flange of the 4wd TH350. The
typical 4wd TH350 has a 27-spline male shaft that sticks out past the
face of the flange about 3/4". This was all very important for me
to know when researching this adapter because the spud shaft that I knew
I would need had to be the proper length.
heart of the whole conversion, and the most expensive part! I paid
2.5 times as much for this as I did the for entire transfer case!
(When looking for Ford NP205's I was able to find them at the average
pull your part-type yards for $100 each. I pulled mine as a
ready-to-use unit, it had no leaks, shifted smoothly, and operated
smoothly in both ranges. I am not even going to replace any
seals.) This is the spud shaft that comes in the Advance Adapters
conversion kit, PN
They sell the entire adapter kit for around $550 + tax and shipping.
The shaft alone ran me about $250 + tax and shipping. It is almost
exactly 6" long overall. The male-spline portion is 31 splines and
is longer than the female-splined portion, which has 27 splines.
Needless to say, the idea here is to get the male end to slide into the
female Ford NP205 input shaft with plenty of engagement while also
allowing the male TH350 output shaft to slide all the way into the
female end of the coupling. The fact that the male end of the
coupling is longer becomes important later on in the whole conversion.
difference between the Ford NP205 adapter and the Chevy NP208 adapter is
that the Ford adapter has a portion cut out of it to clear the range
selector rod near the input of the Ford NP205. This is obviously
very important because you want to be able to shift your transfer case.
Thus, I used the grinder to cut out a chunk of the Chevy adapter to
clear the rail. Not bad, very easy, took 10 mins of grinding and
test-fitting to get it right. But once I was actually able to get
the adapter to sit up against the input of the 205 I realized that this
conversion wasn't going to be as easy as the previous Atlas II
conversion for two reasons. One, the input flange bolts hit the
inner diameter of the Chevy adapter, preventing it from seating
properly, making it sit exactly 1/4" away from the face of the NP205.
There was not enough meat in the adapter to safely grind clearance holes
in it for the 4 bolt heads. Two, the bolt pattern did not line up
as well as I had hoped it would. Four out of the six bolts lined
up perfectly, leaving one bolt hole slightly off from the its respective
hole in the NP205 and the other about 1/2" away! The upper left
hole in this pic shows how I used a plasma torch to elongate the hole
closest to its mark, and then completely cut the lower left hole away
just to get a bolt to go into the 205. Basically, this means that
one out of the six holes is virtually useless in terms of structural
integrity. I'm banking on 5
bolts being enough at this point, but I do have a solution for the whole
problem, which is below. (After all, the entire assembly bolts to
the trans using only 4 bolts.).
The 1/4" gap
left by the 205 input housing actually worked out in my favor in the end
and here is why: The gap forced me to make a 1/4" spacer to go
between the NP208 adapter and the NP205 t-case. This allowed me to
make a plate, seen to the left, that had bolt holes accurately placed to
mate up against the 205. The reason this is nice is because one
can then bolt the adapter onto the plate, only bolting the odd-ball
hole through the space plate. One can then put some short welds
on the adapter, welding it to the spacer plate and thereby utilizing the
odd-ball hole for strength again! While it has not yet been done
in any of these pics, believe me, I intend to. The stock Ford
NP205 adapter came in handy both in tracing the pattern to cut this
plate with and drilling the required holes in the plate.
In this pic
you can see how the spacer plate fits between the adapter and 205,
completing the seal between the two and enabling the adapter to clear
the input flange bolts.
This is what
my "oddball" bolt looks like right now. Not very structurally
useful to the entire assembly, but the t-case would leak through that
bolt hole if nothing was there, so it has to be addressed.
calculations I knew that the spud shaft would not be quite long enough
to fit perfectly between the t-case input and the trans output, but it
was only by 1/4", which is extremely negligible. However, once the
spacer plate is added to the equation, there is about 1/2" of play
between the shafts and the spud shaft. Since the male portion of
the spud shaft was longer than the female portion anyway, I chose to
take the entire 1/2" out of the t-case end of the spud shaft by
inserting a small length of heater hose in the female NP205 input shaft
to provide a soft, noiseless spring force to shove the spud shaft toward
the trans at all times. Before you say this is bootilicious,
consider that many factory applications do something similar using
formed rubber pucks at the tips of the inputs, I assume to utilize
slightly short shafts that they already have in production, so this is
not that different. Besides, no one would ever know that it was in
there unless they read this article. ;) And don't worry
about the 1/2" loss of contact area inside the t-case input, there's
still a good 2.5", which is about .5" more than the original C6 trans
had that was mated to this t-case.
Final pic of
everything with the spud shaft installed to show how far out it will
stick. Note that I can push it in and compress the rubber hose
inside for about 3/8", which will be nice when installed onto the trans.
I plan to make a shifter for it all before I bolt it to the trans, but I
will post pics later of everything bolted together. Stay
Everything assembled as it will be in the buggy. As you can see,
it fits. The spud shaft and everything mated up perfectly.
There is going to be an issue with clearance at the side of the engine /
trans that will force me to run a 2-piece front driveshaft. I
don't think this would be as big a problem with a normal Ford axle
because my Unimog axle pinion is not offset as far as a Ford axle.
One more ...
I plan to do something about the odd-ball bolt soon ...