How to:

 

Adapt a Ford NP205 Transfer Case to a Chevy TH350 Automatic Transmission Using Mostly Factory Parts


 
First is 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 pattern.
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.
Both 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.
This photo 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".
Just for 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.
 

Finally, the 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 50-6904.  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.

 

 

A major 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.

 

Here is everything assembled.
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.
 

During my 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 tuned ...
***Updated 2/12/2006***  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 ...