Author Topic: Newby question on Tcase and transmission  (Read 2328 times)

Offline dustygriley

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Newby question on Tcase and transmission
« on: January 29, 2018, 07:56:05 PM »
Currently in my 78 K20 I have a 350 engine paired with a TH400 transmission and a 208 transfer case.  Obviously none of this is original but I would like to have it as close to original as possible.  That being said, I have a guy selling a TH 400 transmission and he says it's set up for an NP203 transfer case.  So my question is....  What exactly does that mean?  What pairs the transmission with the transfer case?  Is there an adapter plate or is it the spline count?  Sorry I'm sure some are thinking what kind of an idiot doesn't know this....  I can tear down and rebuild an engine with my eyes closed but the transmission and Tcase is something I've never dealt with.  Any words of wisdom would be appreciated.

Offline Rapid Roy

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Re: Newby question on Tcase and transmission
« Reply #1 on: January 29, 2018, 08:27:35 PM »
Welcome from DFW Texas.

1974 Cheyenne 10 LWB 350/TH350/AC/4 BBL Quadrajet
Mopar by Birth
Chevy by Choice

Offline donnyb406

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Re: Newby question on Tcase and transmission
« Reply #2 on: January 29, 2018, 08:47:31 PM »
you just need to get the proper adapter for a 400/208 combo. the 203 has a much different plate to mount up to a 350/400 trans. if you already have a 400/208 in your truck you should have the proper adapter already.

Offline dustygriley

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Re: Newby question on Tcase and transmission
« Reply #3 on: January 29, 2018, 09:39:59 PM »
That completely makes sense.  I obviously wasn't thinking that one all the way through.  I didn't realize I already have the correct set up for the 208 to the 400 and if I get this other transmission then I would have the set up for a 203 also just short the 203 transfer case.  Thanks for your help this is a learning process for me!!

Offline Iceman3005

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Re: Newby question on Tcase and transmission
« Reply #4 on: January 30, 2018, 03:30:00 PM »
An np 203 was a full time transfer case that bolted to a turbo 350.  Turbo 400 came with a np205.  The np203 was chain driven, the np205 was gear to gear.   Np205 is far better.

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Offline 75gmck25

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Re: Newby question on Tcase and transmission
« Reply #5 on: January 30, 2018, 09:09:25 PM »
The NP203 is a chain driven case, but it has an iron case and is nearly the same strength as the NP205.  However, the key difference is that the NP203 is a full-time transfer case, and in its original form did not have a 2WD position.   It could be run in either all wheel drive (high and low range), or 4WD high and low. 

Having the NP203 drive all 4 wheels all the time had an effect on gas mileage, and many were "converted" with kits by Milemarker so they could run in 2WD.  However, the conversion reduces oil distribution when in 2WD position, so they must be run about once a month in 4WD to keep everything working right.   Some also have some issues with chain stretch, although it usually just gets noisy, and is still unlikely to fail.

For most applications, the NP205 is better than the NP203.  However, the NP203 is very popular for plow trucks, since in the full-time position the transfer case differential allows the truck to be run between snow and dry pavement without causing any driveline binding.

Bruce

Offline Jason S

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Re: Newby question on Tcase and transmission
« Reply #6 on: January 30, 2018, 10:49:24 PM »
An np 203 was a full time transfer case that bolted to a turbo 350.  Turbo 400 came with a np205.

That statement is incorrect.  The NP203 could be found behind a SM-465 (4-speed manual), TH-350 (3-speed auto) and TH-400 (3-speed auto).

I have a guy selling a TH 400 transmission and he says it's set up for an NP203 transfer case.  So my question is....  What exactly does that mean? 

The NP203 for GM trucks had a few different input styles. There were differences over the years and what transmission the T-case was behind.  The NP203 on GM trucks could have a 10, 27 or 32 count spline with a male input on the input shaft and use a sleeve to mate to the transmission along with the T-case to transmission adapter. The output shaft on the transmission would have a certain spline count and length to match the mating T-case. What output shaft and length does the TH-400 have?   
1973 GMC K2500, Super Custom, Camper Special, 350, TH350, NP203, 4.10's
1974 Chevrolet K10, Custom Deluxe, 350, SM465, NP203, 3.73's

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Offline Irish_Alley

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Re: Newby question on Tcase and transmission
« Reply #7 on: February 01, 2018, 05:23:21 PM »
x2 what jason said. the th400 could have a couple different shafts
If you canít tell yourself the truth, who can you tell it to?~Irish_Alley

When you have eliminated the impossible, whatever remains, however improbable, must be the truth ~Sherlock Holmes

Offline dustygriley

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Re: Newby question on Tcase and transmission
« Reply #8 on: February 02, 2018, 09:56:59 PM »
Thanks for sharing your knowledge everyone I truly appreciate it.  I am not sure the spline count or shaft info on the current transmission.  I'm still hauling wood with it and I've got a few more loads to go before I tear into this project.  I want to make sure I have all the wood for next year ready since that's all I really do with the truck.  I am picking up a new bed for it this weekend so it's going to be a busy summer working on it.  I'll post what I have in a few weeks when I get started on it.  Thanks again!!!!

Offline Irish_Alley

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Re: Newby question on Tcase and transmission
« Reply #9 on: February 03, 2018, 03:51:29 PM »
to my knowledge every transmission with the except of the sm465 always came with the same spline count, but could have different lengths depending on year and application. the th400 could have the 203, 205 or 208. if memory serves me right the two common shafts in our trucks is the "short"  and "mid" and you can cut the "mid" down to the "short" size.

how much is he asking? and whats wrong with your trans?
If you canít tell yourself the truth, who can you tell it to?~Irish_Alley

When you have eliminated the impossible, whatever remains, however improbable, must be the truth ~Sherlock Holmes