Turbo 4x4 Transmission - how do I know ?

[koolray09]

I am new to the 4x4 thing and I purchased a 1978 GMC 4x4 1/2 ton, with a turbo 350 automatic transmission with NP203, from an estate auction, I got it running and it does not move, I dropped the pan and there are metal frags in the bottom.  I pulled the transmission to get it rebuilt, but the shop tells me that I have a 2wd auto trans case doing duty as a 4x4 auto case. The tail shaft is 27 spline and the np203 married up with stock transmission adapter/support mounting plate.

The cover which allows access to the torque converter, has large holes and the cover has two places on the bottom where to mount support brackets that come from the engine.  The transmission originally had small bolt holes to receive a tin cover plate, three of them were drilled out to accept a larger bolt which was required to marry the torque cover plate. 

I am being told by a GM tech who has long time experience with transmissions that in order to be a true 4x4 transmission case, their should be large bolt holes in the case to receive the torque convertor cover. The small tapped hole probably recieves a 3/16 or 1/4 bolt, the large receives something close to a 3/8 inch bolt.

How the heck does a guy know ?  I've seen hundreds of pictures on google that show guys proudly showing off their rebuilt turbo 350 transmission for 4x4 application - no tail, shaft sticks out 1" past housing - and the bolt holes for the torque cover are small bolt holes. 

I did see one "K" case - It look just like the 350 case I have and had a K in large letter, top right of case bell housing.    Is this the TRUE 4x4 case I seek ?  I know that this case is thicker.

how does a guy know if a transmission is a true 4x4 or simply a reused 2wd case ? 

Is there such a thing as a true 4x4 case ? does the main body of all 4x4 cases look like the main body of a 2wd ?  do the bolt holes at the converter cover a dead giveaway ?  inquiring minds want to know.  Is there a definitive answer to this question ?

Pictures are always appreciated.

[1967KaiserM715]

I believe K cases were the 700R4 trannys, HD should be stamped on all HD TH400s/TH350s

I don't believe bolt size changed alot, 1/4  to 5/16 maybe, but would use an aluminum flex cover, rather then stamped steel

the HD trannys had several more ribs in the bell housing, and a thicker case

beyond that, if it bolts up to a transfer case it is 4x4, wether it was built with a 2wd case(just need to swap tailshaft)

[Irish_Alley]

the only k i ever have heard of is on the 700r4s/4l60. yeah you can convert a 2wd t case to 4wd in some applications or swap tail shafts. the th350 is a weaker trans compared to the th400 but it takes a little less power to move it and is a popular drag trans just cause of that.  but even if there was a difference in the 2wd vs 4wd i wouldnt worry about i would just rebuild it or replace it with a overdrive trans. if they havnt rebuilt it i would find a trans with over drive like the 4l60 or 4l80 and find a transfer case to work with it. but then you have to think about drive shafts. i would also think about finding a different shop if this shop tried to get more money

[koolray09]

I ran across a transmission with "K" markings, modulator sticks out the back like all TH350s, has a 32 spline and it has six large opening bolt holes to recieve the dust cover. My dust cover has 6 large openings.  This is not a 700 and it has the K marking.  I am told this a more desirable transmission as the case is thicker and the 32 spline is more desired over the 27. 

I have NEVER seen a 350 case marked HD. and I've looked at a few. They all appear to be cases with four small bolt ups for a tin dust cover, which I am told is a dead give away that the case came off a 2wd application.   Can anybody out there verify this information. The matter is not whether I can use the 2wd case for a 4x4 application, the matter is whether or not a th350 case for a 4x4 application is different, and if so, in what way.  This is the heart of the matter I need to answer as it will help me make a buying decision.

the 4x4 application has two supporting arms that marry to the bottom of the dust cover, the other ends bolt to the engine either side, below the motor mounts, for added support.  From what I gather, you can use the 2wd case for a 4x4 application, but in order to reuse the beefy dust cover, you have to drill out the four small bolts holes and drill in two more.   the material at this point is thinner and more prone to breakage.  the th350 K marked ones have thicker material at this junction.

[enaberif]

I am absolutely and totally confused by the things being said in this thread.

If you have a TH350 the dust shield isn't going to make a difference between a 2wd and a 4wd th350.

There are differences between a 2wd and 4wd TH350 like the output shaft but that is about it. I have a TH350 with a dust shield that only bolts up with 2 bolts and have never seen a 4 bolt dust shield ever.

And I have a 4wd Th350

[VileZambonie]

I think he is confusing the tc cover for the 700R4. K case is used in truck applications

[rich weyand]

I have a 1978 K-10 350/TH350/NP203 sitting in the garage.  I believe the trans and xfer case are original.  You tell me what pics you want, and I'll take them.

[Irish_Alley]

I ran across a transmission with "K" markings, modulator sticks out the back like all TH350s, has a 32 spline and it has six large opening bolt holes to recieve the dust cover. My dust cover has 6 large openings.  This is not a 700 and it has the K marking.  I am told this a more desirable transmission as the case is thicker and the 32 spline is more desired over the 27. 

I have NEVER seen a 350 case marked HD. and I've looked at a few. They all appear to be cases with four small bolt ups for a tin dust cover, which I am told is a dead give away that the case came off a 2wd application.   Can anybody out there verify this information. The matter is not whether I can use the 2wd case for a 4x4 application, the matter is whether or not a th350 case for a 4x4 application is different, and if so, in what way.  This is the heart of the matter I need to answer as it will help me make a buying decision.

the 4x4 application has two supporting arms that marry to the bottom of the dust cover, the other ends bolt to the engine either side, below the motor mounts, for added support.  From what I gather, you can use the 2wd case for a 4x4 application, but in order to reuse the beefy dust cover, you have to drill out the four small bolts holes and drill in two more.   the material at this point is thinner and more prone to breakage.  the th350 K marked ones have thicker material at this junction.

the th350 was never a 32 spline output shaft it only came in 27
take some pictures 

[enaberif]

What really needs to be checked is the transmission pan and the transfer case before any other assumptions are made.

[HAULIN IT]

Yep, Sounds like you're getting some good info here. What you describe sounds like a mid '80's 700 from in front of a 208 case... K, 32 spline. Roll that bugger over...it's goin' have a big rectangle pan, not a square with a corner "cut off".

I've had 2wd trans cases in both of my 4x4's (to use the multi-bell TH350 case) without a problem. Look at it this way...What is wrong with the tranny from the truck? The insides are wore (metal in the pan) not a broken case! Do I think the braces are a good idea...yea, but not needed for the average truck.
My Thoughts, Lorne

[koolray09]

I have a 1978 K-10 350/TH350/NP203 sitting in the garage.  I believe the trans and xfer case are original.  You tell me what pics you want, and I'll take them.

take pictures of the th350 where the dust cover bolts to the transmission.  top down view is ok.

[koolray09]

Here is a picture of the transmission I speak.   This is NOT The tranny from my truck.  This is the tranny i want to purchase to replace mine. Look closely at the bolt holes where the dust cover mounts.  You will notice that the bolt holes are large...you will also notice the "K" on the upper housing.  Notched pan.  Modulator out the rear.  this is turbo 350.

I misplaced my camera, else I would take pictures of my dust cover, it is cast aluminum and it is thick, it is definitely not tin and thin.

[koolray09]

another image

[enaberif]

Very interesting! What are the numbers/letters on either the governor cover or the round cover on the passenger side.

[Irish_Alley]

maybe i never paid that much attention to my th350s about the k lol. but it shouldnt be a 32 spline and if it is you will need to get an adapter to mate your t case to the new trans output shaft.