GM truck - 70s-80s transfercase swapping

Got a question about swapping a transfercase between 4WD GM trucks, Blazers, Suburbans, etc.

I was kind of hoping someone out there might know this - so I don't have to find out the hard way.

I have a 78 K5 Blazer - 350 V8 with a turbo-350 auto trans. It's a farm snow-plow truck. It has lost 4WD. I never paid close attention to it until now. I see it's been converted a long time ago, from full-time 4WD to part-time. Lockout hubs added and a conversion kit installed into the transfercase. It now does not work. If I really jam on the transfercase lever - it will pop in and out of 4WD, but won't stay there. So, I'm looking for the easiest fix. This is a farm truck and will never go on the road.

I have many junk 4WD trucks in my fields with many extra transfercases - so my question is about what might fit.

I have many 80s to 90s trucks with New Process 205, 208s and 241 transfercases. The problem is this. GM puts different splines in transfercases depending if for HD use or LD use. I know the following for newer partime transfer cases - e.g. 205, 208, and 241:

Transfercase splines to fit a heavy duty four speed manual (SM 465 cast-iron with granny low) are the same as for a turbo-400 automatic. Shaft is 32 splines.

Transfercase splines to fit a light duty four-speed manual overdrive (New Process 833) or a light duty turbo-350 or 700R4 auto are 27 splines.

Now, I've got this truck with the full-time 4WD NP 203 transfercase mated to a turbo-350 trans. Anybody have any idea what splined shaft it's going to have?

I'm also wondering if there might be a way to pull the lockup guts out of this 203 and maybe fuse them together someway to make it 4WD forever?


I'm just trying to figure which will be the easiest since this truck is basically just a farm truck. I was considering taking the plow off and just sticking it on to another truck - but that's a lot of work too. Especially with a Fisher plow that requires a belt-driven hydraulic pump mounted on the engine.

"Transfercase splines to fit a light duty four-speed manual overdrive (New Process 833) or a light duty turbo-350 or 700R4 auto are 27 splines.

Now, I've got this truck with the full-time 4WD NP 203 transfercase mated to a turbo-350 trans. Anybody have any idea what splined shaft it's going to have?"

You answered your own question... The THM-350 and the 700R-4 share the same tranny output spline congiuration.

The THM-400 and the later 4L80's share a heavier shaft/spline configuration.

First of all, have you cleaned and lubed the 4X4 shifter and adjusted the linkage rods?

If that doesn't help, it's really NOT too tough to access the lockup parts on the UPPER shaft at the rear of the unit.

The problem may be as simple as a worn fork, or the parts added to eliminate the lockup function MAY be torn up.

There were GOOD kits, using a one-piece steel splined part to accomplish the "full-time 4X4 defeat" function, and less tough kits that used 4 toothed blocks to replace the spider gears in the interaxle differential. You may want to grab a book and tear the back upper cover off of the case and see what's in there before going to all the trouble to change out the entire transfer case.

Here's what the little blocks look like:

<img src = "http://www.off-road.com/trucks4x4/data/articlestandard/chevy/472005/199953/crowngear1.jpg">

Those spline counts are for the 80s and up trucks. I don't have the info for the 70s trucks - thus my question. Maybe they're the same, but I don't know for sure. In the 70s, GM did things different. Many of my 70s Blazers have the HD cast-iron 205 transfercases - and in the 80s those cases were only used in HD applications.
Often in 1 ton trucks. I've got a 79 Blazer here with the 205 transfercase and a 305 engine - along with the four speed top-loader HD trans.

With this 78 - I know the linkage isn't the problem. I know it's internal - but have not peaked inside yet. To tell you the truth - I bought the truck last year just to get the camper off of it. A local girl drove it here to NY from Colorado and it had an odd-ball Hallmark pop-up camper on it - specially made for K5 Blazers. So, I used the camper on one of my diesel Blazers - and then stuck a plow on this 78. Put chains on all four wheels and was just keeping it for the worst conditions. First time I used it, it plowed OK but things weren't too icy and snow wasn't real deep. 2nd time I went to use it, I discovered it didn't even have 4WD. Not so easy to notice when you've got good chains until you hit some deep snow or get stuck in a snowbank and only see the back wheels turning. So, that's where I'm at. It's something internal - and if I put a lot of pressure on the shift-rod - it will work 4WD in high range, but not at all in low range.

I've never installed one of those conversion kits, but will assume they were not all of the same quality? Who knows, guess I'm going to find out.

I'm 99.9% certain the THM-350 output shaft spline counts remain the same for the 70's and 80's, 2WD OR 4WD.

You can replace an 80's 700R-4 with a 70's THM-350 in a 4X4 application and keep the driveshaft length the same by using a tranny with a short 2WD driveshaft, and cutting off part of the splined area, then mate it to the transfer case using an adapter that's about 1-2/2" or 2" thick. So the splines ARE the same. (You wind up with an output shaft that's a little longer than the factory part, but shorter than the shortest 2WD version.)

I work in a transmission parts supply warehouse and have sold literally thousands of the parts AND units in question here. If you want some good straight info, call us Mon-Fri 8:00 to 5:30 MST.
Idaho Transmission: 1-800-225-6568. There's lots more to be said than I want to type here, but I'll give you some basics.
The early SM465's had 10 spline outputs They didn't go to the 32spl until 1982, the first year for the NP208.
The NP203 T-case you have presents the biggest problem. Nothing else will fit in it's place without major modifications, but it can be done. Putting a NP205 in it would be the best, but here's what you have to do: Having a spare truck with all the right stuff is handy. First, you have to have a 205 with the right input in it, 27spl to mate up to your 350. You can change the input gear in one, but not one that was behind a TH400.
Having the right t-case, you need the adapter between trans & t-case. Also you will need the sleeve that couples them together, and the complete shifter for the 205. Next, because of length differences you will have to move the crossmember, use different mounts(maybe), and when you're all done with that you will need to lengthen one drive shaft and shorten the other. Hence my comment about having a spare thuck with all that stuff on it.
It's possible that you can fix the 203 without much trouble. There were 3 designs of the conversion kits made. The one that Bob has pictured is made by Milemarker, and is a good design. The crappy ones were the ones using a one piece gear instead of the 4 blocks. The most common problem with those kits(and this may be your problem) is that over time the thrust bearing behind the rear yoke would shell itself out and cause too much end play in the shaft. That usually resulted in the shaft moving backward just enough that the sliding collar wouldn't engage all the way and they would pop out of gear. If you take it apart and find everything all right there, then you may have fork problems or something more serious, but it can be checked out fairly easily by taking off the yoke and rear housing. If you're not familiar with this stuff, I wouldn't do it without first looking at some instructions for the conversion kit, though.
Call and ask for me & I'll discuss it with you, or any of our salesmen can answer your questions, too. I hope this helps.
Jeff B.

Thanks, that all makes sense. Feels like there's something internal allowing too much slop and preventing full engagement.

I didn't know about the older coarse spline shaft. So, my 79 K5 Blazer that has that cast-iron HD four-speed and the cast-iron NP 205 transfercase is 10 spline? I was kind of wondering if there were different standards pre-80s.

It wouldn' be worth all the effort to swap in a transfercase. I've done enough 700R4-removal conversions back in the early 80s. Changed a lot over in diesel trucks - to TH400s with mechancial modulators.

I'm hoping that there's some way to perhaps weld some parts together inside the transfercase to make it 4WD all the time. It will never be driven on the road again - it's strictly a farm snowplow rig. If it's too much work to fix, it would be easier to take the plow off and mount it on another Blazer or Suburban - I have many good runnig 4WD Blazers and Burbs - partially retired and parked out in my field.

Yea, back in the '70s you had 10spl outputs on the 4 speeds, 27spl on the 350/700R4 and 32spl on the TH400 and that's about it. The one exception was that dodge 833 that they used for a few years. If I remember right, that one was a 27spl also.
Depending on the conversion kit you have, you shouldn't have to do any welding. The absolute best one was the one they called the Doug Nash conversion. It used a one piece gear/output shaft so there were no pieces to come apart. The next best is the Milemarker kit with the square blocks. When it's put together it is nearly like the nash style. There's really nothing to break. The bad one was one that used a "crown gear", a toothed piece that just slipped over the cross in the planetary. They were clunky, and those gears would actually break into pieces sometimes. They were sold under several names, but the Wolverine brand comes to mind.
All of them require shimming of the output shaft to get proper end play. It would be easy to pull the back housing and check that torrington bearing and the conversion. If everything looks intact, just slap it back together and put your plow on another truck. If not, it could be a simple fix to get it going again. Good luck.