NP203 transfer case 101

[LinemanL.U.47]

The NP203 Transfer case. I had some confusion lately with my 203 t-case. After doing some research and talking to a t-case guru I think I have sorted it out. Correct me if any of this is wrong.

STOCK NP203 W/ STOCK HUBS
The 203 is a chain driven full time 4 wheel drive t-case.
Trucks manufactured with the 203 came stock with always locked hubs.
The shift pattern is forward 4 lo lock, 4 lo, N, 4 hi, 4 hi lock closest to seat.
In 4 hi,lo the t-case works like an open diff sending power to the least resistance.
In 4 hi,lo lock the t-case sends power equally to the front and rear.

Many trucks with the 203 t-case have been converted to after market manual locking hubs. The most common reasons for this are price and availability. When your stock always locked hubs fail it can be expensive and hard to find stock replacement parts. I’ve also been told that good after market hubs are stronger than the stock ones. Another common reason is that the t-case has been converted to part time 4 wheel drive (more on that later).

STOCK NP203 W/ AFTER MARKET HUBS
Has all the same characteristics as the all stock set up with a few exceptions.
In 4 hi, lo with hubs unlocked the truck will not move. The T-case would send all power to the least resistance, only the front shaft would spin turning the axles into unlocked hubs.
In 4 hi,lo lock with hubs unlocked the truck will move. Both shafts will turn at the same rate but only the rear will be driving due to the unlocked hubs this is not recommended for daily driving.
A truck with manual locking hubs and a stock 203 t-case should be operated in 4 hi with hubs locked for all normal driving conditions (this is the same as the stock configuration).

Many truck owners to save on fuel cost and wear and tear have converted their full time 4x4 203 t-cases to part time 4x4 cases. The Mile Marker kit is one of the more popular kits.

CONVERTED PART TIME 4x4 NP203 W/ AFTER MARKET HUBS
This NP203 is a chain driven part time 4 wheel drive t-case
Almost all part time conversions are done in conjunction with after market manual hubs.
The shift pattern is forward 4 lo lock, 2 lo, N, 2 hi, 4 hi lock closest to seat.
In 2 hi with hubs unlocked the front drive shaft, diff and axels do not spin. This would be used for normal driving conditions.
4 hi,lo lock with hubs locked would be used for 4x4 driving conditions.

If your truck has manual hubs and your unsure weather or not the t-case has been converted use this simple test. Unlock the hubs and position the t-case one notch down(toward the seat) from neutral. Put it in gear. If the truck moves its been converted, your in 2 hi. If it just revs up it has not been converted, your in 4 hi spinning the front end into unlocked hubs. Hope this info might help someone else out there.

[OrrieG]

There are two basic conversion types out there. One locks the existing spiders with wedges and uses a wafer bearing. This was the original Mile Marker kit. Over time the wafer bearing compresses, fails, causing excessive movement and the case hand grenades. The second is a full shaft conversion kit that gets rid of the spiders. It was originally offered by Doug Nash (DNE2) in the mid 80's. I've got one in my 203. It essentially changes the box to a true in or out box. Nash quit making them and the patent circulated around for a while. Mile Marker now has the rights and sells the shaft kit using the same design. I can attest that it strengthens the set up, the weak link is still the chain. Even with the shaft kit that allow the complete front wheel drive to be disengaged at the transfer case in 2wd (no power to drive shaft) bug allows the chain to slowly circulate for lubrication the chain stretches. When stretched enough under loading it will skip teeth, very noisy and disconcerting. Eventually something breaks, a tooth or the chain. When the chain is tight all works well, not a good as a gear driven 205 but I always ran out of chones before the xfer case gave up. I am still running it because the truck is set up for it and replacement 203's are cheap up here. Right now I could get one in good working condition on craigslist for $50.

[LONGHAIR]

Originally Posted by LinemanL.U.47

When your stock always locked hubs fail it can be expensive and hard to find stock replacement parts.

This part is not correct. There really is no "locked hubs", it is a simple solid steel "slug". It is about 3/4" thick with splines on the outer edge to match the hub and inside to fit the axle. It is virtually impossible to ruin one of these "drive flanges".
They might be slightly hard to find because they have not been installed from the factory for about 30 years. There should be thousands of them out there though in wrecking yards and probably some sitting around every 4 wheel drive shop in the country. I have personally thrown away hundreds of them after doing the conversion.

[silver7]

Great thread. Although I'm still a ways off from installing my transfer case this helps me understand it a lot more. Thanks.

[Adams74Chevy]

The '73 I bought it supposed to be converted now I know what position is what and plan on checking it out this week.

Thanks

[LostMy65]

I have a NP203 left over from my donor.

I forgot to check to see if it works before dropping it.

Is there a way to test it to see if it works?

[K10-Kansas]

Thanks for the great info in this old thread. I've got a 203 and am reading all of the threads diving into all the things I don't yet know about.

[Getter-Done]

Originally Posted by K10-Kansas

Thanks for the great info in this old thread. I've got a 203 and am reading all of the threads diving into all the things I don't yet know about.

Here is a good place to get parts and info on transfer cases.

I used an 90 degree (NPT) elbow to raise the oil level.

Link: https://torqueking.com/category/1973...transfer-case/


The 1979 Oil Crisis had one good benefit in that it ended the use of the inefficient, gas guzzling NP203 transfer case by all vehicle manufacturers. To increase the fuel economy of their 4x4s, many owners of vehicles equipped with the NP203 installed "Part Time Conversion Kits" in the transfer case and locking hubs in the front axle. These conversion kits basically eliminated the inter-axle differential. With a Part Time Kit installed and the lockouts in free, fuel economy did go up, however lubrication problems related to the conversion destroyed many converted transfer cases. Since the NP203 does not have an oil pump, it relies on the rotating chain to splash oil on the upper shaft components. When operating a converted NP203 in 4x2 mode with the hubs in Free, the chain does not rotate which leads to oil starvation and failure of the upper shaft components (Raising the oil level in modified NP203 Transfer Cases helped, but did not solve the lubrication issue) Relatively few vehicles equipped with the NP203 remained in service beyond the mid 1980's.

[K10-Kansas]

Originally Posted by Getter-Done

Here is a good place to get parts and info on transfer cases.

I used an 90 degree (NPT) elbow to raise the oil level.

Link: https://torqueking.com/category/1973...transfer-case/


The 1979 Oil Crisis had one good benefit in that it ended the use of the inefficient, gas guzzling NP203 transfer case by all vehicle manufacturers. To increase the fuel economy of their 4x4s, many owners of vehicles equipped with the NP203 installed "Part Time Conversion Kits" in the transfer case and locking hubs in the front axle. These conversion kits basically eliminated the inter-axle differential. With a Part Time Kit installed and the lockouts in free, fuel economy did go up, however lubrication problems related to the conversion destroyed many converted transfer cases. Since the NP203 does not have an oil pump, it relies on the rotating chain to splash oil on the upper shaft components. When operating a converted NP203 in 4x2 mode with the hubs in Free, the chain does not rotate which leads to oil starvation and failure of the upper shaft components (Raising the oil level in modified NP203 Transfer Cases helped, but did not solve the lubrication issue) Relatively few vehicles equipped with the NP203 remained in service beyond the mid 1980's.

Is there a better transfer case today that K10 owners go to when their np203 t-case fails?

[kwmech]

Originally Posted by K10-Kansas

Is there a better transfer case today that K10 owners go to when their np203 t-case fails?

Yup, back to the original beast....the NP205 an old tried and true bullet proof gear drive.

[K10-Kansas]

I found a great video on changing the lubrication on a transfer case.

https://www.youtube.com/watch?v=QjOYhVLwhxA

[Accelo]

Originally Posted by kwmech

Yup, back to the original beast....the NP205 an old tried and true bullet proof gear drive.

I second Kwmech post. The 203's are a pain to work on and heavier than the 205. One has to put money into them to get them to be reliable. It's a heavy turd.

I have converted a few of the 203's in the past. If I had one I would either replace it with a 205 or a modern aluminum case transfer case. Converting is a good method to loose about 50-75 pounds of excessive weight.
Nothing good to say about them. They were not great when new and the only reasons to keep one is they are cheap.
Cheers.

[Getter-Done]

Originally Posted by K10-Kansas

Is there a better transfer case today that K10 owners go to when their np203 t-case fails?

Just seeing this.

Yes as said above the 205 is the better choice.

[cadillac_al]

I would never give up the AWD of my 203 brute. I usually only use it in winter but If I need to haul something 6 hours down the highway it still works just fine and gets 10 mpg like any other truck.