NP203 Differences Early vs Late Design

[dhenderz]

Looking for an NP203 expert. I got the differential from Offroad Design for converting the 77 K10 back to full time 4WD. Interestingly they sent me the entire tail housing and extension. Putting the original side-by-side with the unit they sent me shows a few differences (original on left, new on the right).

Most notably you see the speedo driven gear on the new one is not supported at the internal end. It is simply supported by a longer section of casting where it threads into the case. Also interesting is the tooth count is lower, 14T versus original at 18T. I really need a 16T for proper speedo calibration with the 33inch tires that are planned. In the original design (left pic) 17T is the lowest you can get. Actually the available tooth count goes the other way - up to 22T. I am guessing this upper tooth count availability was to accommodate taller rear end gearing (I have a 3.73)

To other differences worth noting are the orientation of the housing and bearing retainer snap ring. These two tailhousing are very close. But if you look at the mounting holes it is evident they will 'clock' slightly differently when installed in the truck. Also, the one on the right does not have a snap ring to retain the bearing. It simply press fits into place and is retained by the assembly.

So now I have just learned from Offroad Design (good guys by the way) that the one on the left is from the early design. Not exactly sure of the cutoff year, but the guess is 74 and later. After that the design changed to the one on the right. So now I need to see if there are driven gears in 16T available for the older design. If not I will just reassemble with the original one and get an adapter to correct the speedo.

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[75gmck25]

I have a stock late '75 NP203, but haven't had to do much to keep it working. I've never had a reason to open it up.

I do know there are two different sizes for the chain; however, I don't know if that is part of the early/late version changes.

Bruce

 

[dhenderz]

OK, now I am starting to wonder about the info I received indicating the differences in the 2 tailhousing was due to early vs late design. I was able to locate the exact speedo driven gear online. It even has the same part number stamped on it. And according to the site selling the part this is for a 72 - 93 Dodge NP205. What gives? Are the tailhousings the same between an NP203 and NP205? Is the difference here that I received an NP205 tailhousing? That doesn't make any sense to me since a 205 does not have a differential. Confused!

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[HotRodPC]

I don't know that we have an Tcase Experts but @crazy4offroad or @bucket might know a bit more about those NP203's than most of us.

 

[HotRodPC]

OK, now I am starting to wonder about the info I received indicating the differences in the 2 tailhousing was due to early vs late design. I was able to locate the exact speedo driven gear online. It even has the same part number stamped on it. And according to the site selling the part this is for a 72 - 93 Dodge NP205. What gives? Are the tailhousings the same between an NP203 and NP205? Is the difference here that I received an NP205 tailhousing? That doesn't make any sense to me since a 205 does not have a differential. Confused!

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I was going to make that suggestion, that it might not be a difference of late or early models, but possibly a difference in breeds. As in Chevy vs Ford or Dodge. This here you're showing a difference in models of NP203 and NP205.

 

[crazy4offroad]

I wont be much help here, the ORD guys would be the best source. I know there's a year break but not sure when. I think 76 was the last year for the yoke style front output. As long as ORD says it's safe to run I would go with it. I know you can get external speedo correctors that may fill the difference but honestly 3.73 with 33 inch tires is as close to stock ratio as you can get. The next best answer is, ditch the 203 for a 205.

 

[bucket]

I was told by a knowledgeable fella that most of the 203 design changes happened around '76. I've never really opened up t-cases before, I've always avoided that, just like transmissions. But I do know that the 203 and the 205 use a different tail housing.

 

[Dooley]

I was told by a knowledgeable fella that most of the 203 design changes happened around '76. I've never really opened up t-cases before, I've always avoided that, just like transmissions. But I do know that the 203 and the 205 use a different tail housing.

Some already beat me to it, I think the one sent is non GM application. I will try to dig out my extra speedo gear shafts as I think I have several tooth counts. There were several changes beginning with the second year. The first year units had the oil drive gear as a machined part of the rear output shaft instead if the plastic gear later on. They also had a one piece tali section instead of the 2 piece design. I felt the biggest change to the 203's came mid model year 1978 when they changed the drive gears and chain size. The narrower chain and gears were less likely to have chain slippage when worn or stretched due to the steeper pitch of the teeth. The 79 units were all good.
I believe the H code gear has 18 teeth and the J code gear has 19 teeth.
I nave several 203's and a couple 205's in the back garage. I do not know what if any they have currently.