Tightening down front yoke nut stops NP205 from turning

[Mark80K15]

Hello, 1980 GMC K15 SM465/NP205. I broke the SM465/NP205 adapter, so I took the time to apply new NP205 seals/gaskets and the JB Fab twin stick setup. I didn't intend a full rebuild, but I wound up removing everything except the idler gear assembly. In fact the front shaft just kinda fell out (gently) at one point. I've never worked on NP205 before.

Replacing the shift rails went well. Since I had both the front and rear output assembly out, I could clearly see the 2 rails from the inside, the 2 shift forks, and the interlock rod. Seeing these and understanding how they work helped a lot in going through the rail replacement procedure. Once the roll pin is knocked out of the rail (3/16" punch), and the rail is clear of the interlock, there is nothing else touching the rail but the fork and the case itself. I found the passenger rail was kind of stuck in the fork, which probably causes the difficulty that others have reported. Just keep working the fork off the rail, it will come eventually. Brand new and without the roll pin, the rail slides through the fork effortlessly.

So I finished reassembling, applied gaskets and seals, and measured how far the modified rails (that came with the twin stick setup) protrude from the case in each NP205 shift position. The measurements were correct, (and the same as stock). I drew a guide which I posted here:

 

[Mark80K15]

I confirmed that for each NP 205 driving position (4H, 2H, N, 4L), the rail position was correct, and the input/output shafts turned properly. For example, N has each shaft turning (or stationary) independently, 4L has input turning twice for every front/rear output turning once, etc. Everything moved smoothly, minimal noise, etc.

Here's the PROBLEM: The last thing I did was tighten down the nut on the front output yoke. On the bench, I could only get it to about 60 ft-lbs (I think it has to be 120-150 ft-lbs). At that point, no shaft would turn at all. Everything was locked up. I couldn't even move the shift rails. If I simply finger tightened the nut, it would still turn.

If no one has any ideas, I guess I'll have to open the front drive back up (remove front yoke and seal, and rear cover for the front output assembly) and see if anything is reassembled wrong.

Thanks in advance!

 

[PrairieDrifter]

Sounds like maybe a bearing issue. Thrust washer maybe.

Check out GebBuilt on YouTube, he goes through one by the book. He also became a member pretty recently.