Hey I'm currently rebuilding a nv4500 for a 1992 c3500. It is 2wd with round top shift cover. So I went through and replaced all the bearings, synchros, a couple of worn gears, etc. Got the shaft end play set for the reverse countershaft, and the endplay set for the main shaft as well. The range for endplay according to the rebuild manual is 0.002-0.006". So after all of that I got to the point of putting the shift cover back on. The forks all line up. After bolting the shift cover back on, I verify that it does appear to shift into all five forward gears plus reverse. My question for anyone that messes around with rebuilding these types of transmissions is on the bench after you put it into gear, should you be able to turn the the input shaft by hand?
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Yes, as long as you see rotation at the output shaft as well when it’s in gear.

 

Okay so in all forward gears it won't rotate. If I put it into reverse it'll rotate. And I'm testing this on the bench.

 

With the shift cover off , run it thru all the gears to see if your getting a tie-up for some reason

 

So with the shift cover off, when I put it into any of the forward gears, it locks up. When I put it into reverse, that's the only time it'll rotate. So I'm thinking at this point I'll need to fully disassemble this and see what I overlooked. The main shaft still has endplay, so there is something that I didn't assemble correctly is my guess.

 

With the trans together and the top cover off can you watch inside to see what is happening? When in neutral the input and main shaft should turn together and the output should be free. Is the reverse idler gear completely dis engaged when in 1st - 4th gears? are the syncro's free and able to operate correctly? Before disassembly I would try to figure out the cause. Probably not telling you any thing new, just thought's that go thru my head.

 

It’s possible to install the gears and synchronizers backwards.

If you took pictures on disassembly, reference them to verify, otherwise you’ll need to hunt down an exploded view.

 

Funny thing is I didn't take nearly enough pictures. I did finally find the factory service manual for that year Chevy pickup. And it does have the exploded parts diagram of that nv4500.