I took my spare S20 tranny I had and tore it apart to replace the Input Shaft Bearing. I get everything put back together, case and all, and I check the gears. I can get into every gear including reverse but not fifth. So I thought, maybe you need to spin the spline to ease it into gear. Well I can't spin it at all no matter if I'm in neutral or in a gear.

What could be the issue here?

 

I had the same problem, and according to a tech bad motor mounts will cause strain on the shift linkage and 5th gear assy to wear out prematurely. Causing the dreaded 5th gear pop out.
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Drive Safely and Smile Freely

 

The tranny is still out of the car. I can spin the shaft but its VERY hard to. takes two hands.

Is this natural?

 

BUMP

 

no, it is not normal.

Open it back up, or have someone else do it for you.