6 Speed Parts Interchange (revisted)
 

After months of on and off again working on trying to put together a working 6 speed, here is what I've learned so far about mixing and matching parts between the NB1 and MSM transmissions. I can only ASSUME at this time the the NB2 and MSM share the same shift linkages as noted below. I haven't had the opportunity to tear down a NB2. I will add pictures later, but I wanted to get this post up while everything was fresh on my mind.

Gears:
The gears between the NB1 variation and MSM cannot be interchanged. We didn't have the proper tools to measure the gear diameters and the pitch but there is something different between the two. We tried to use a MSM main input/main shaft assembly and NB1 counter shaft assembly together. It would assemble, but the gears weren't meshing properly and created a lot of drag. Also, we found that the MSM gears did not have "nubs" on the end to prevent the 3-4 selector ring from being able to over travel. If it over travels, the 3-4 hub detent balls and springs can become loose and damage gears.

Shift Linkage:
The shift forks, cams, and rods are all the same, EXCEPT the shift rod with the 1st-2nd and 5th-Rev shift forks sliding on it (upper shift fork assembly). The NB1 has 3 detent grooves machined into it and no c-clip to retain the assembly. The MSM has no dent grooves and a c-clip on one end. The addition of the c-clip requires additional machining on the center housing.

The shifter rod in the tail shaft housing is interchangeable between the two variations in some situations. The NB1 shifter rod only has locating holes for the cast ends. The MSM shifter rod has 3 detents grooves machined into it near the shifter. These grooves match up with a detent in the tail shaft that was added for the NB2.

For my transmission, I used the NB2/MSM shifter rod and tail shaft on a NB1 center section. We tested the shifter with and without the tail shaft detent. My friend who was helping has driven on stock NB1 6 speeds in the past said that without the detent, the shifter felt the same. So we added the detent and it felt much better. The best way to explain it, you can feel the shifter being "pulled" into each gear as the detent goes over the crests. I suspect this is where the improved shifter feel comes from on the NB2/MSM. On the NB1, the fore/aft detents are down in the center section, so there is the ball/cup/spring of the shifter rod damps the movement. With the NB2/MSM the detents are closer to the shifter without much damping.

Bell Housing:
No differences noted.

Center Housing:
There are machining differences to accommodate different shift linkage designs (c-clip NB2/MSM vs. no c-clip NB1). There is also a difference in the number of detent balls and springs used. The NB1 has a 3 ball dent setup in the rear of the center section to match the grooves on the 1-2,5-R shifter rod. The NB2/MSM seems to have omited some of these detents because of the detents on the tail shaft shifter rod.

Tail Shaft Housing:
All the same except for the additional dent. A NB2/MSM shifter rod could be used in tail shaft (with or without the detent). The NB1 shifter rod may not work with the NB2/MSM tail housing unless you removed and capped the dent hole.