What Transmission Is This In Pic?

The T-90, T-86 and I assume T-84 all use a sliding gear for 1st and reverse. That slides on splines on the main shaft. 2 and 3 operate as above, where the synchro hub slides and locks the hub to the main shaft and the gear to the hub, engaging the gear. I think all the modern manual transmissions use some scheme where the cluster engages the input gear, and the gear that is selected is locked to the main shaft. You have to go from the input gear, through the cluster, and back to the main shaft to get speed reduction. The 3-speeds have a bronze sleeve bearing inside second gear, which seems sufficient. It only turns on the shaft when there is no load on the gear, so there's little wear.

JMO - you should be able to find a truck 4-speed that does not have a lot of wear. They are tough, and I'd think you'd need to put a lot of miles on them to wear them out, especially if they came from a pickup originally. Any of them (T-18, T-19, NP435, SM420, SM465) will need an adapter kit, and I expect the transmission must come apart completely for installation of the new main shaft.

 

That holds true for all BW manual transmissions and also the SM 420/465 for that matter.

Caution: reading the following may cause your head to spin.

One thing I failed to mention in detail is how the 1st/reverse gear operates.
This gear in some non synchro 1st gear transmissions like the BW T90 is also splined direct to the main shaft yet it does not always spin from rotation of countershaft gear.
That’s because the sliding gear (the largest of gears on top) is not always aligned to the counter shaft gear teeth below it.
So if you have a non synchronized BW like the T90 then the Ist reverse slide gear only spins when it is slid foreword or rearward from it’s middle or neutral position.

If you have a synchronized 1st like the BW T14 then the first reverse slide gear is not direct splined to the mainshaft.
It must also then also be engaged to the main shaft only via a synchronizer assembly.