Well here is the problem. The old 1940 tranny runs great except that it will pop out of high gear when climbing up a hill. The transmission has 27,000 miles on all NOS absolutely everything in the transmission. I mean everything.
I figured that the second/third shifting fork was worn, which it was, so I replaced it with another NOS fork. All of that work resulted in NO resolution of the problem.
The wear in the groove on the shifter fork is about 0.070" - alot. It appears that there is minimal wear on the main synchronizing unit.

dtm

Dave,

I had the same problem (popping out of high gear when pulling up hill or drifting down hill) on my 40 1/2 ton 3-speed. I'm pretty sure my problem was due to misalignment of the transmission to the crankshaft centerline. The internal teeth at the front of the synchronizer unit and the mating teeth on the input shaft (clutch gear or mainshaft) were abnormally worn. The truck 3-speed uses a top-mounted shifter, so the shifter forks are different than yours, however, I did not see any abnormal wear on the shifter fork face or the collar on the synchronizer.

I checked the alignment of the transmission with a dial indicator and ended up adding shims under three mounting ears on the transmission. I installed a NOS clutch gear and a good used synchronizer unit and after about 4,000 miles, have not had any more trouble with popping out of gear.

Your problem appears to be different than mine, though the result is the same. Is that worn face on the shifter fork the face that pushes the synchro toward the back of the transmission? If so, that would indicate that the shifter fork is constantly trying to shift the transmission out of high gear. It seems like maybe there is a problem with the shift linkage somewhere that is causing that shifter fork to rub continously against the synchro collar.

If you find the problem, I'd be interested in knowing what it is.

Mark

Do you still have the vacuum assist?

Dave - As you may remember, my '40 has over 400,000 miles on it. During that 400,000 miles I have had 6 events of the transmission popping out of gear. Some with vacuum shift and some without. The first five times I changed just the clutch input shaft and was able to get 40 - 60,000 miles before it stared popping out of gear again. Changing the clutch shaft is the easy way out because you can change the clutch shaft without disassembling the transmission. Finally, on the sixth event the clutch shaft did not fix it and I had to change the synchonizer barrel along with the clutch shaft. The teeth at the high gear spot in the sync. barrel were almost completely worn off. The pop outs were never consistent. Sometimes it would pop out on a pull and sometimes on a coast or downhill. Each time I could compare the teeth on the old (popping out) clutch with the neww one and could see the wear (the clutch shaft wear round and is very visible). If you want to talk give me a call. 318.798.0493

Dave,
By far the most common reason for coming out of high gear is uneven wear between the inside of the input shaft gear and the sliding gear that meshes with it. The wear is typically caused by either misalignment of the transmission and engine or wear in the pilot bearing/bushing causing the shaft to wobble. Correct alignment of those gears is critical to long term life.

It is also common for the pop out to occur when power is reapplied. What typically happens is that the gears start to come apart when decelerating (with a floor shift you can feel the stick move a bit toward second). When power is reapplied nothing happens. Then deceleration again moves the gears a tiny bit again. Finally when power is applied the gears "pop" apart. It can happen when decelerating but not as common. Just depends on how much "mesh" when force (either deceleration or acceleration) is applied.

The old 40 still has the original vacuum canister, which works very well. The transmission shifts with little effort brtween all of the gears.

dtm