Today I spent most of my Saturday helping the shop manager at AAMCO put my transmission back together. It's apparently hard to find good help and he had been stuck for a few days, needed a fresh perspective.
Diagnosis: The magnesium casing I purchased had one of the bearing seats ovaled out, which I should have noticed - one of the bearings went in suspiciously easy. This is apparently common with magnesium cases, as it's a weaker metal in most ways than aluminum is. I had also shimmed the transmission wrong which was letting the input shaft move laterally a bit, and between the two of these, that's probably why it was popping out. Driving around holding it in 6th occasionally put some wear on one of the shift sleeves, so I bought a hardened replacement for that from Synchrotech.
Rather than get another magnesium case, I took it on authority from a few sources I felt I could trust that the Accord 5 speed aluminum case was identical on the outside, that the internals were (mostly) interchangeable, that Accord parts can be put into a TSX case so therefore I should have no problems putting the TSX gears into the Accord case, which is less likely to wear out due to it being made of aluminum, as well as being cheaper and easier to acquire.
WRONG!
Turns out the internal castings of the two cases are different. The 5-6 shift fork on the Accord case has the fork for reverse welded in place at the bottom of the fork shaft, and when the shifter is moved right and down (into reverse) the fork moves a blank (with a syncro, amusingly) into the position that has 6th gear in the TSX case, it also engages reverse.
In the TSX gearset, the reverse fork rides on the 5-6 fork shaft but is not welded in place, as shifting into 6th would put the transmission into both reverse and 6th gear - which would be bad. Reverse has its own ... erm, not sure the proper name, but it's what the shift lever presses on. So anyhow, the reverse fork from the TSX contacts some of the structural ribbing in the Accord case when not engaged, and that needed to be cut/ground out of the way.
Transmission will likely go back in the car on Monday. And, I will have the weirdest K series box: Accord case, TSX 1-5, 2012 Civic Si 6th gear, 2006 Civic Si limited slip differential, and a custom final drive originally intended for Mini Cooper K swaps.
