The 280z came with a later 5-speed transmission already in it when I acquired the car. And ever since I've owned it, the car has been popping out of 2nd and 4th gear under engine braking. For years I figured it was just a sign that a transmission rebuild was in the cards eventually.
Then I came across an article that reframed the whole thing. The claim was that if you're running the original shift rod with a later 5-speed, the lower portion of the rod can contact the transmission housing where it enters - and that contact is what's causing the pop-out, not anything internal. The fix is to pull the rod, look for contact marks, and grind relief pockets at those points to give some clearance. I pulled the rod and sure enough, the wear marks were right where the article said they'd be. I clamped it in the vise and ground the pockets.
While I had everything apart I also replaced the outer shift boot. The original vinyl unit was 50 years old and looked every bit of it - cracked, stiff, and past its useful life. The replacement isn't an OEM replica, but at this point I've already swapped in modern gauges and a classic-looking modern radio, so close enough is good enough.
After reinstalling everything and taking the car out, no pop-outs. After all these years, it was the rod contacting the housing - not the gearbox.
