NB 5 speed 5th gear problem.
 

I picked up this NB 5 speed of unknown miles from a friend as part of a big parts swap. It was stored upright, dry, on the bellhousing. No shift lever installed so I couldn't test shift it. Installed it in my 93 NA, behind a fresh 1.8 BP4W build. It snaps nicely into 1-4 and R, but immediately pops out of 5. I've searched a bit finding threads about syncro wear, but not the popping out of gear. I've had other cars do this under load, but never when just sitting on the lift statically.

Anyone with some btdt experience to share?

https://photos.app.goo.gl/z45wCjFJmH7beXW38
https://cimg3.ibsrv.net/gimg/www.mia...baf8628e01.jpg
Engine pic just for fun.

Apparently the car is self healing. Or possessed. When I got home from work yesterday, I pulled off the center console and upper dust boot, to be sure there was no restriction in the throw of the lever. And I still had a 4 speed trans. I was pretty bummed, thinking I'd have to source something different, or fix this one, and lose more time to the lift. Had some dinner, watched a movie with my wife, went back in the garage about 4-5 hrs later to re assemble it all in sadness and shame. Before starting to re assemble, I gave it one more test shift. And had all 5 gears solidly, + reverse. What??? Tried it again. And again. And again. Same result, it all worked as it should.

So if someone else has this problem, as much as I'd love to share a solution and maybe get a cat, I got nothing for ya. Other than I'm stopping at the gas station on the way to work to pick up a lottery ticket.

I have also faced the same problem with my gear box.

I have taken apart and rebuilt several spec-miata-raced 5 speed transmissions, and a couple of well used street trans. Several of them had a broken 5th gear synchro. They either just crack in one spot, or they break into pieces. Not sure why, especially since at our track a Spec Miata never even uses 5th gear. And earlier RX7 trans I rebuilt never showed this problem.

But since it is a trend, it could easily explain your problem. A broken snych could push back and disengage the gear, and it could shift around, and then allow it to work for a while.

An alternative is that one of the springs that the selector keys ride on is broken or popped out of place. So the tension on the selector sleeve is not correct, and affects how it engages with 5th gear.

If it is either of these problems, it ultimately needs a partial rebuild, or replacement.

Oh, one more simple thing to check that does not involve taking the trans apart.
I had trouble with one trans slipping out of second too easily, and discovered the detent ports were not tightened up. So the springs did not push the detent balls against the shift levers properly. There are three on top and left side of the centre plate of the case. The lowest one is 5/R. And there is another 3 screws on the back, bottomish side of the tail housing that controls return pressure on 5/R. (Sort of looks like a Darlington transistor)

Thanks for that info! I'll check the screws you mention in the prior post, and if for some reason they are good and the trans goes bad again, I'll have a plan "B" to check the 5th gear syncro.