NB Miata Wont Shift into reverse and no 5th.

Hello, I have an 03 Miata with a 5 speed. I'm stock powerwise and have never beat on it. One day I was driving along in 5th and it popped out of 5th. Now it will go back into the 5th spot but it pops right back out and it will not go into reverse. I have opened up the shift turret and found nothing and replaced the shifter bushing, I have also pulled the reverse switch out and the 5th/r detent spring. Nothing I've done has changed what it is doing. Is there anything else I should try before pulling the transmission? Thank you.

I'm not all that familiar with the internals, but that sounds like a bent shift fork. 

It's broken. Pull it out and have it fixed.

In reply to Keith Tanner :

Okay, some people have also said it could be a clutch hub or synchro. I don't think it's a synchro because there was no crunching or grinding and it won't shift with the car off either.

A failed shift collar will cause this.

The shift collar is the bit that slips over the synchros and engages the splines on the side of the gear.  It uses the same splines to slide back and forth over the hub that is attached to the shaft.  What can happen is, if the splines on the end of the collar are bent or have large burrs on them (such as what happens if you miss a shift or it pops out of gear), in addition to not being able to engage the corresponding gear because the splines are messed up, you won't be able to slide the collar back the other way to engage the other gear used by the collar, because the burrs/bent splines will not pass over the hub to allow it to move that far.

I have done this with a really, astoundingly bad 2-3 shift, leaving me with 1,2,5, and R.  I was able to gently work the shifter back and forth to where I massaged the collar smooth enough that I could engage 4th gear, but 3rd gear would always pop out if I could even get it to engage in the first place.

RX-7 transmission but smoothcases is smoothcases.  The 5-R collar is in Reverse and the 1-2 collar is in 1, to lock the gearset in place so I can torque the mainshaft nut.  The 3-4 collar is in neutral position.  You can see how much of the hub splines are exposed when a collar is in gear, the splines that engage the gear have to cross over all of that to allow engagement.

There is also a possibility that the reverse blocker detent mechanism in the shift tower is jammed.  This is a bit unlikely if the problem started with popping out of gear.    Most likely the parts were worn to begin with and popping out of gear under load was the final thing that led to where you are now.

The transmission needs either major surgery or possibly the car needs a transmission transplant...

In reply to Pete. (l33t FS) :

Alrighty, thats what i was thinking I would have had to do. Just wasn't sure if there was anything else to check before i pulled it.

So then, rebuilding the trans. What is the minimum cost of opening it up? Do I need gaskets and seals or rtv?

In reply to car_006 :

5 speed Miata transmissions are pretty inexpensive, it might be cheaper to just chuck another one in there.

If you want to open it up, and would like a taller overdrive, now would be the time to do a 5th gear swap.  This would require obtaining an entire nonturbo FC transmission for parts since I highly doubt that Mazda sells individual gears for a nearly 40 year old non home market car.  

Likewise, you will probably find that it is cheaper to buy an entire used transmission than to buy the new parts for yours, if you just want to keep it as stock. 

If you are only going to address the 5th/Reverse problem you will only need sealant.  And a lot of good circlip pliers.  And a 16" deep 40mm socket, if you need to remove the 5/R hub.  And a bearing puller and a way to make it longer.  And good pin punches because a lot of components are secured with roll pins.  And probably a new shift collar and 5th driven gear, possibly synchros and synchro keys.

Edit: If you are more curious, I tore into a smoothcase (type M) starting about halfway down the page here.  Mazda came out with the trans in 1978 and did not really change it much other than minor details until they stopped making it with the introduction of the NDs.  A friend with a fiche says that the NC mainshaft is the same part as an '82 RX-7 

Alrighty, thank you.