Holdfast

I'm currently rebuilding a Miata transmission for my rx7. This transmission I got for free because it was stuck in 5th & Rev. It was only used for one race after a complete rebuild before I got it. Everything inside looks brand new. Now my question. Should I or can I leave out the reverse shift rod spring on the Miata transmission? This is the cause for the stuck in reverse 5th gear problem. None of the rx7 transmissions I've taken apart have this spring . Only Miata. I was going to leave it out but thought I should ask first. Anyone with transmission rebuilding experience specifically with the Miata transmission with some insight?

swbtm

iTrader: (2)

Responding more to ask a question than with an answer to your specific one.



How are you going about the swap? Are you using an FB trans or a T2 trans?

Are you taking the Miata tailshaft housing off and swapping the FB one on?

What are you doing about the bellhousing pattern?



Are you going to swap the miata input shaft into the FB box? (this would need the following input shaft mod)

https://www.mazdatrix.com/miata-inpu...-modification/

I think the guy in this thread tried this with some issues with the gears causing side loads on the fifth/reverse shaft.

https://www.rx7club.com/race-car-tec...ft-cut-319981/



Some info about the stuck in reverse issue.

https://www.miata.net/garage/stuckreverse.html

https://forum.miata.net/vb/showthread.php?t=629551



Are you thinking the spring not being there would help in not allowing the shaft to get stuck in reverse?

That doesn't seem to have been addressed by the NA guys.



I'm curious to know which route you are taking. I have my spare NA Miata 94 box apart now. I was going to swap over the gearset and synchros only (I've read those are a direct fit) and leave everything else as is for the FB box. I have 4 other FB transmissions sitting around, that I'll eventually get around to tearing into and comparing things directly.

Holdfast

This is the 3rd Miata swap ive done. Using a box from a 96 Miata with bell housing and tail housing from a fb rx7. U have to completely disassemble the trans to get the input shaft off. To have it machined. Few write ups on here on how too. Mazda gearbox are all the same from 81up to 97 or 98. Could be other years also. That's for rx7, b2200 truck and Miata. With just minor differences inside, and of course Input shaft, bell housings and tailshaft are different. I just kept the reverse spring in there. Miata 5th gear is .814 the 84 rx7 5th gear is .758. Right now with the Miata transmission and r&p 4.10 final drive on the hwy it's 4k 4.5k rpm at 80mph used to be 3.5k rpm. So I'm hoping that the rx7 5th gear will help.

j9fd3s

iTrader: (3)

the miata FIVE speed guts are the same up to 2015...

amcrx7

That last got my attention!
How sure are you that all the 5 speeds use the same internals?

j9fd3s

iTrader: (3)

same part numbers.

swbtm

iTrader: (2)

j9, if the part numbers are the same, how is there room for the double cone synchros for the Miata 1.8 gear stack in the fb housing? I've also seen that the 1.6 boxes are a little different with how the gears line up. I'll eventually just measure all these things side by side when I'm swapping over the 1.8 gear set. Can I use the fb input shaft with the Miata 1-2-3-4 gears and fb 5th? That's where I think the previous post I copied ran into issues, just asking to confirm (that's it's not possible). Thanks.

Holdfast

​​​​​​ I found a lot of good info at this site. It explains everything Miata Drivetrain Interchange Guide
As for the 5th gear swap I found out after I put it together that it does nothing to raise the gear ratio. It actually lowers it. The Miata trans uses a different number of teeth on the input and counter shafts than the RX-7 trans so swapping to the RX-7 5th gears creates a different ratio all together. So now I have .816 5th gear ratio. Before the stock Miata 5th gear ratio was .814. I installed the trans already and it works great. But screams a little more on the freeway.

j9fd3s

iTrader: (3)

the double cone synchros are like a twin plate clutch, its more surface area in the same size package. they are maybe 1-2mm wider, and 1-2mm bigger in diameter.

the FB input shaft has a different gear pattern, and will not mesh with the miata stuff, you can try it

swbtm

iTrader: (2)

Thanks, one last question. Does that mean i can take the Miata second gear synchro and put it in the fb box (and fb gears). This is my plan moving forward.

Thread jack complete. Thanks for the info.

j9fd3s

iTrader: (3)

you'd have to measure it, its not impossible, but i wouldn't expect it to fit. Mazda has their own transmission factory, and they changed a lot of stuff between the Rx7 and Miata

just a for instance the Rx7 has 1-4 synchros the same size, but the FC got larger 2nd and 3rd gear synchros, and then the miata upgrades to the double cone. its possible that something weird would work too, maybe the Rx7 2nd gear is the same as the FD 4th or something.

Holdfast

The Miata and rx7 synchros are not interchangeable. The cone shape is different. The 2nd gear on the counter shaft does not come apart. The only two gears that can be swapped around is 5th and Rev.

swbtm

iTrader: (2)

Awesome, thanks. Machining the na8 input shaft it is 👍