Gearbox swap for MX5/Miata

[biznatch]

I've read elsewhere that the MX5 1.8 5 speed is a viable swap for the SA/FB box. The SA/FB Bellhouse and tail shaft are retained and apparently bolt up to the MX5 box using identical holes.

The input shaft is slightly longer than ours so needs to be shortened by grinding off a small amount. Gear ratios and smoothness are said to be superior by all accounts. Firstly, does anyone else on here have first hand experience of this swap?
Any problems, pitfalls or tips? What year MX5 box is required, or are they all the same?

Could be a good alternative given the age and condition of our boxes.

[Steve-A]

I've built a hybrid mx5/rx7 gearbox, though as mine was to fit a rotor into my mx5 it wasn't quite the same hybrid as you're talking about.

All the mx5 5 speeds, from '89-'05, will work with Rx7 s2 + s3 and n/a s4 + s5 boxes. Each box is made of 3 main sections, the bell housing, gear set and the tail housing, and all 3 sections are pretty much interchangeable.



In my case I bolted a '96 mx5 tail housing, with the shifter housing and correct gearbox mounts, onto an s4 rx7 gear set and bell housing. As you mentioned to fit the rotary bell housing to the mx5 box you need to have the mx5 input shaft machined down a little. Watch out, because in my experience you need to make sure you take the both the speedo gear for the output shaft and the speedo drive from the same box, the internet told me they are interchangeable for the mx5/rx7 s4 boxes, but they weren't and after ruining 2 new speedo drives I ended up having to take the gearbox out of the car and rebuild it to solve the problem….

As to there being any difference in smoothness? They should be the same box, so there shouldn't be, I have to say I thought the box in my s2 rx7 was nicer than the one in my mx5, but it was lower milage than the mx5 one. The long first gear in the mx5 box is the biggest difference I notice between the two.

Note the mx5 6 speeds, available in some mx5s from '98 onwards, are a different internal design more closely related to the rx8 gearbox.

[biznatch]

Thanks mate, i think i will go ahead with this swap when i finally get around to fitting my new engine (next year judging by my current progress). The possibility of an 05' gearbox in my 85' car is quite compelling!

Thanks for the tip with the speedo drive, should i just reuse my S3 one in the new assembly?

[ian65]

make sure the pitch of the teeth on the speedo drive corresponds with those in the gearbox.....

viewtopic.php?f=11&t=376&p=3010#p3010

[Steve-A]

Yeah, as long as you fit the circular speedo gear from your S3 gearbox to the mx5 output shaft, the cream/purple gears visible in the picture I posted, then you can use the normal your original speedo drive. The mistake I made was that there are some internet sources that say you can mix and match the speedo drive parts, which as the link in Ian's post shows isn't always the case.

As to the appeal of the '05 gearbox, is IS the same gearbox as any other mx5 5 speed, the only difference will be wear. So a high millage 2005 gearbox would probably be a step backwards in terms of feel compared to a low millage 1990 box.

[Arvika]

Ratios for Rx7 is: 3.622 2.186 1.419 1.00 .758
and for the Mx5: 3.136 1.888 1.330 1.00 .814