I'm hoping to skip some of the noise, and get some solid GRM opinions on the NC miata 5 vs. 6spd transmissions. The application is for a 2.5 swap into a rallyx car. I understand the 2009 and later transmissions are supposed to have stronger forks, but is that for both the 5sp and 6spd? The gearing of the 6 spd is a bit better (2nd is almost a match to what I currently have, and the miata 3rd is closer, which I'd like), but there's a fair jump in prices as you go from '06-08 6spd to '09+ 5spd to '09+ 6spd. If I need to get the '09+ 6spd, that's ok, I just want to make sure before spending the money.
thanks, all!
A friend of mine said all NC 5 speeds have identical internals to NB 5-speeds, same part numbers for everything.
The first 6-speeds had a lifespan of 200 laps before the shift forks broke.
Keith Tanner said:
The first 6-speeds had a lifespan of 200 laps before the shift forks broke.
Noted, thanks! Big difference in durability between the 5spd and later 6spd?
I never did any endurance racing with the later ones :) Sorry.
Keith Tanner said:
I never did any endurance racing with the later ones :) Sorry.
Sounds like you never got to do any endurance racing with the earlier ones, either!
:)
200 laps at a time ;) This was the race debut of the NC using the MX5 Cup prototype cars on the Mazda team at the 2006 25 Hours of Thunderhill. We found out fairly quickly about the shift fork "fuse" and had to limit the shifting our drivers did. The information went all the way to the top of the Mazda Japan R&D food chain before the race was finished...
Keith Tanner said:
200 laps at a time ;) This was the race debut of the NC using the MX5 Cup prototype cars on the Mazda team at the 2006 25 Hours of Thunderhill. We found out fairly quickly about the shift fork "fuse" and had to limit the shifting our drivers did. The information went all the way to the top of the Mazda Japan R&D food chain before the race wa finished...
THIS is why I ask questions like this here. Thanks again, Keith!
I'm actually going to go with the 5spd anyway. Anyone have a shifter they feel up to throwing in a flat rate box and selling?
Keith, with that info in my mind, who is/are the reputable Miata salvage(s) now that FM has been out of the game for a while?
I want to bolt up the later NC trans when I do the 2.5 swap next year or the year after.
I don't think I've ever had bad dealings with a Miata salvage shop. I could name names but I'd leave someone out :)
The one exception was an "American Import" yard in CA. We bought a head from them and asked them to check the condition of the cam journals first. They didn't, the journals were trashed, waste of everyone's time. But they're not a Miata salvage shop and it was a long time ago.
Keith Tanner said:
I don't think I've ever had bad dealings with a Miata salvage shop. I could name names but I'd leave someone out :)
The one exception was an "American Import" yard in CA. We bought a head from them and asked them to check the condition of the cam journals first. They didn't, the journals were trashed, waste of everyone's time. But they're not a Miata salvage shop and it was a long time ago.
I checked the usual players that google presented, but seems like NC specific is thinner. Do you recall any particular good source for NC parts? I did just check online, and haven't tried calling anyone yet.
Sorry, no. I rarely have to source salvage parts, I have a pretty good stash on my own. And my NC trans is a core only, so it wouldn't be much help to you.
Bumping this for a follow-up: can anyone confirm if the rx8 transmission output shaft size matches that of the nc 5-spd? I know the spline counts match, just a question of diameters.
In reply to bluej (Forum Supporter) :
Are you SURE the spline counts match?
People use Turbo II driveshafts when swapping RX-8 transmissions into FCs, the Turbo II had a larger output than smoothcase, and the NC 5 speed is a smoothcase.
I do have an RX-8 transmission I can try to plug a smoothcase driveshaft into (I have SA, FB, NA6, and NB to choose from - and they're all the same)
Are you thinking of the RX-8 STYLE 6 speed from the NC? They're the same transmission family but everything is bigger in the RX-8. Input, output, most bearings.
Thanks for chiming in, Pete. I figured you'd have some of this info handy! I counted 22 teeth on the transmission output shaft of the NC 5spd I purchased.
And this is the pic of an rx8 transmission slip yoke I was able to find where you could actually count. Though the part number is for an auto transmission. I had read somewhere about the MT and AT versions actually being interchangeable if not ideal (slight length difference?) between the rx8 driveshafts. It's 23-2 teeth, so would match the 22 teeth of the miata trans if diameters match as well.
Edit: my quick check of the outer diameter of the nc 5spd output shaft (outside of teeth) is in the 25-26mm range. Didn't have good access or calipers handy. Does that sound right?
Yes, the RX-8 (and FD and Turbo II and FC auto) are 23 spline. Smoothcases (and FB-earlier auto) are 22 spline. I'm not sure how you think they are the same? You just counted and showed they are different.
All slip yokes have a missing tooth to allow fluid to move when the driveshaft plunges. It could have sixteen missing teeth but it will still be larger 
The same size difference is at the other end, too. The smoothcases have a 22 spline input shaft. All ribcases* and RX-8 have a 23 spline input shaft.
(*) Not the ones made before '79. The RX-3/4/5/REPU ribcases were 22 spline)
I counted 22 teeth on the trans output and 23 on the rx8 auto yoke. Think of it as if they had 5 and 6 teeth respectively. The 5 hills of the output fit in the valleys between the 6 hills of the yoke. So the rx8 auto should be a 22 spline output.
Edit: this is the site the pic above is from:
https://autoparts.beforward.jp/detail/Transmission---Drivetrain/Propeller-Shafts/PA01782260/
I found it doing an image search of the rx8 auto driveshaft pn: PA4025100
This is not an academic question, I did end up ordering a used rx8 auto ds taking a chance for $60. It was supposed to arrive early this coming week, but the seller was feeling shiny and shipped it late and slow, so not till Friday now. If it's not going to work, I'd like to know before then if possible.
Found my calipers, measured 24mm diameter from outside of nc 5spd output shaft teeth.
bluej (Forum Supporter) said:
I counted 22 teeth on the trans output and 23 on the rx8 auto yoke.
Yes, those are different numbers and therefore won't work together. I'm not sure how you think they can.
Pete. (l33t FS) said:
bluej (Forum Supporter) said:
I counted 22 teeth on the trans output and 23 on the rx8 auto yoke.
Yes, those are different numbers and therefore won't work together. I'm not sure how you think they can.
Pete. You're killing me bud. Think about it this way, the rx8 yoke i reference here has 22 "valleys", to match 22 "hills" of the output shaft. That means 23 "hills" on the yoke. The numbers reference different things. Draw it out.
Edit: would you agree or not that the pic I posted earlier of a slip yoke needs a 22 spline output shaft to mate with it?
In reply to bluej (Forum Supporter) :
A 23 spline yoke will need a 23 spline shaft. 22, as Brother Maynard would say, is right out.
I count 23 valleys in the slip yoke, needing 23 spines to fill them.
