Miata Ring & Pinion Swap: How DIY-able?

So, after blowing up the 5-speed in my Miata, I'm going to a 6-speed. Problem is, its mostly an autocross car and the 6-speed with any of the normal gear ratios (3.90, 4.10, 4.30) destroys your 2nd gear top speed. So the solution is the 3.63 ring & pinion swap (only available in RWD 5-speed 626s and some Australian-market Miata SE Turbo), which lengthens 2nd-gear almost to 5-speed/4.10 and nets you a bonus deep cruising gear. It's pretty rare though, so it's not case of just buying a complete centersection from a dismantler or trading rear ends with someone.

How easy is it to install a new ring & pinion in a Miata? Any crazy special tools like case-spreaders required? Or should I just leave this one to the pros? Keep in mind, I'm a GM tech for a living, my father was a machinist for 25 years and is a manufacturing engineer for the past 15, and I built the engine that's in the car right now, so our talent/experience is a bit above the normal guy.

Only special tools I can think of at the moment are a hydraulic press and the right separators and installers for the pinion and other bearings, otherwise it requires a bit of patience and attention to detail to get the backlash set up properly. 

For examples of the procedure, http://gearinstalls.com/ has been helpful to me.

Well, I have access to a big 20-ton OTC press at work

Nothing special about the job, other than you have to set up the gears.

That 3.6 rear is a little more common than you think - it was Euro-spec as well, and Mazda Competition did bring in a number of them.

For an autox car, I'd be tempted to stick with the 5-speed. How much power are you running?

In reply to Keith Tanner :

I'm making 245hp, although I'm considering adding a little more. I had a "1989" 5-speed in it, which I guess is supposed to be stronger than the later 5-speeds according to Planet Miata, but it absolutely came apart on me at The Tail Of The Dragon this fall. Also, because the car is super loud and I do a lot of highway miles, I like the idea of turning ~500rpm less at cruise.

Fair enough, sounds like a 6-speed is a good call.

There was a design change in the 5-speeds with the NB, but it was mostly in the synchros. I hadn't heard anything about the early ones being stronger.

You really don't need a separator unless you plan on changing the differential side bearings. Use new bearings and crush collar on the new pinion. You will need a dial indicator and a 7"-8"micrometer, along with a torque wrench and common hand tools. The bearing can be carefully driven onto the pinion with a pipe, the fit is not heavy- a bit of heat helps. You may need to buy a new pinion shim.

Here's a good tutorial from the Miata.net website-

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

In reply to ChasH :

You will need a separator to get the shim out from under the old pinion bearing.  Modern manufacturing is generally so repeatable that the original shim is not only a good starting point, it is 99% of the time the correct one.

If you do not do this, you will spend a good amount of time juggling shims... which requires a bearing separator.

Proper pinion depth is the most critical part plus bearing preload.

You could also swap to the bmw tranny using one of the kits.  Might open more ratios for you.

The ZF320 has a mucher deeper 2.49 2nd gear which doesn't help him. The miata 2nd gear would be 1.88 in the 5speed and 2.27 in the 6speed.

FieroReinke said:

You could also swap to the bmw tranny using one of the kits.  Might open more ratios for you.

Looked at the BMW trans kit. Lot of money and it caused more issues than it solved. $2500 for the swap kit, then $500-$1000 for the trans, then if I used the stock rear diff I have to buy the $600 PPF delete kit, plus the delete kit doesn't work with the '90-'93 rear subframe, so I would have to swap to the '94-'05 subframe. And the BMW trans ratios don't work at all with the Miata gear ratios, I think one combo had a 2nd gear top speed of like 45mph. Could do the Getrag rear diff as well, but then that's another $1460 for the kit plus whatever that diff costs. So I could spend $4000-6000 for that, or $750 trans + $750 rear gears.

In reply to Knurled. :

Given the difference in ratio I think a different shim will be needed.

The thickness of the shim can be calculated without trial and error using the number on the nose of the pinions and the thickness original shim.

In reply to ChasH :

...and you can't know the thickness of the original shim, unless you have a bearing separator

This is kind of my day job, I'm the guy who does the R&P swaps.  Trust me, modern manufacturing is that good.  I can't remember the last time I had to play with pinion shims on anything that started with an OE-assembled gearset.  Might have been maybe ten years ago, putting gears in a late 60s GM.

Hell, Ford doesn't even machine the end of the pinion gear anymore, for their Motorsport gearsets.  There's nothing to measure, the end of the pinion is rough and there are no markings.  You just put it together with the old shims and it doesn't even "pattern", it goes full swipe across the tooth, drive and coast.  Freaked me out the first time I did one.

In reply to Knurled. :

Ford isn't Miata and the R&P we're talking about was made years ago, before any of this cost cutting business.

I've done many Mazda R&P changes so you got nothing to tell me.

I personally think we should fight about it some more! I also think in general it is good policy to tell people to take their differentials apart without having a bearing separator at hand! Un-cover your bases, leave no base unturned! Harbor Freight is too expensive anyway. 

Well, decided to tackle this ourselves. I was uncertain, but my father was a machinist for years and has done a ton of research on the process as well, and feels it's within our capabilities. Dropped the differential out Saturday and had it apart by lunch. Hardest parts were separating the aluminum and steel case halves that haven't been apart in 26 years, and getting the pinion nut off (Ended up dropping two throwaway bolts through the flange, sticking a piece of bar steel between them to hold it, and then removed it with the Snap-On electric half-inch impact). Now he has to get a 7-8" micrometer (his set only goes up to 6-7") from his work, already have multiple dial indicators with magnetic bases.

And the new holy grail 3.63s, straight from Hiroshima

Also, while putting the Miata up on wooden blocks to be able to jack it up (it's real berkeleying low and I don't have a lift), I discovered that when I swapped the 5-speed transmission wiring connectors onto the 6-speed, I goofed something up and my Reverse lights come on in Neutral and go out when it's put in gear. Whoooops

Easy and common mistake. Mazda used the same connectors on the reverse and neutral switches. Just swap the wires between the two switches.

If you want to use the micrometer for adjusting the carrier bearing preload, the magic number is 1.5 holes tighter than "snug".

I think you're going to like the 3.63s! How long have your reverse lights been flashing every time you go across neutral to shift? That's awesome! 

Probably helps with tailgaters. 

Also, the car may have had some idle problems occasionally.

Vigo said:

 How long have your reverse lights been flashing every time you go across neutral to shift? That's awesome! 

Well, it's winter here in NY and this car gets parked in the winter, so I have yet to drive it with the new transmission. My 5-speed died in late November (one of the input shaft plate bolts fell out and ran it out of fluid) and it's been sitting since.

Keith Tanner said:

Probably helps with tailgaters. 

Also, the car may have had some idle problems occasionally.

Between the on/off 1.6L IAC, MegaSquirt, and the Rotrex blower the idle is pretty funky already, so I likely wouldn't have noticed. 

The tailgater comment makes me think of how my friend discovered that the Fiat 500 Abarth has a lower rev limit when you have it in Reverse with the clutch in, but that it builds boost at the RPM it holds it at. So we were trying to think of how to put the Reverse switch on a handheld momentary contact switch so that he could use it as launch control in forward gears at autocrosses. Then we realized it would likely freak out the people in line behind him when the reverse lights came on while he's at the starting line, and they would think he was about to Marty McFly into them

So, here's a fun little fact I've discovered: RockAuto lists a Timken 2800S as the pinion seal for a 7" Mazda rear diff. This is in fact, not correct. You could practically throw a Timken 2800S through the opening in the case for a pinion seal. And RockAuto lists dimensions for the part under the More Info segment, and I measured the seal, and it was not boxed correctly. So rather than risk getting another wrong seal from a different manufacturer from Rock, I instead just ordered a new Mazda seal from Priority Mazda, which is what I should have done in the first place.

The frustrating part was that I needed to have it together before Saturday, because Saturday my father was going to take it to his work and use the 7-8" micrometers that they have there to set up the backlash/pattern. I discovered the wrong seal Friday. Quick call to all the local auto parts stores showed that it was either not listed in any of their warehouses or was halfway across the country. So, hopefully can get it set up this weekend.

Well, it's winter here in NY and this car gets parked in the winter, so I have yet to drive it with the new transmission. 

Booooring. I forgot winter was a thing (because Texas). I know at one point i figured out that on my old Dodges if you lift the lockout ring and slide over towards reverse in the neutral gate the light would still light up, so you could light the reverse lights while moving forward in neutral. Sucks on the seal snafu!